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The 26.5 ka Oruanui eruption, New Zealand: An introduction and overview
Abstract
The 26.5ka Oruanui eruption, from Taupo volcano in the central North Island of New Zealand, is the largest known ‘wet’ eruption, generating 430km3 of fall deposits, 320km3 of pyroclastic density–current (PDC) deposits (mostly ignimbrite) and ∼420km3 of primary intracaldera material, equivalent to ∼530km3 of magma. Erupted magma is >99% rhyolite and 90%). PDC deposits range from mm- to cm-thick ultra-thin veneers enclosed within fall material to >200m-thick ignimbrite in proximal areas. The farthest travelled (∼90km), most energetic PDCs (velocities >100ms−1) occurred during phase 8, but the most voluminous PDC deposits were emplaced during phase 10. Grain size variations in the PDC deposits are complex, with changes seen vertically in thick, proximal accumulations being greater than those seen laterally from near-source to most-distal deposits. Modern Lake Taupo partly infills the caldera generated during this eruption; a ∼140km2 structural collapse area is concealed beneath the lake, while the lake outline reflects coeval peripheral and volcano–tectonic collapse. Early eruption phases saw shifting vent positions; development of the caldera to its maximum extent (indicated by lithic lag breccias) occurred during phase 10. The Oruanui eruption shows many unusual features; its episodic nature, wide range of depositional conditions in fall deposits of very wide dispersal, and complex interplay of fall and PDC activity.
... Taupō is known as the site of the most recent supereruption on Earth at 25.5 ka. The Oruanui supereruption is the largest phreatomagmatic event documented and occurred within the Last Glacial Maximum (Wilson 2001;). It is found as far away as the Chatham Islands (~18 cm thick; Figure 4) and glass shards have even been found in an Antarctic ice core ). ...
... The northern end of this cutting exposes only a small portion of primary deposits from the Oruanui supereruption and it is difficult to gain an appreciation for the complexity and volume of this eruption at any one location. The Oruanui eruption was prolonged and episodic with 10 phases: a total of >530 km 3 of magma (>1100 km 3 of pyroclastic material) was discharged over a total period of several months (Wilson 2001;Wilson et al. 2006;. Pyroclastic density currents were generated throughout the eruption, reaching peak distances of ∼90 km from source during phase 8 (Wilson 2001; Figure 4) and the ignimbrite is exceptionally thick in proximal areas ( Figure 12). ...
... The Oruanui eruption was prolonged and episodic with 10 phases: a total of >530 km 3 of magma (>1100 km 3 of pyroclastic material) was discharged over a total period of several months (Wilson 2001;Wilson et al. 2006;. Pyroclastic density currents were generated throughout the eruption, reaching peak distances of ∼90 km from source during phase 8 (Wilson 2001; Figure 4) and the ignimbrite is exceptionally thick in proximal areas ( Figure 12). The eruptive products are fine grained and wholly nonwelded, with abundant accretionary lapilli, particularly in more distal locations . ...
... This stop provides a good context to look at one of large volume rhyolite lavas that erupted in this area; the ~180 ka Rangitukua flow (Figures 28 and 31), which has been truncated by slumping The southern western shoreline of Lake Taupō is remarkably linear, and is mapped as a major normal fault (the Waihi Fault). This fault is inferred in this area to have largely developed in association with collapse accompanying the 25.5 ka Oruanui eruption (Wilson 2001) although continued ruptures along its extension to the south can be associated with rifting (Gómez-Vasconcelos et al. 2017). Three other isolated domes (Pukekaikiore, Kuharua, Maunganamu) are evident on our drive back around to Turangi and these represent some of the southernmost silicic lavas of the TVZ. ...
... Key volcano-tectonic features of the Taupō-Maroa area. Caldera structure and vent envelope for phases 1 and 2 of the Oruanui eruption are from Wilson (2001). The northeastsouthwest alignment of "young" faults (ruptures younger than 20 ka) highlight the axis of regional extension in the modern TVZ. ...
... However, excavation of the quarry here has exposed a basal contact of Oruanui. Of the different eruptive units described by Wilson (2001), units 1, 2 and 3 are represented above the quarry (depending on exposure and vegetation cover). These represent the early phases of the Oruanui supereruption from vents to the north and east of the main Oruanui collapse structure (Figure 45). ...
... This range is seen in SiO 2 , K 2 O and CaO values and most obviously in FeO t (Fig. 4). The range in geochemistry in all elements most closely reflects the composition range of the lower tephra of Schwarze et al. (2022), but is also reflective of a very small number of samples of glass separates presented in Wilson et al. (2006), Allan (2013), and tephra from Pank et al. (2023) as seen and highlighted in Fig. 4. The lower tephra identified by Schwarze et al. (2022) was correlated through AMS 14 C dating, stratigraphy, and morphology of the deposit to the KOT, however the "quite complex" anomalous geochemistry of the deposit was discussed with correlation to alternative phases of the eruption as identified in proximal, terrestrial studies (Wilson, 2001;Allan, 2013) being considered, and discussed further below. ...
... Fig. 4A shows there are two apparent trends in the KOT reference data when FeO t is plotted against CaO, both of which are reflected in our new Yellow Marsh glass shard data. Wilson (2001) and Allan (2013) recognise ten major phases of thē Oruanui eruption, which were linked to the development of zones within the melt body. Phases 3 and 6 were shown to have significant LSR components, linked to a deeper, hotter, more crystal-rich mush-root of the magma chamber, but making up <1% of the total erupted material. ...
... Phases 3 and 6 were shown to have significant LSR components, linked to a deeper, hotter, more crystal-rich mush-root of the magma chamber, but making up <1% of the total erupted material. As the most explosive and largest phase of theŌruanui eruption was the high-silica (77-78 wt%) phase 10, and not phases 3 or 6 (Wilson, 2001;Allan, 2013;Barker et al., 2021) it might seem unlikely that the geochemical composition of some of the Yellow Marsh cryptotephra could be explained by an 'early phase' contribution. However, modelling by Dunbar et al. (2017), showed that tephra from theŌruanui eruption may have reached eastern Australia directly from across the Tasman Sea after only a few days, followed subsequently (after about 2 weeks) by an ash cloud carried via the westerly airstream. ...
Cryptotephra subsampling techniques were used to identify a high concentration (c. 700 shards/g) of glass shards within the Yellow Marsh sediments in northwest Tasmania, Australia. Radiocarbon dating from the overlying sediments coupled with geochemical analysis of the glass shards indicate their similarity to the Kawakawa-Oruanui Tephra (KOT), derived from the Oruanui supereruption of 25,568 ± 232 cal yr BP (±2sd) from the Taupo Volcanic Zone, New Zealand. Although cryptotephra from this eruption has previously been identified in Antarctica and modelled to have been transported over parts of southern and eastern Australia, to date glass shards from this eruption have not been identified in Australia. If the correlation of the cryptotephra to the Oruanui supereruption is correct, this finding has the potential to allow Last Glacial deposits in the SW Pacific (including those in Australia, New Zealand and Antarctica) to be irrefutably linked.
... Detailed described examples of phreatoplinian eruptions around the world are the Hatepe and Rotongaio ashes of the Taupō eruption in New Zealand (Walker, 1981;Smith and Houghton, 1995), the Oruanui eruption of Taupō volcano (Self and Sparks, 1978;Wilson, 2001), the Hachinohe eruption of Towada caldera in Japan (Hayakawa, 1983), the Neapolitan Yellow Tuff from Campi Flegrei caldera in Italy (Cole and Scarpati, 1993), and the Kos Plateau Tuff from Eastern Aegean (Allen and Cas, 1998). These previous studies have illustrated that the sequences of phreatoplinian eruption styles, depositional processes, and magma fragmentation processes are diverse and very complex. ...
... In addition, Volcanic Explosivity Index (VEI; Newhall and Self, 1982) of these phreatoplinian eruptions were classified as 5 to 6 or more, and eruption column heights were estimated to be ~20 to 40 km (e.g., Allen and Cas, 1998;Van Eaton and Wilson, 2013). In fact, in the case of the Oruanui eruption (750 km 3 ), huge amounts of fine ash were widely dispersed from the vent over several hundred kilometers (e. g., Wilson, 2001). Some of these large-scale (VEI > 5) eruptions were associated with caldera formation. ...
... For the ash units, there is a clear trend toward the upper units (i.e., Unit 2 to Unit 6), which become finer in Md ϕ and smaller in σ ϕ (Fig. 7a). Md ϕ and σ ϕ values of Unit 6 are finer and relatively better sorted than those of the fine-grained pyroclastic fall deposits of the Neapolitan Yellow Tuff (Member B; Cole and Scarpati, 1993) and the Kos Plateau Tuff (Unit A; Allen and Cas, 1998), and overlap the finer part of the pyroclastic fall deposits of the Oruanui eruption (Wilson, 2001) (Fig. 7a). Focusing on Unit 6, the Md ϕ of the cross-laminated part is coarser (Md ϕ = 3 ϕ) than the massive and the clast-supported accretionary parts. ...
... The central North Island of New Zealand hosts the most productive and frequently active Quaternary region of explosive silicic volcanism on Earth, the central Taupō Volcanic Zone (TVZ). This region has produced four of the planet's 13 known supereruptions over the past 2.6 Myr , including the most recent that ejected 1100 km 3 of ash during the Ōruanui event (Wilson, 2001;Barker et al., 2021) at~25.5 ka Dunbar et al., 2017). The Ōruanui fall deposit covers a large area across New Zealand and adjacent ocean floor and is commonly referred to as the Kawakawa-Ōruanui Tephra (KOT; Fig. 1A). ...
... Onepoto is located~240 km upwind from Taupō volcano and is of particular value because Auckland is far enough away from the eruption centre for the site to have preserved the top contact of the KOT, and therefore the immediate post-eruptive effects. This is in contrast to the more proximal environments where the upper surface of the KOT has been eroded (Wilson, 2001;Manville and Wilson, 2004). The preservation of an intact 3-cm-thick KOT layer, coupled with the small, deep, closed basin and relatively high sedimentation rate at Onepoto (e.g. ...
... (A) Location of Onepoto maar palaeolake, Auckland (red star), relative to the distribution of the ignimbrite (blue outline) and the 10-cm tephra fallout isopach from the Ōruanui supereruption (afterWilson, 2001;Manville and Wilson, 2004). (B) Onepoto maar in the context of the Auckland Volcanic Field, New Zealand. ...
The ~25.5‐ka Ōruanui supereruption (Taupō volcano, New Zealand) erupted >1100 km3 of pyroclastic material during the Last Glacial Maximum. The impacts of this event on climate and the New Zealand environment remain unresolved, particularly on ecological timescales. Using sediment cores from Onepoto maar palaeolake, Auckland (~240 km upwind from source), we have analysed pollen assemblages at contiguous 1‐mm intervals, around an intact 3‐cm layer of the Kawakawa‐Ōruanui Tephra to resolve and assess post‐eruption vegetation impacts and landscape recovery. Sediments immediately above the tephra record a decline in the relative abundance of the dominant canopy species of Fuscospora, and concurrent increase in the abundances of grasses, herbs, ferns and shrubs. These changes reflect a brief (<10 years) part‐defoliation of canopy trees, permitting more light to penetrate and to encourage sub‐canopy vegetation growth. A short‐lived volcanogenic cooling inferred from Antarctic ice core records may have contributed to the changes but cannot be separated from the immediate and direct ecological impacts of ashfall on vegetation following the eruption. Our results, here applied to the world's most recent supereruption, more generally demonstrate the value of millimetre‐scale stratigraphic pollen analysis from non‐varved lacustrine sediments as a tool for assessing past eruptive impacts on sub‐decadal timescales.
... Eruptions in this period have mostly occurred from two independent silicic magmatic systems: one (by far the larger, and our focus here) spatially overlapping with the Oruanui caldera, and a second focused to the northeast of the modern lake (Sutton et al. 1995;Wilson and Charlier 2009). The Oruanui event discharged ~ 530 km 3 of rhyolite magma (> 1100 km 3 of pyroclastic material) in a prolonged, multiphase eruption (Wilson 2001;Wilson et al. 2006;Allan et al. 2017). Recognition of Oruanui glass shards accompanied by a major non-sea-salt sulfate anomaly in an Antarctic ice core (Dunbar et al. 2017) implies that ash and aerosol dispersal was at least hemispheric (and possibly global: Svensson et al. 2020;Lin et al. 2022). ...
... Samples investigated cover a broad range of magma compositions, representing the deep to shallow magmatic system beneath Taupō volcano. For the upper level silicic part of the system, three LSR and seven HSR clasts from the Oruanui ignimbrite (eruptive phase 10 of Wilson 2001) were selected. Five post-Oruanui eruptive units were sampled: dacite units Ψ and Ω, and three rhyolite eruptions including units C, S (Waimihia) and Y (Taupō), which cover the compositional range of the Holocene events (Table 1). ...
... Two additional sources may contribute to eruptive S budgets at Taupō. First, up to 5 km 3 of S-rich mafic magma may have recharged the magmatic system prior to the Unit Y (Taupō) eruption (Barker et al. 2016) and 3-5 km 3 of mafic magma was co-erupted with rhyolite in the Oruanui event (Wilson 2001;Wilson et al. 2006;Allan et al. 2017;Rooyakkers et al. 2018). Assuming K D (S) gas/melt ≈ 10 and a gas content of 1 wt. ...
The transport and degassing pathways of volatiles through large silicic magmatic systems are central to understanding geothermal fluid compositions, ore deposit genesis, and volcanic eruption dynamics and impacts. Here, we document sulfur (S), chlorine (Cl), and fluorine (F) concentrations in a range of host materials in eruptive deposits from Taupō volcano (New Zealand). Materials analysed are groundmass glass, silicic melt inclusions, and microphenocrystic apatite that equilibrated in shallow melt-dominant magma bodies; silicic melt and apatite inclusions within crystal cores inferred to be sourced from deeper crystal mush; and olivine-hosted basaltic melt inclusions from mafic enclaves that represent the most primitive feedstock magmas. Sulfur and halogen concentrations each follow distinct concentration pathways during magma differentiation in response to changing pressures, temperatures, oxygen fugacities, crystallising mineral phases, the effects of volatile saturation, and the presence of an aqueous fluid phase. Sulfur contents in the basaltic melt inclusions (~ 2000 ppm) are typical for arc-type magmas, but drop to near detection limits by dacitic compositions, reflecting pyrrhotite crystallisation at ~ 60 wt. % SiO2 during the onset of magnetite crystallisation. In contrast, Cl increases from ~ 500 ppm in basalts to ~ 2500 ppm in dacitic compositions, due to incompatibility in the crystallising phases. Fluorine contents are similar between mafic and silicic compositions (< 1200 ppm) and are primarily controlled by the onset of apatite and/or amphibole crystallisation and then destabilisation. Sulfur and Cl partition strongly into an aqueous fluid and/or vapour phase in the shallow silicic system. Sulfur contents in the rhyolite melts are low, yet the Oruanui supereruption is associated with a major sulfate peak in ice core records in Antarctica and Greenland, implying that excess S was derived from a pre-eruptive gas phase, mafic magma recharge, and/or disintegration of a hydrothermal system. We estimate that the 25.5 ka Oruanui eruption ejected > 130 Tg of S (390 Tg sulfate) and up to ~ 1800 Tg of Cl, with potentially global impacts on climate and stratospheric ozone.
... However, most of the young activity has been focused in the area that is defined by and concealed beneath Lake Taupō . Although Taupō volcano is situated at the southern end of the~340 ka Whakamaru caldera , the modern caldera structure at Lake Taupō can be largely attributed to the 25,580 ± 258 years BP (rounded to 25.5 ka hereafter) Oruanui eruption, which discharged~530 km 3 DRE (dense-rock equivalent) of magma in a complex 10-phase phreatoplinian eruption yielding widespread fall deposits and ignimbrite, together with thick (2-3 km) intracaldera fill (Wilson, 2001;Van Eaton et al., 2013). Since the Oruanui eruption, activity has been characterized by three small volume (<0.1 km 3 ) dacites (units Ψ, Ω and A, from~20.5 to 17 ka) from vents at the northern end of the lake, and then 25 rhyolite eruptions between~12 and 1.8 ka, that range in volume over several orders of magnitude (units B through Z: Wilson, 1993;Sutton et al., 1995;Sutton et al., 2000;Barker et al., 2015;Barker et al., 2019;Barker et al., 2021). ...
... The expression of Lake Taupō south of the Oruanui structural caldera resembles a typical linear rift lake (e.g., Lake Tanganyika; Wright et al., 2020;Shaban et al., 2021). Subsidence in southern Lake Taupō occurs largely on the downthrown side of the Waihi fault, with the footwall of this structure forming the NE-trending Karangahape Cliffs on the western edge of the lake (Wilson, 2001) (Figure 2). Similarly, a series of NE-trending faults control the eastern expression of the Lake Taupō shoreline. ...
... Similarly, a series of NE-trending faults control the eastern expression of the Lake Taupō shoreline. Although the structural expression of Lake Taupō in many places has the trademarks of a NE-trending rift system, much of this structural relief, particularly since the Oruanui eruption, is inferred to have developed in response to caldera collapse (e.g., Davy, 1993;Wilson, 2001;Barker et al., 2021). ...
Silicic caldera volcanoes are frequently situated in regions of tectonic extension, such as continental rifts, and are subject to periods of unrest and/or eruption that can be triggered by the interplay between magmatic and tectonic processes. Modern (instrumental) observations of deformation patterns associated with magmatic and tectonic unrest in the lead up to eruptive events at silicic calderas are sparse. Therefore, our understanding of the magmatic-tectonic processes associated with volcanic unrest at silicic calderas is largely dependent on historical and geological observations. Here we utilize existing instrumental, historical and geological data to provide an overview of the magmatic-tectonic deformation patterns operating over annual to 10⁴ year timescales at Taupō volcano, now largely submerged beneath Lake Taupō, in the rifted-arc of the Taupō Volcanic Zone. Short-term deformation patterns observed from seismicity, lake level recordings and historical records are characterized by decadal-scale uplift and subsidence with accompanying seismic swarms, ground shaking and surface ruptures, many of which may reflect magma injections into and around the magma reservoir. The decadal-scale frequency at which intense seismic events occur shows that ground shaking, rather than volcanic eruptions, is the primary short-term local hazard in the Taupō District. Deformation trends near and in the caldera on 10¹–10⁴ yr timescales are atypical of the longer-term behavior of a continental rift, with magma influx within the crust suppressing axial subsidence of the rift basin within ∼10 km of the caldera margin. Examination of exposed faults and fissures reveals that silicic volcanic eruptions from Taupō volcano are characterized by intense syn-eruptive deformation that can occasionally extend up to 50 km outside the caldera structure, including ground shaking, fissuring and triggered fault movements. We conclude that eruption and unrest scenarios at Taupō volcano depend on the three-way coupling between the mafic-silicic-tectonic systems, with eruption and/or unrest events leading to six possible outcomes initially triggered by mafic injection either into or outside the magma mush system, or by changes to the tectonic stress state.
... To acquire the most complete record of highly explosive and long-lasting volcanic history of the source regions, as well as to elucidate the nature of individual eruptions, volcaniclastic deposits are frequently studied in distal archives (e.g., marine and lacustrine) and correlated with their proximal counterparts if these are available (e.g., Wilson 2001;Kutterolf et al. 2016Kutterolf et al. , 2023Albert et al. 2019;Pearce et al. 2020;Cisneros de León et al. 2021Prentice et al. 2022;Brlek et al. 2023;Trinajstić et al. 2023;Harmon et al. 2024). The Lower and Middle Miocene volcaniclastic horizons within the Dinaride Lake System (e.g., Sinj Basin and Livno-Tomislavgrad Basin; Fig. 1) have played Extended author information available on the last page of the article a pivotal role in resolving the chronostratigraphy and geodynamic evolution of intra-montane basins and associated Miocene Climatic Optimum-related lacustrine deposits (e.g., de Leeuw et al. 2010de Leeuw et al. , 2011de Leeuw et al. , 2012Brlek et al. 2021;Mandic et al. 2023). ...
... The question arises: do the recorded ~ 14.32 Ma volcaniclastic deposits of the Livno-Tomislavgrad Basin represent primary ash fallout, or was the initial tephra fallout deposited in the distal basin in the Dinarides subsequently modified? The grain-size differences and distributions of the ~ 14.32 Ma volcaniclastic deposits in the Livno-Tomislavgrad Basin, and particularly within the BD succession, could result from various processes from the initial volcanic eruption to the final deposition within a lacustrine environment (Figs. 2, 5, 7;Supplementary Table 2;e.g., Wilson and Walker 1985;Wilson 2001;Matthews et al. 2012;Engwell et al. 2014;Cioni et al. 2015;Houghton and Carey 2015;Engwell and Eychenne 2016;Scarpati and Perotta 2016;Freundt et al. 2022;Major 2023). However, due to lack of data it is currently not possible to make further correlations between the volcanic mechanisms during the ~ 14.32 Ma eruption and distal volcaniclastic record in the Dinarides. ...
Reliable reconstructions of tephrostratigraphy and emplacement mechanisms of Early to Middle Miocene volcaniclastic deposits across the Alpine-Mediterranean region may yield important clues as to the nature, spread, volume, magnitude and frequency of large silicic eruptions of the Carpathian-Pannonian Region. Here we report on a sequence of Middle Miocene volcaniclastic deposits intercalated with lacustrine strata from the Livno-Tomislavgrad Basin, part of the Dinaride Lake System. We applied a multi-proxy approach to elucidate their source, decipher their emplacement mechanism, and evaluate their basin-scale and regional correlativity. New high-precision zircon geochronology (~14.32 Ma) reveals their simultaneity with numerous volcaniclastic deposits (and their alteration products) widely spread across the Alpine-Mediterranean region. Additionally, their correlativity is confirmed at the scale of the Livno-Tomislavgrad Basin, based on similar litho-stratigraphy, mineralogy and volcanic glass geochemistry. Newly obtained zircon Hf isotope data imply that Livno-Tomislavgrad Basin distal volcaniclastic deposits and ~14.36 Ma Harsány ignimbrite of the Carpathian-Pannonian Region had shared a parental eruptive center. However, different volcanic glass geochemistry, bolstered by the high-precision geochronology, suggests distinct eruption events, implying more frequent explosive volcanism of the Carpathian-Pannonian Region during Middle Miocene than previously recognized. We suggest that the ~14.32 Ma fine fallout tephra, deposited in the distal basin in the Dinarides (>400 km from the source), was likely re-mobilized by water-driven hillside erosion from the extensive paleo-relief, and further transported via land-derived gravity flows. Upon entering the lake, the gravity flows likely transformed into subaqueous sediment density flows. These deposited ~1 to 7-m-thick overall graded volcaniclastic turbidites, thinning away from the presumed source of tephra re-mobilization. Although over-thickened, the ~14.32 Ma Livno-Tomislavgrad Basin volcaniclastic deposits can still serve as a reliable tephro- and tectono-stratigraphic markers due to their rapid mode of accumulation.
... Taupō volcano is a large silicic caldera located within the Taupō Volcanic Zone (TVZ) rift in New Zealand's North Island (Te Ika-a-Māui) and largely obscured by Lake Taupō (Fig. 1; Barker et al., 2021). The caldera was formed by structural collapse associated with the ∼25.4 ka Oruanui supereruption (Wilson, 2001;Vandergoes et al., 2013), with the spatial extent defined by a large negative (-70 mGal) gravity anomaly ( Fig. 1; Davy and Caldwell, 1998;Stagpoole et al., 2020). 28 post-Oruanui eruptions have been recognised (25 during the last 12 ka) with great variation in size and eruption style, the latest and largest of which occurred at 232 ± 10 CE (Hogg et al., 2012), causing further collapse beneath the lake ('Taupō eruption'; Barker et al., 2015Barker et al., , 2021. ...
... We recognised four phases of activity within a 13 month period of unrest, each distinguished by differing earthquake occurrence rates, magnitudes, and locations (Figs. 2, 3). Earthquake locations mostly coincide with a significant overlap of the Taupō caldera and residual gravity anomaly associated with the Oruanui caldera ( Fig. 1; Davy and Caldwell, 1998;Wilson, 2001;Stagpoole et al., 2020) as well as a geothermal field and active hydrothermal venting (Bibby et al., 1995;de Ronde et al., 2002). Most of the moment tensor estimates in this area feature double-couple normal-fault solutions on rift-aligned planes, with a few showing NW-SE alignment instead (Fig. 3); the main exception is the non-double couple reverse faulting moment tensor solution for the M L 5.7 event which is discussed in more detail below. ...
Taupō is a large caldera volcano located beneath a lake in the centre of the North Island of New Zealand and most recently erupted ~1800 years ago. The volcano has experienced at least 16 periods of unrest since 1872, each of which were characterised by increased seismic activity. Here we detail seismic activity during the most recent period of unrest from May 2022 to May 2023. The unrest was notable for the highest number of earthquakes detected during instrumented unrest episodes, and for one of the largest magnitude earthquakes detected beneath the lake for at least 50 years (ML 5.7). Relocated earthquakes indicate seismic activity was focused around an area hosting overlapping caldera structures and a hydrothermal system. Moment tensor inversion for the largest earthquake includes a non-negligible inflationary isotropic component. We suggest the seismic unrest was caused by the reactivation of faults due to an intrusion of magma at depth.
... Taupōis a silicic volcano that last erupted ∼1,800 years ago (Hogg et al., 2012;Piva et al., 2023;Wilson & Walker, 1985), expelling ∼35 km 3 of material (Davy & Caldwell, 1998). Despite Taupo's propensity for large and violent eruptions (Wilson, 2001), in historic times the caldera has undergone periods of unrest only, with no accompanying eruption Illsley-Kemp et al., 2021;Journeau et al., 2022;Peltier et al., 2009;Potter et al., 2015). The most significant unrest episodes in the last 150 years took place in 1897, 1922, 1964-1965, 1983-1984(Potter et al., 2015, and more recently in 2019, and 2022-2023. ...
... It is also possible that there was accompanying low-magnitude seismicity, below the detection limit. While the location of the modeled dike intrusion lies outside of the estimated silicic magma reservoir (Figure 1), it is in a similar location to an inferred intrusion in 2001(McGregor et al., 2022, and further modeled magma intrusions (Smith et al., 2007). Dike intrusions are usually accompanied by earthquakes, however it is not unknown for them to propagate aseismically (Belachew et al., 2011;Grandin et al., 2011). ...
Several studies suggest that large earthquakes (M > 7.0) can act as external triggers of volcanic unrest, and even eruption. This triggering is attributed to either ground shaking (dynamic stresses) or to permanent ground deformation (associated with static stress changes). However, large earthquakes are rare and testing triggering hypotheses has proven difficult. We use geodetic data to show that the 13 November 2016 Kaikōura earthquake (Mw 7.8) triggered local deformation of up to 11 mm at Taupō volcano, 500 km away, which lasted for approximately twelve days. Using elastic geodetic models, we infer that the observed deformation was caused by either aseismic fault slip or a dike intrusion. We then use strong motion data from the surrounding area to show that the Kaikōura earthquake caused maximum dynamic stress changes in the range of 0.15–0.9 MPa in the vicinity of Taupō volcano and conclude that these dynamic stress changes triggered local faulting or dike activity and the associated deformation at Taupō volcano.
... (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) scale of recurrent moderate volume silicic explosive events known worldwide, such as those along the Taupo Volcanic Zone in New Zealand with at least seven caldera-forming events accompanied with several dozens of silicic lava dome over 350 ky time span (Wilson, 1993(Wilson, , 2001. The distinguished 'units' of the BFVA were named mainly as 'ignimbrite units' (Lukács et al., 2018). ...
... A comparable, active volcanic area, which is thought to be analogous to the BFVA from a volcanological and paleoenvironmental point of view is the Taupo Volcanic Zone (TVZ) in New Zealand (as suggested by Biró et al., 2020). Phreatomagmatism was recognized and documented in the case of TVZ in early works (Self and Sparks, 1978;Self, 1983;Wilson, 1993Wilson, , 2001Van Eaton and Wilson, 2013). These publications highlighted the possible causes of phreatomagmatism, i.e., the origin of the external water, which has been explained by caldera lakes in most cases, based on, among others, the presence of diatomites in the pyroclastic deposits (Van Eaton and Wilson, 2013). ...
... Higher eruption columns that collapse generate a larger flow mobility and, therefore, a more widely distributed ignimbrite with a low aspect ratio can be created (Francis and Baker, 1977;Baker, 1981;Lube et al., 2019). The height of eruption column that controls flow mobility is ultimately controlled by the mass eruption rate hence this parameter plays a major role in determining the final runout, as well as air entrainment and depositional rate (Fisher et al., 1993;Wilson, 2001;Shimizu et al., 2019;Roche et al. 2021;Biró et al., 2022). ...
... and are probably most comparable to the Ongatiti Ignimbrite. The Oruanui ignimbrite(Wilson, 2001) on the other hand, is more comparable to the Kidnapper's eruptionboth are non-welded and have underlying fall deposits. ...
Pyroclastic flows are the most devastating phenomena of explosive volcanic eruptions. These hazardous fastmoving,
hot, concentrated density currents are able to travel several tens of kilometers radially away from their source. Due to an enveloping ash cloud, it is still impossible to directly study pyroclastic flows. However, their deposits (i.e., ignimbrites) provide useful insight into their internal processes of emplacement. This study focuses on the Ongatiti Ignimbrite, which is sourced from Mangakino caldera (i.e., the oldest volcanic centre in the Taupo Volcanic Zone, North Island, New Zealand dated at 1600–950 ka) and offers a unique opportunity to
understand the emplacement processes of an ancient and large-volume pyroclastic flow. It is a welded to nonwelded, columnar-jointed, cliff-forming deposit, that has been divided into nine facies based on the variation in pumice and lithic clast abundance, and degree of welding. Our results constrain the minimum deposit volume for the Ongatiti Ignimbrite to ca. 1400 km3, or 1000 km3 dense-rock equivalent. Zircon (U–Th)/He data suggest an eruption age of 1.37 ± 0.04 Ma, which is in good
agreement with the previous proposed eruption age. The topographic controls on the spatial distribution of the
ignimbrite have been determined to understand pyroclastic flow pathways through valleys and over hills. The ignimbrite covered hills up to ~900 m (about 650 m above the caldera height) to around 40 km from the Mangakino volcanic centre (MVC) and, the pyroclastic flow travelled to beyond 90 km from the vent. The
Ongatiti Ignimbrite was a landscape-modifying event that covered at least the western North Island and as far away as Auckland and Wellington.
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... km Sparks, 1978 VEI 6 , , 1983;Allen and Cas, 1998;Wilson, 2001 Unit 6 ...
... ,Allen and Cas, 1998; Aravena et al., 2018Hayakawa, 1985Allen and Cas, 1998;Wilson, 2001;Hiroi ...
We studied the 40 ka Kp I eruption deposits of Kutcharo volcano to unravel its eruption sequence and generation mechanisms. Previous studies have suggested that Kp I is the youngest caldera-forming eruption in this volcano and is characterized by large-scale phreatomagmatic activity. We divided Kp I eruption deposits into 7 units (Units 1 7, in ascending order). Units 1~6 consist of alternating thin pumice and thick fine ash layers. Units 1, 3, and 5 are pumice falls (totaling 1.6 km 3), while Units 2, 4, and 6 are ash falls (totaling 52.2 km 3) with abundant accretionary lapilli. Stratigraphically higher ash fall units are larger in volume, finer in grain size, and more widely distributed (e.g., Units 2, 4, and 6 are 0.2 km 3 , 13 km 3 , 39 km 3 respectively). Unit 7 is a climactic ignimbrite (76 km 3) that subdivides into lower (Unit 7-L), and upper (Unit 7-U) parts based on the pumice size and the existence of a lithic concentration zone (LCZ). Considering its wide dispersion, high fragmentation, and existence of abundant accretionary lapilli, Unit 6 can be considered to have been deposited by a phreatoplinian style eruption. Even though the ejected magma volume increased during the eruption of Unit 1 to 6, interaction between ascending magma and ground water caused maximum explosivity during the eruption that deposited Unit 6. Highly fragmentated magmas might have promoted vaporization and mixing with surface (lake) water to form the buoyant eruption column of Unit 6 eruption phase. Unit 7 is the most voluminous and the richest in lithic fragments at the LCZ, suggesting caldera collapse that generated a climactic pyroclastic flow. In addition to glass shards of bubble wall and pumiceous types, Kp I eruption deposits also commonly contain flake-, and blocky-shaped glass shards produced by phreatomagmatic (quenching) fragmentation. For both types of glass shards to have been generated, part of the ascending magma would have interacted with ground water before and/ or during the magmatic fragmentation (vesiculation) that generally occurs below a depth of approximately 1,000 m in felsic H 2 O-saturated magma systems. In conclusion, a large and deep (1,000 m) aquifer in the former caldera basin was sustainably supplied with ground water through the conduit system. Generation of the phreatoplinian eruption seems to have been controlled by a plumbing where conduits penetrated the huge aquifer of a pre-existing caldera structure that preserved/hosted a large amount of external water.
... km Sparks, 1978 VEI 6 , , 1983;Allen and Cas, 1998;Wilson, 2001 Unit 6 ...
... ,Allen and Cas, 1998; Aravena et al., 2018Hayakawa, 1985Allen and Cas, 1998;Wilson, 2001;Hiroi ...
We studied the 40 ka Kp I eruption deposits of Kutcharo volcano to unravel its eruption sequence and generation mechanisms. Previous studies have suggested that Kp I is the youngest caldera-forming eruption in this volcano and is characterized by large-scale phreatomagmatic activity. We divided Kp I eruption deposits into 7 units (Units 1 7, in ascending order). Units 1 6 consist of alternating thin pumice and thick fine ash layers. Units 1, 3, and 5 are pumice falls (totaling 1.6 km 3), while Units 2, 4, and 6 are ash falls (totaling 52.2 km 3) with abundant accretionary lapilli. Stratigraphically higher ash fall units are larger in volume, finer in grain size, and more widely distributed (e.g., Units 2, 4, and 6 are 0.2 km 3 , 13 km 3 , 39 km 3 respectively). Unit 7 is a climactic ignimbrite (76 km 3) that subdivides into lower (Unit 7-L), and upper (Unit 7-U) parts based on the pumice size and the existence of a lithic concentration zone (LCZ). Considering its wide dispersion, high fragmentation, and existence of abundant accretionary lapilli, Unit 6 can be considered to have been deposited by a phreatoplinian style eruption. Even though the ejected magma volume increased during the eruption of Unit 1 to 6, interaction between ascending magma and ground water caused maximum explosivity during the eruption that deposited Unit 6. Highly fragmentated magmas might have promoted vaporization and mixing with surface (lake) water to form the buoyant eruption column of Unit 6 eruption phase. Unit 7 is the most voluminous and the richest in lithic fragments at the LCZ, suggesting caldera collapse that generated a climactic pyroclastic flow. In addition to glass shards of bubble wall and pumiceous types, Kp I eruption deposits also commonly contain flake-, and blocky-shaped glass shards produced by phreatomagmatic (quenching) fragmentation. For both types of glass shards to have been generated, part of the ascending magma would have interacted with ground water before and/ or during the magmatic fragmentation (vesiculation) that generally occurs below a depth of approximately 1,000 m in felsic H 2 O-saturated magma systems. In conclusion, a large and deep (1,000 m) aquifer in the former caldera basin was sustainably supplied with ground water through the conduit system. Generation of the phreatoplinian eruption seems to have been controlled by a plumbing where conduits penetrated the huge aquifer of a pre-existing caldera structure that preserved/hosted a large amount of external water.
... It was inferred that even the super-sized magmatic systems, such as the Mangakino Volcanic Center of the Taupō Volcanic Zone, can produce geochronologicaly indistinguishable, geochemically partly overlapping but stratigraphically separated ignimbrites, such as Kidnappers and Rocky Hill ignimbrites (e.g., Cooper et al., 2014Cooper et al., , 2016 for other examples see Wotzlaw et al., 2015;Swallow et al., 2019;Kutterolf et al., 2020). The stratified ignimbrites (e.g., Supplementary Table 1) associated with KNOL-3B and KAL-1 massive ignimbrites could have also been derived by separate eruption(s), although they likely shared a common magmatic source based on similar geochemical, radiogenic isotopic signatures and in situ geochronology, and therefore could have also been produced during single eruption (e.g., Wilson and Walker, 1985;Wilson, 2001;Allen et al., 1999;Hildreth and Firestein, 2012;Druitt et al., 2019). In conclusion, based on similar and overlapping isotopic compositions of zircon (KNOL-3B, KAL-1 and Csv-2) and volcanic glass (KNOL-3B and KAL-1), and especially , respectively, based on combined zircon petrochronological and volcanic glass compositional fingerprints used in this study (also including data from Lukács et al. 2018Lukács et al. , 2021Brlek et al., 2020. ...
... In contrast, the larger size of pumices and lithic clasts (10-15 cm) recorded in $ 17.3 Ma massive ignimbrites from the BVF , in comparison to KVC massive ignimbrites (Fig. 2), along with their greater thickness (up to $ 80 m according to Karátson et al., 2022, in comparison to max. $ 20-m-thick KVC massive ignimbrites; Table 2; Supplementary Table 2), suggests more proximal deposition of the BVF $ 17.3 Ma massive ignimbrites relative to the source (see Karátson et al., 2022; for more complex scenarious from the geological record see e.g., Wilson, 1985Wilson, , 2001. In summary, the maximum runout distances of pyroclastic flows which formed Kalnik and Eger ignimbrites could have roughly ranged from < 100 km up to approximately 150 km. ...
The Carpathian-Pannonian Region (CPR) hosted some of the largest silicic volcanic eruptions in Europe during the Early and Middle Miocene, contemporaneously with major lithospheric thinning of the Pannonian Basin. This was recorded as an ignimbrite flare-up event from approximately 18.1–14.4 Ma. To gain in-depth perspectives on the eruption chronology, tephrostratigraphy, and petrogenesis at the onset of CPR silicic volcanism, we applied a multi-proxy approach to Lower Miocene rhyolitic ignimbrites and pyroclastic fall deposits from the northern CPR to the Dinaride Lake System. High-precision zircon U-Pb geochronology distinguished two Lower Miocene groups of volcaniclastic rocks at ∼18.1 Ma and ∼17.3 Ma. Based on combined tephrostratigraphic signatures we propose that the ∼18.1 Ma Kalnik and ∼17.3 Ma Eger eruptions produced widespread (intermediate to) large caldera-forming massive rhyolitic ignimbrites, deposited across northern and southwestern regions of the CPR. Due to easterly winds that carried volcanic ash hundreds of kilometers to the southwest, Eger eruption products also reached distal intra-montane Dinaride lacustrine basins, recorded as pyroclastic fall deposits. Heterogeneous major and trace elemental compositions of ∼18.1 Ma volcanic glass shards suggest that the Kalnik eruption was sourced from complex silicic magmatic systems, with simultaneous tapping of two discrete melt bodies during the eruption. The homogeneous geochemical composition of ∼17.3 Ma glasses is distinct from the older glasses. Integrated zircon and bulk glass Nd-Hf isotope compositions have a positive correlation, defining a regional mantle array, and are more radiogenic in the younger phase of volcanism. The recorded systematic isotopic change, moving from older more crustal signatures to younger more juvenile compositions, imply that during the period of lithospheric thinning of the Pannonian Basin the region underwent more complex variations in the interaction between metasomatized lithospheric mantle-derived magmas and various crustal components than previously recognized.
... t Hogg et al. (2012). u Wilson (2001). ...
... The 25.4 ka Ōruanui eruption was by far the largest in the modern TVZ. About 530 km 3 of magma was discharged, culminating in a major caldera collapse centered in the northern part of Lake Taupō (Davy & Caldwell, 1998;Wilson, 2001). More than 98% of Ōruanui products derive from a homogeneous, crystal-poor high-silica rhyolite body (HSR), which was extracted from the same parental mush body that fed the earlier "Ōruanui type" magmas (Allan et al., 2017). ...
Oxygen isotopes are useful for tracing interactions between magmas, crustal rocks and surface‐derived waters. We use them here to consider links between voluminous silicic magmatism and large‐scale hydrothermal circulation in New Zealand's central Taupō Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass δ¹⁸O values from 40 eruptions from three discrete magmatic systems: Ōkataina and Taupō calderas, and the smaller Northeast Dome system. For each mineral, mean δ¹⁸O values vary by ∼1‰ (δ¹⁸Oplag = +6.7–7.8‰, δ¹⁸Oqtz = +7.7–+8.7‰, δ¹⁸Ohbl = +5.4–+6.4‰, δ¹⁸Oglass = +7.1–+8.0‰), and inter‐mineral fractionations mostly reflect high‐temperature equilibria. Outliers (e.g., ∼+6‰ or >+10‰ plagioclase) represent contaminants incorporated on short‐enough timescales to preserve disequilibrium (∼10² yrs for plagioclase). Melt δ¹⁸O values calculated from phenocrysts are ∼+7.3–+8.0‰. Where multiple magmas were involved in the same eruption their δ¹⁸Omelt values are indistinguishable, implying that their parental mushes were isotopically well‐mixed. However, small (≤0.5‰) but consistent δ¹⁸Omelt value gradients occur over millennial timescales at Ōkataina and Taupō, with short‐term ∼0.4–0.5‰ decreases in δ¹⁸Omelt values over successive post‐caldera eruptions correlating with increases in ⁸⁷Sr/⁸⁶Sr. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher‐⁸⁷Sr/⁸⁶Sr greywacke. These examples represent the first evidence for assimilation of altered crust into TVZ magmas. The subtle and short‐lived isotopic signals of these interactions are only recognized through the high temporal resolution of the TVZ eruptive record and complementary radiogenic isotope data. Similar interactions may have been obscured in other nominally high‐ or normal‐δ¹⁸O magmatic systems.
... The Oruanui eruption ejected > 530 km 3 DRE of magma (> 1100 km 3 of pyroclastic material) over several months (Davy and Caldwell, 1998;Wilson, 2001;Wilson et al., 2006). Caldera collapse occurred in the climactic Phase 10, which has now infilled and reflects the modern shape of Lake Taupō (Davy and Caldwell, 1998;Wilson, 2001;Stagpoole et al., 2021). ...
... The Oruanui eruption ejected > 530 km 3 DRE of magma (> 1100 km 3 of pyroclastic material) over several months (Davy and Caldwell, 1998;Wilson, 2001;Wilson et al., 2006). Caldera collapse occurred in the climactic Phase 10, which has now infilled and reflects the modern shape of Lake Taupō (Davy and Caldwell, 1998;Wilson, 2001;Stagpoole et al., 2021). The 232 AD Taupō eruption was the most recent known explosive eruption from the volcano, producing > 35km 3 DRE of magma and a further caldera collapse (Bibby et al., 1995;Davy and Caldwell, 1998). ...
Taupō volcano, located within the Taupō Volcanic Zone (TVZ) in the central North Island of Aotearoa-New Zealand, is one of the world’s most active silicic caldera systems. Silicic calderas such as Taupō are capable of a broad and complex range of volcanological activity, ranging from minor unrest episodes to large destructive supereruptions. A critical tool for volcanic risk management is eruption forecasting. The Bayesian Event Tree for Eruption Forecasting (BET_EF) is one probabilistic eruption forecasting tool that can be used to produce short-term eruption forecasts for any volcano worldwide. A BET_EF model is developed for Taupō volcano, informed by geologic and historic data. Monitoring parameters for the model were obtained through a structured expert elicitation workshop with 30 of Aotearoa-New Zealand’s volcanologists and volcano monitoring scientists. The eruption probabilities output by the BET_EF model for Taupō volcano’s 17 recorded unrest episodes (between 1877 and 2019) were examined. We found time-inhomogeneity in the probabilities stemming from both the changes over time in the monitoring network around Taupō volcano and increasing level of past data (number of non-eruptive unrest episodes). We examine the former issue through the lens of the latest episodes, and the latter by re-running the episodes assuming knowledge of all 16 other episodes (calibration to 2021 data). The time variable monitoring network around Taupō volcano and parameter weights had a substantial impact on the estimated probabilities of magmatic unrest and eruption. We also note the need for improved monitoring and data processing at Taupō volcano, the existence of which would prompt updates and therefore refinements in the BET_EF model.
... These calderas have together produced >770 km 3 of rhyolitic magma in the past 60 kyr . The modern Taupō Volcano has experienced 28 eruptions at Taupō since the Oruanui supereruption at 25.5 ka (Dunbar et al., 2017;Wilson, 2001), including 25 in the last 12 kyr Wilson, 1993). ...
... The origin of this intrusion is a key question, considering the location of the Western Bay seismicity. The majority of known post-Oruanui volcanic vents are situated east of the Western Bay seismic cluster (Barker et al., 2015;Wilson, 1993Wilson, , 2001Wilson & Charlier, 2009). In addition the Western Bay seismic cluster occurred outside the Figure 4). ...
Plain Language Summary
The Taupō Volcanic Zone (TVZ) is a large area of volcanism that spans the central North Island of New Zealand. Active volcanism coincides with continental extension, named the Taupō Rift and both are the result of the interaction between the Australian and Pacific plates occurring offshore. At the center of the TVZ is Taupō volcano, a large caldera which undergoes regular phases of seismic unrest involving large numbers of earthquakes and ground shaking. A period of unrest in 2001 contrasts with the other unrest episodes in that most of the seismicity was concentrated in the Taupō Fault Belt rather than within the adjacent caldera, and we hypothesize that its occurrence may provide insight into the interaction between magmatic and tectonic processes around Taupō volcano. Our analysis suggests that the earthquakes in 2001 first occurred beneath Taupō volcano, initiating subsequent seismic activity in the Taupō Fault Belt and temporarily altering the surrounding stress state. We infer that the initial group of earthquakes beneath Taupō volcano coincided with and were triggered by a magmatic intrusion. Therefore, the seismicity in 2001 highlights an example of interaction between the tectonic and deep magmatic systems at Taupō volcano.
... The red triangles denote active volcanoes. The thick black lines denote plate boundaries (Bird, 2003) experienced a youngest VEI (volcanic explosivity index) 8 super-eruption ∼26 ka (Barker et al., 2020;Wilson, 2001) and an estimated VEI 7 eruption ca. CE 233 (Newhall et al., 2018). ...
Australia, New Zealand, and the surrounding regions have experienced complex plate interactions with significant seismic and volcanic activities. The Taupo volcano on the North Island of New Zealand has experienced multiple catastrophic eruptions. Although Australia is known as a stable landmass with low seismic and volcanic activity, intraplate volcanoes along its eastern coast are considered to be caused by hot mantle plumes. To better understand the seismic and volcanic activities in the region, it is necessary to study the detailed 3‐D structure of the crust and mantle. Here we apply a well‐established global tomography method to reveal the 3‐D P‐wave velocity (VP ) structure of the whole mantle beneath this region. We used ∼7 million P, pP, PP, PcP, and Pdiff wave arrival times of 23,666 earthquakes recorded at 14,181 seismograph stations worldwide. The resulting VP tomography clearly shows high‐VP subducted slabs, and low‐VP anomalies above and below the slabs, which may reflect corner flow in the mantle wedge and subslab hot mantle upwelling (SHMU), respectively. A slab window is revealed beneath the North Island of New Zealand. Given the development of SHMU beneath this region, the catastrophic eruptions of the Taupo volcano might be powered by a mixture of island arc magma and SHMU through the slab window. Beneath the intraplate volcanoes along the eastern coast of Australia and the Tasman Sea, a thin low‐VP zone exists and extends down to the core‐mantle boundary, suggesting that the intraplate volcanoes might be, at least partially, fed by a plume rising from the lower mantle.
... The most common criteria used to distinguish phreatomagmatic from dry magmatic eruptions are the proportion of juvenile fines, the occurrence of ash aggregates, lithic fragment abundance and the presence of deposits from PDCs, particle shape and degree of vesiculation, degree of quenching and particle aggregation (e.g., McPhie 1986;Wilson 2001;Ellis and Branney 2010;White and Valentine 2016). However, when dealing with very old eruptive products, like the Upper Cretaceous volcaniclastic deposits of the Haţeg Basin, determining the "fingerprints" of magmawater interaction can be a demanding task. ...
Upper Cretaceous volcaniclastic deposits of the Haţeg Basin (Southern Carpathians, Romania), exposed around Densuş and Răchitova localities, are interpreted as three different eruptive sequences, the fragmentary record of multiple subaerial explosive-effusive volcanic eruptions of relatively long-lived stratovolcanoes. This volcanological study offers the first detailed description of the lithology and lithofacies of these deposits, with the aim of gaining an insight into the nature of the eruptive processes that led to their formation and, subsequently, into the dynamics of these Late Cretaceous volcanic eruptions. All types of volcanically-derived deposits have been identified in these volcaniclastic successions: primary, secondary, and epiclastic. Interbedded shales, which occur in the Densuş successions, were deposited during periods of volcanic repose, in a wetland. Lithological characteristics indicate that the volcaniclastic deposits were primarily produced by phreatomagmatic volcanism, possibly of Phreatoplinian type. The three successions were part of different composite volcanoes that likely formed a volcanic field near a coastal area. The largest part of the Haţeg Basin, including the outcropping area of the volcaniclastic deposits, forms the Haţeg Country UNESCO Global Geopark. As part of a UNESCO Geopark, these remnants of former volcanoes also have a geoheritage significance and despite their old age and poor exposure, can serve as an excellent location for community geoeducation. Although ancient volcanic features are usually underrated, scientific data derived from the study of such features can help identify important volcanic geoheritage elements. A more extensive use of these elements in the fields of geoeducation and geotourism can support the popularization and valorization of the Haţeg Country volcanoes and their significance in the geological history of this region.
... F I G U R E 1 North Island, New Zealand palaeogeography against the modern coastline, indicating major geophysical features since late Pliocene time. Tectonic uplift (Trewick & Bland, 2012), lowered sea level (~28 kyr; Rother et al., 2014) and continuous forests contraction at the last glacial phase (Newnham et al., 2013), with area affected by widespread pyroclastic flow from Oruanui eruption in the Taupō Volcanic Zone (TVZ) at ~25.5 kyr (Wilson, 2001). Sampling of Sigaus piliferus location names are available in Table S1.1 in Appendix S1. ...
Aim
Species responses to global warming will depend on intraspecific diversity, yet studies of factors governing biogeographic patterns of variability are scarce. Here, we investigate the evolutionary processes underlying genetic and phenotypic diversity in the flightless and cold‐adapted grasshopper Sigaus piliferus, and project its suitable space in time.
Location
Te Ika‐a‐Māui Aotearoa—North Island of New Zealand.
Methods
We used mitochondrial sequences to investigate population connectivity and demographic trends using phylogeographic tools and neutrality statistics. Metric data were used to document phenotypic variation using naïve clustering. We used niche metrics to assess intraspecific niche variation, and niche modelling to investigate suitability under past and future scenarios. Multiple matrix regressions with randomization explored the processes contributing to phenotypic differentiation among grasshopper populations.
Results
Niche models and demographic analyses suggest suitable space for this grasshopper was more restricted during glacial than interglacial stages. Genealogical relationships among ND2 haplotypes revealed a deep north–south split partly concordant with phenotypic and niche variation, suggesting two ecotypes that have mixed during recolonisation of the central volcanic region. Multiple matrix regressions with randomization indicate a link between climate and phenotypic differentiation inferred from leg and pronotum dimensions but not pronotum shape. Niche projections predict severe habitat reduction due to climate warming.
Main conclusions
The current distribution and intraspecific diversity of S. piliferus reflect complex biogeographical scenarios consistent with Quaternary climates and volcanism. Phenotypic divergence appears adaptive. Current levels of genetic and phenotypic variation suggest adaptive potential, yet the pace of anthropogenic warming over the next 50 years could result in small populations that may collapse before adapting. Differences in niche features between diverging intraspecific lineages suggest distinct responses to climate change, and this has implications for prioritising conservation actions and management strategies.
... The Hinuera C deposits largely comprise the reworked products of the voluminous break-out flood event from the Taupō volcano (Manville and Wilson 2004;Manville et al. 2007) that occurred soon after the Oruanui supereruption c. 25,400 cal yr BP (Wilson 2001;Vandergoes et al. 2013;Barker et al. 2021) but before the Waikato River's avulsion into the Hamilton Basin, which occurred between c. 24,000 cal yr BP (Lowe and Green 2024) and c. 23,500 cal yr BP (Peti et al. 2021) (see also section on this topic below). The land surface in the southern Hauraki Plains (on the Hinuera C deposits), therefore, has an estimated age of c. 24,000 to c. 23,500 cal yr BP (Lowe and Green 2024), consistent with calibrations on several radiocarbon dates obtained by Houghton and Cuthbertson (Hogg et al. 2020) (see Supplementary material S2). ...
In this study, we provide the first field-based assessment of the seismic potential of the Te Puninga Fault, Hauraki Plains, Waikato region. Initially considered to be part of the nearby Kerepehi Fault, our new mapping and field data suggest the Te Puninga Fault is independent. A new net slip rate value of 0.25 mm/yr, based on geomorphic data and evaluations from two paleoseismic trenches, is slightly higher than previously considered. Comparisons of geomorphic expression between the two faults suggest that the slip rate currently assigned to the Kerepehi Fault could be underestimated. The earthquake magnitude estimated here for the Te Puninga Fault (M w 6.9 ± 0.35) is based on a characteristic earthquake model. New PGA and MMI estimates here are only slightly larger than those published prior to this study. Although ruptures of the Te Puninga Fault are infrequent (derived recurrence range of 3000-11,500 years), and thus its hazard is low, with this paper we wish to enhance the community awareness to prepare for the rare large earthquake in the region. We also recommend that this new information is added to fault databases used for seismic assessment.
... 1030including the largest on Earth within the last 5 ka (e.g.,Wilson, 2001). A dense network of1031 sensors takes continuous readings via seismometers, GNSS stations, and hydrothermal fluid 1032 and gas vents analysis. ...
Submarine landslides can generate destructive tsunamis. Yet their recurrence intervals and tsunamigenic mechanisms are poorly understood, hampering quantification of global exposure and risk. With growing coastal populations and climatic changes, the impacts of tsunami hazards will increase, and will disproportionately affect peoples in underdeveloped countries (e.g., Small Island Developing States). Specific hazard characteristics of submarine landslide-tsunami include the potential for: short tsunami travel times, highly directional waves, locally extreme wave amplitudes and lack of forewarning. Probabilistic tsunami hazard assessments (PTHA) have emerged as the gold standard for tsunami hazard quantification, enabling science understanding to be translated into policy and decision making. However, our knowledge gaps and research challenges largely prohibit PTHA for submarine landslide sources. In this review, we reference Aotearoa New Zealand, as a western Pacific Island nation vulnerable to tsunami hazards, to illustrate the fragmentary nature of the evidence for, and perceived threat of submarine landslide-tsunami typical of many regions globally. We present an overview of geoscience approaches for the identification and assessment of submarine landslide-tsunami hazards, which include construction of submarine landslide databases, source-specific numerical tsunami simulations, successful approaches to submarine landslide-specific PTHA, submarine landslide susceptibility mapping and emerging data science techniques. We recommend a sequence of logical and accessible steps for enhanced submarine landslide identification and hazard assessment aimed at geoscience disciplines. Our recommendations promote strategic approaches to future data collection and enhanced multi-disciplinary collaboration between marine geoscientists, and tsunami, hydrodynamic, hazard and risk specialists.
... Moreover, the Koya pyroclastic flow deposit also consists of several flow-units indicative of a pulse-like ignimbrite eruption. The Oruanui eruption of Taupō volcano, Aso-4 eruption of Aso volcano are another such cases for which a large ignimbrite was produced by multiple pulses separated by time breaks 14,30 . The shallow depth to the high-silica magma chamber (H ~ 3 km for Aso 30 and H ~ 3.5 km for Taupō 31 ) compared with the large caldera size for the Oruanui eruption of Taupō may have allowed caldera subsidence with a small underpressure, resulting in several breaks in the caldera collapse sequence. ...
Explosive caldera-forming eruptions eject voluminous magma during the gravitational collapse of the roof of the magma chamber. Caldera collapse is known to occur by rapid decompression of a magma chamber at shallow depth, however, the thresholds for magma chamber decompression that promotes caldera collapse have not been tested using examples from actual caldera-forming eruptions. Here, we investigated the processes of magma chamber decompression leading to caldera collapse using two natural examples from Aira and Kikai calderas in southwestern Japan. The analysis of water content in phenocryst glass embayments revealed that Aira experienced a large magmatic underpressure before the onset of caldera collapse, whereas caldera collapse occurred with a relatively small underpressure at Kikai. Our friction models for caldera faults show that the underpressure required for a magma chamber to collapse is proportional to the square of the depth to the magma chamber for calderas of the same horizontal size. This model explains why the relatively deep magma system of Aira required a larger underpressure for collapse when compared with the shallower magma chamber of Kikai. The distinct magma chamber underpressure thresholds can explain variations in the evolution of caldera-forming eruptions and the eruption sequences for catastrophic ignimbrites during caldera collapse.
... Explosive rhyolitic events range widely in size and vary greatly in the patterns of their initial behaviour, such as the degree of episodicity in their initial explosions, and whether they stall between their slow initial ascent and final rise to the surface (e.g. Wilson and Hildreth 1997;Wilson 2001;Myers et al. 2016Myers et al. , 2018Swallow et al. 2019). However, comparisons between the youngest Ōkataina eruptions studied here and data published for other events of widely varying eruptive volume show that the volume of magma being erupted, and the overall pattern of magma ascent, has practically no bearing on the rates or timings of magma rise through the mid-upper part of the conduit prior to fragmentation (Fig. 8). ...
Volatile measurements in mineral-hosted sealed melt inclusions, and open-ended embayments, have previously been used to study magma ascent dynamics in large rhyolitic eruptions. However, despite occurring more frequently, smaller-volume explosive events remain under-studied. We present magmatic volatile data from quartz-hosted melt inclusions and embayments for eight post-25.4 ka rhyolitic eruptions at Ōkataina Volcanic Centre, Aotearoa New Zealand. Seven originated from within the main caldera, and the other erupted from the associated Ōkareka Structural Embayment. Melt inclusions preserve volatile contents of 2.92–5.82 wt% H 2 O and 13–126 ppm CO 2 , indicating pre-eruptive storage depths of 4.5–7.4 km, with younger eruptions being more shallow. The lack of correlation between H 2 O, CO 2 , inclusion size or distance to the crystal rim suggests magma bodies experienced variable degrees of degassing during magma storage, with some amount of post-entrapment volatile modification prior to and concurrent with final magma ascent. Diffusion modelling of measured H 2 O gradients in melt embayments indicates ascent rates of 0.10–1.67 m.s ⁻¹ over time spans of 20–230 min for the intra-caldera events. In contrast, ascent rates for the eruption from the Ōkareka Structural Embayment may be more rapid, at 1.59–4.4 m.s ⁻¹ over a time span of 22–34 min. Our findings imply that the final, pre-eruptive magma movement towards the surface could be less than a few hours. Comparisons with published data for caldera-forming explosive events reveal no clear relationships between final ascent rate, eruption size or initial volatile content, implying that other factors besides eruption volume control rhyolite magma ascent.
... One of the volcanoes in this system, the Taupō volcano, is responsible for the youngest known supereruption, the Oruanui eruption at ∼ 25.5 ka, which produced over 530 km 3 dense-rock equivalent (DRE) of magma. This eruption culminated in a caldera collapse of the local area which, after infilling, became part of the modern lake (Davy and Cald- well, 1998; Wilson, 2001;Vandergoes et al., 2013;Allan, 2013). In the time since, smaller eruptions of a wide range of eruptive volumes (across 4 orders of magnitude) have occurred within a relatively concentrated vent location range (shown in Fig. 1), with at least 25 identified within 12 kyr (Wilson, 1993;Barker et al., 2020). ...
Volcanogenic tsunami and wave hazard remains less understood than that of other tsunami sources. Volcanoes can generate waves in a multitude of ways, including subaqueous explosions. Recent events, including a highly explosive eruption at Hunga Tonga-Hunga Ha'apai and subsequent tsunami in January 2022, have reinforced the necessity to explore and quantify volcanic tsunami sources. We utilise a non-hydrostatic multilayer numerical method to simulate 20 scenarios of sublacustrine explosive eruptions under Lake Taupō, New Zealand, across five locations and four eruption sizes. Waves propagate around the entire lake within 15 min, and there is a minimum explosive size required to generate significant waves (positive amplitudes incident on foreshore of > 1 m) from the impulsive displacement of water from the eruption itself. This minimum size corresponds to a mass eruption rate of 5.8×107 kg s-1, or VEI 5 equivalent. Inundation is mapped across five built areas and becomes significant near shore when considering only the two largest sizes, above VEI 5, which preferentially impact areas of low-gradient slope. In addition, novel hydrographic output is produced showing the impact of incident waves on the Waikato River inlet draining the lake and is potentially useful for future structural impact analysis. Waves generated from these explosive source types are highly dispersive, resulting in hazard rapidly diminishing with distance from the source. With improved computational efficiency, a probabilistic study could be formulated and other, potentially more significant, volcanic source mechanisms should be investigated.
... *Terminology is based mainly on Froggatt and Lowe (1990) and Wilson (1993Wilson ( , 2001. Bayesian-modelled ages mainly from Lowe et al. (2013), Vandergoes et al. (2013), and Peti et al. (2021). ...
Hands-on session to trial StraboSpot with a field excursion to tephra section on Okareka Loop Road near Rotorua. The guide documents the stratigraphy of the section and also includes information on the wider picture of rhyolitic volcanism in the Taupo Volcanic Zone and pedogenic upbuilding of soils on tephra deposits and the formation of clay minerals in them. Supported by the Commission on Tephrochronology (IAVCEI).
... A later compilation (Rust and Cashman 2011) showed this to be among the most fine-enriched deposits yet sampled. It is now thought that the fine-ash component of the 1980 fall deposit was enhanced by the substantial addition of fragments milled during blast co-PDC activity (Eychenne et al. 2015), similar to other large, co-PDC deposits (e.g., Sparks and Huang 1980;Wilson 2001;Wiesner et al. 2004;Engwell et al. 2014;Engwell and Eychenne 2016). ...
Discoveries made during the 18 May 1980 eruption of Mount St. Helens advanced our understanding of tephra transport and deposition in fundamental ways. The eruption enabled detailed, quantitative observations of downwind cloud movement and particle sedimentation, along with the dynamics of co-pyroclastic-density current (PDC) clouds lofted from ground-hugging currents. The deposit was mapped and sampled over more than 150,000 km2 within days of the event and remains among the most thoroughly documented tephra deposits in the world. Abundant observations were made possible by the large size of the eruption, its occurrence in good weather during daylight hours, cloud movement over a large, populated continent, and the availability of images from recently deployed satellites. These observations underpinned new, quantitative models for the rise and growth of volcanic plumes, the importance of umbrella clouds in dispersing ash, and the roles of particle aggregation and gravitational instabilities in removing ash from the atmosphere. Exceptional detail in the eruption chronology and deposit characterization helped identify the eruptive phases contributing to deposition in different sectors of the distal deposit. The eruption was the first to significantly impact civil aviation, leading to the earliest documented case of in-flight engine damage. Continued eruptive activity in 1980 also motivated pioneering use of meteorological models to forecast ash-cloud movement. In this paper, we consider the most important discoveries and how they changed the science of tephra transport.
... The volcanic edifice reaches an elevation of 354.3 m above sea-level with an estimated 15km-wide base rising from c. 420 m depth of the continental slope (Kósik et al. 2022). It is typical of volcanic systems surrounded by large lakes or occurring in a marine environment that the volcanic activity is heavily influenced by the availability of external water to fuel explosive eruptions (Houghton and Nairn 1991;Wilson 2001;Kósik et al. 2021;Brenna et al. 2022). The largest, 7.6 ka Plinian eruption of Tuhua was influenced by magma-water interaction, resulting in the Tuhua Tephra deposit, one of the most important Holocene marker horizons in the region (Lowe et al. 2008;Lowe et al. 2019). ...
Mayor Island is located approximately 45km off the North-East coast of the North Island of New Zealand. This island was formed by various explosive and effusive volcanic eruptions commonly influenced by magma-water interaction eruption events occurring since the Pleistocene. The wider area of the SW Pacific contains numerous volcanic islands with a similar type of volcanic evolution. Mayor Island should be studied in more detail to understand underlying volcanic mechanisms and apply this research to other volcanic islands in the SW Pacific. Mayor Island, also known by its indigenous Māori name Tūhua , provides an ideal site for studying current volcanism. The present day-island was formed around 150 thousand years ago and contains several rhyolitic lava-flows from different time periods, pyroclastic-flow deposits generated by small volume localized eruptions, and ignimbrite deposited by large explosive eruptions. Our research utilized a qualitative-quantitative assessment of geodiversity estimates to highlight possible geosites for collection of precise information about geological evolution of this area demonstrating potential of geoeducational sites. The term geodiversity recognizes geological and geomorphological elements, which have shaped the Earth's surface and underly our abiotic environment. Additionally, volcanic heritage was included in our equation specifically tailored for Mayor Island and based on expert views (qualitative model). This model allows for a wider diversity for the area of research compared to the original method, which utilized only geological elements. The results show that areas with pyroclastic deposits exposed on the cliffs and in the center of the collapse caldera should be considered for the further study for geosites' planning.
... The range of aspect ratios (thickness over diameter) for the reconstructed Kızılkaya magma reservoir is between 0.02 and 0.3, supporting a pre-eruptive pancakeshaped magma body (Fig. 6b). These aspect ratios are in excellent agreement with reservoir shapes inferred from geophysical studies and modelling of other silicic systems worldwide (Bachmann and Bergantz, 2008;Gregg et al., 2012;Wilson, 2001). ...
Large silicic explosive eruptions are one of Nature's most hazardous phenomena. Very few have been witnessed and unravelling the complex conditions that lead to these eruptions remains a difficult task because the characteristics of the feeding magma reservoirs are still insufficiently constrained from geophysical imaging. Here we show that a barometer based on the composition of amphibole in equilibrium with plagioclase and biotite, common minerals in the eruptive products of large silicic eruptions, records the thickness and depth of magma reservoirs with uncertainties of 0.8 and 2.7 km, respectively. Pressures are given by the equation: P (MPa) = 892 · VIAl + 101, where VIAl is the octahedral aluminum content of the amphibole. With the example of a Miocene Turkish ignimbrite, we show that reservoirs feeding large silicic eruptions can be pancake-shaped. Our new barometer, valid between 650 and 950 °C, can be used in combination with volcanological and geophysical data to infer the size, shape and depth of magma reservoirs and may serve as a tool for monitoring future activity. This temperature-independent barometer is also applicable to any volcanic or plutonic rock containing amphibole + plagioclase + biotite and is in excellent agreement with previously published temperature-dependent barometers within their calibration range.
... At Yellowstone dozens of lava eruptions have occurred since the supereruption 631,000 years ago, and until as recently as~70,000 years ago with the most recent having a volume of 70 km 3 ( Table 1; Christiansen et al., 2007;Watts et al., 2012). The Taupo supervolcano in New Zealand has had one supereruption, the 26.5 ka Oruanui eruption (Wilson, 2001), but has had many smaller eruptions before and since including the famous Taupo Eruption~1800 years ago that measures approximately as an M 6.9 and VEI 5. Small effusive post-climactic eruptions as seen at Toba, are referred to as ring-fracture or post-caldera eruptions (e.g. Smith and Bailey, 1968) and represent leaks of new or remnant magma as the caldera system relaxes after the catastrophic disruption of the climactic eruption. ...
Although evocative, the term supervolcano has a checkered history of hyperbole and misuse to the point that it seems unprofessional. However, “supervolcano” is firmly embedded in volcanological discourse and we make the case that it is useful if defined and used correctly. To this end we examine the etymology of supervolcano and demonstrate its’ dependence on the term supereruption. We build on the work of colleagues to propose that supervolcano be restricted to a volcano that has been the site of at least one silicic explosive eruption of Magnitude of 8 (M 8) or greater. Based on this, nine active supervolcanoes are found on the Earth today and although all are calderas, we contend that referring to them simply as large calderas or caldera complexes obviates clear magmatic, volcanological, and structural extremes that distinguish supervolcanoes from other caldera complexes. Such supervolcanoes may produce eruptions that exceed M 9 but we stress that most eruptions from supervolcanoes are actually small effusive eruptions. Basaltic explosive supereruptions remain enigmatic on Earth and therefore we advise against the use of supervolcano for any basaltic volcano or province on Earth.
... The entrainment rates and collapse conditions in the No-L d -No-L X scenario are therefore likely inconsistent with real hydrovolcanic eruptions. For example the~24,000 BP Oruanui hydrovolcanic eruption in New Zealand had estimated magma mass fluxes of 10 8 -10 9 kg/ s and is recognized for its remarkably wide dispersal of airfall deposits (Wilson, 2001). This eruption emerged through Lake Taupo, which in modern times has water depths averaging about 150 m, and is believed to have had depths of at least 100 m at the time of the eruption (Nelson and Lister, 1995). ...
Explosive volcanic eruptions can inject sulfur dioxide (SO2) into the stratosphere to form aerosol particles that modify Earth’s radiation balance and drive surface cooling. Eruptions involving interactions with shallow layers (≤500 m) of surface water and ice modify the eruption dynamics that govern the delivery of SO2 to the stratosphere. External surface water controls the evolution of explosive eruptions in two ways that are poorly understood: 1) by modulating the hydrostatic pressure within the conduit and at the vent, and 2) through the ingestion and mixing of external water, which governs fine ash production and eruption column buoyancy flux. To make progress, we couple one-dimensional models of conduit flow and atmospheric column rise through a novel “magma-water interaction” model that simulates the occurrence, extent and consequences of water entrainment depending on the depth of a surface water layer. We explore the effects of hydrostatic pressure on magma ascent in the conduit and gas decompression at the vent, and the conditions for which water entrainment drives fine ash production by quench fragmentation, eruption column collapse, or outright failure of the jet to breach the water surface. We show that the efficiency of water entrainment into the jet is the predominant control on jet behavior. For an increase in water depth of 50–100 m, the critical magma mass eruption rate required for eruption columns to reach the tropopause increases by an order of magnitude. Finally, we estimate that enhanced emission of fine ash leads to up to a 2-fold increase in the mass flux of particles < 125 μm to spreading umbrella clouds, together with up to a 10-fold increase in water mass flux, conditions that can enhance the removal of SO2 via chemical scavenging and ash sedimentation. On average, compared to purely magmatic eruptions, we suggest that hydrovolcanic eruptions will be characterized by reduced climate forcing. Our results suggest one possible mechanism for volcano-climate feedback: temporal changes with climate in surface distributions of water and ice may modify the relative global frequency or dominance of hydrovolcanic eruption processes, modulating, in turn, global patterns in volcano-climate forcing.
Explosive silicic volcanism of the Carpathian-Pannonian Region (CPR) is increasingly recognized as the primary source of tephra across the Alpine-Mediterranean region during the Early and Middle Miocene. However, the tephrostratigraphic framework for this period of volcanic activity is still incomplete. We present new multi-proxy data from Lower Miocene ignimbrites and tephra fallout deposits from the southwestern CPR and the Dinaride Lake System and integrate them into existing datasets to better resolve the regional extent and scale of these eruptions of the CPR. Volcanic glass geochemistry indicates distal fallout tuffs deposited in the Sinj Basin are correlative with the proximal Ostoros ignimbrites from the Bükkalja Volcanic Field, indicative of regionally extensive volcanism at 17.295 ± 0.028 Ma, based on CA-ID-TIMS U-Pb zircon geochronology. Based on integrated tephrostratigraphic data, newly identified 17.064 ± 0.010 Ma massive rhyolitic ignimbrite deposits from the Kalnik Volcaniclastic Complex located in the southwestern CPR are correlative with the 17.062 ± 0.010 Ma Mangó massive ignimbrite found in the Bükkalja Volcanic Field located in the northern CPR. Based on these new observations of its potential areal distribution and estimated thicknesses, these two widespread ~17.1 Ma ignimbrites represent intermediate to large caldera-forming ignimbrites, larger than previously suggested. Finally, volcanic glass geochemistry of fallout deposits from the Dinaridic Sinj and Livno-Tomislavgrad Basins have similar volcanic glass geochemistry as the rhyolitic pumices from the lowermost part of the Bogács ignimbrite unit of the Bükkalja Volcanic Field. However, high-precision geochronology indicates that these distal ashfalls were deposited at 16.9567 ± 0.0074 Ma, significantly predating the 16.824 ± 0.028 Ma emplacement of the fiamme-bearing part of the Bogács ignimbrite. These distinct ages suggest that the Bogács unit represents multiple eruptive events and indicating that further work is required to deconvolve this portion of the CPR volcanic record. Together, these data suggest that large volume CPR ignimbrite volcanism was more frequent and widespread than previously understood, enhancing the existing volcanic framework and history of the source region for this time period.
Explosive eruptions can produce a spectrum of pyroclastic density currents (pdcs) which we describe in this chapter. Pdcs are potentially the most destructive of all volcanic phenomena, due to their high velocities, the large distances that some can travel, the large volume of volcanic debris they can carry, and their high temperature. Pyroclastic flows include pumice/scoria and ash flows, blast flows, and block and ash flows. Pumice/scoria and ash flows are characterised by variably vesiculated pumice and scoria and finer ash and result from partial or wholesale gravitational collapse of initially buoyant explosive eruption columns or directly from a boil-over fountain of the pyroclastic mixture directly from the vent(s). In some cases, column collapses are preceded or accompanied by pyroclastic fallout, but some extremely large volume pyroclastic flow systems, especially those derived from very large, crystal-rich magma reservoirs that source major caldera collapse events and explosive super-eruptions, seem to originate from ring fractures rather than point source vents and immediate eruption column or fountain collapse, as soon as the eruption begins. Pumice/scoria and ash flows can flow from <1 to >100 km from source, and range in volume from <1 to >1000 km3. Deposits, called ignimbrites, range from felsic to mafic in composition, and they are massive to diffusely stratified, non-welded to largely welded. They are generally poorly sorted, and include variable proportions of accessory lithic clasts, including significant breccia horizons. Surface area and volume of ignimbrites are related and reflect variable mass eruption rates and eruption durations, which can be used to distinguish between column collapse and caldera collapse associated ignimbrites. Lateral blast flows originate from sector collapse of parts of high-relief stratovolcanoes, or the margins of high-relief lava domes, which are very polymictic, and can flow several tens of km. Block and ash flows are relatively small in volume, monomictic, lack accessory lithics, originate from lava dome collapse and have limited flow distances (<15 km). Pyroclastic surges range from vent-centric base surges derived from phreatomagmatic-phreatic-hydrothermal explosions with limited flow distances, to marginal, co-pyroclastic flow surges that can continue to propagate for the length of pyroclastic flows. Although we focus on deposit facies characteristics to interpret the flow dynamics of pdcs, post-depositional effects are also considered, particularly welding, which is a common, although not universal characteristic of ignimbrites, but absent or rare in block and ash flow deposits, and absent in blast flow and base surge deposits.
In this chapter, we consider the range of volcanic explosive eruption styles, from the smallest and least intense to the largest and most intense that produce pyroclastic fallout deposits. At the small-scale end of the spectrum of subaerial explosive eruptions, we include strombolian sensu stricto, halema’ma’uan and small-scale vulcanian and hydrothermal explosions. At the next level, there are small-scale hawai’ian magma/fire fountaining and larger scale vulcanian and hydrothermal explosions, followed by large-scale hawai’ian magma/fire fountaining, large vulcanian, and violent strombolian eruptions. We consider the eruptions that produce basaltic scoria through to rhyolitic pumice cones to be micro-plinian, at the small end of the spectrum of sub-plinian, plinian, and ultra-plinian explosive styles. We suggest that violent strombolian eruption style, which also produces cones, represents a transition between open-system degassing and closed-system degassing processes, and also that all the magmatic explosive eruption styles can transition into phreatomagmatic equivalents, irrespective of the scale, as demonstrated by the 2022 Hunga Tonga-Hunga Ha’apai eruption. The dynamics of the explosive eruption columns for all these styles are discussed, as are characteristic erupted masses and mass eruption rates. The characteristics of the deposits from each eruption style are described and comprehensively illustrated, particularly the field facies characteristics. Close to the vent, some proximal fallout deposits are welded, or pass into agglutinated spatter. We also review what is known about pyroclastic fallout processes in aqueous environments, and highlight how eruption dynamics, columns, and dispersal processes are different from subaerial processes. Several new proposed subaqueous eruption styles have recently emerged from recent research, including yalian, poseidic, neptunian, tangaroan-havre, and now hunga tongan. Although not enough is presently known about the fragmentation processes, intensity, and dispersal extent of these subaqueous eruption styles, it is important that they be acknowledged as a basis for driving future research. Finally, we consider how fit for purpose existing classification schemes for pyroclastic fallout deposits are. We suggest modifications to these, including recalibration of the logarithmic VEI, Eruption Magnitude and Explosive Intensity scales that have been applied for several decades, to accommodate smaller scale events that could previously not be included, but can be lethal in their impact, and therefore, need to be included in all schemes that reflect eruption scale and intensity.
Our newly acquired and recently published map, geochronologic, and com- positional data for early intermediate-composition central volcanoes in the northeastern San Juan Mountains provide insights about the broad magmatic precursors to the large continental-arc ignimbrite flare-up in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF). Initial volcanism migrated from central Colorado to northern New Mexico ca. 40–29 Ma, as part of a more regional trend of southward-progressing mid-Cenozoic magmatism in the U.S. segment of the North American Cordillera. Within the San Juan locus, which represents the largest preserved erosional remnant of the SRMVF and site of most intense eruptive activity, new 40Ar/39Ar and U-Pb zircon ages show that eruptions at many individual edifices began nearly concurrently, at ca. 35 Ma, with peak activity at 34–32 Ma. Broadly similar precursor effusive volcanism characterizes other major loci of continental-arc ignimbrite magmatism along the western American cordilleras, but none of these sites records early volcanism as voluminous, spatially widespread, well exposed, or compositionally diverse as the San Juan locus in Colorado.
Early San Juan volcanism was larger in volume than the later ignimbrites, constituting about two thirds the total erupted. Early lava and breccias are as much as 700–900 m thick where exposed along eroded flanks of the San Juan Mountains; drill holes have penetrated sections as thick as 2600 m. The early volcanoes were dominantly andesitic, with lesser dacite and minor rhyolite. Such volcanism is widely interpreted as initiated by basaltic magma from the mantle, but mid-Cenozoic basalt is almost nonexistent at the San Juan locus—an absence inferred to be due to extensive lower-crustal assimilation and fractionation. The early volcanic rocks are calc-alkaline and typical of high-K continental-arc volcanism; they become modestly more alkalic and enriched in trace elements such as light rare earth elements, Zr, Nb, and Th from the San Juan locus northeastward into central Colorado. Such variations may reflect synmagmatic crustal thickening and deeper levels of primary magma generation concurrent with assembly of upper-crustal magma bodies that could support large ignimbrite eruptions. Substantial uncertainties remain for growth histories of the early volcanoes, however, because of unexposed lower parts of edifices, eroded upper parts, and limited availability of mineral phases that could be dated reliably.
Although the early volcanoes are widely distributed within the SRMVF, many are clustered at sites of subsequent ignimbrite calderas. The precursor edifices are inferred to record incubation stages in construction of overall translithospheric batholithic-scale magmatic systems. Prolonged processes of incremental magma generation, accumulation, fractionation, and solidification intermittently generated sufficient liquid to erupt large ignimbrites. Maturation of focused eruptions and intrusions was prolonged, 5 m.y. or more, prior to the culminating ignimbrite at some centers in the San Juan Mountains. Some large-volume ignimbrites and related calderas, including the ~5000 km3 Fish Canyon Tuff and associated La Garita caldera, formed as much as several million years later than peak growth of associated precursor volcanoes, recording a sustained interval of diminished eruptive activity as the magma reservoir increased in volume and evolved to more silicic compositions capable of supporting a subsequent large ignimbrite eruption.
Dike configurations at early volcanoes that were active nearly concurrently in the SRMVF vary from symmetrically radial to more parallel trends. The contrasting dike geometries are inferred to record possible multiple fluctuations from compressive to weakly extensional regional stress, concurrent with destabilization of the prior flat-slab plate configuration that triggered mid-Cenozoic ignimbrite flare-ups along the Cordilleran margin of the North American plate. These apparent fluctuations in regional stress preceded development of substantial extensional strain in the Southern Rocky Mountain region; outflow ignimbrite sheets of the SRMVF spread across subsequent horst-and-graben structures of the Rio Grande rift without complementary thickness variations.
The word “tephra” is an all-encompassing term for the explosively erupted, pyroclastic (fragmental) products of a volcanic eruption. Since the early pioneering work of Thorarinsson and others, the value of tephras in providing time-parallel marker horizons or isochrons is now well understood. Tephras are routinely detected and identified in both visible and non-visible (cryptotephra) forms, and are used in a diverse range of disciplinary fields including stratigraphy, sedimentology, geomorphology, archaeology, and paleoenvironmental reconstruction over a wide range of time scales. Tephrochronology is also an essential tool for establishing the frequency/periodicity of volcanic activity and for assessing volcanic hazards.
Origin, mechanics and properties of the Solar System are analysed in the framework of the Complete Relativity theory (by the same author). According to Complete Relativity, everything is relative. Any apparent absolutism (notably invariance to scale of dimensional constants, absolute elementariness, invariance to time) is an illusion stemming from limits imposed by [or on] polarized observers that will inevitably lead to misinterpretation of phenomena (another illusion) occurring on non-directly observable scales or even on observable but distant scales in space or time. If everything is relative, reference frames will exist where particles are planets and where planets are living beings. Earth is, therefore, analysed here in more detail, both as a particle and, as a living evolving being (of, hypothesized, extremely introverted intelligence). The analysis confirms the postulates and hypotheses of the theory (ie. existence of discrete vertical energy levels) with a significant degree of confidence. During the analysis, some new hypotheses have emerged. These are discussed and confirmed with various degrees of confidence. To increase confidence or refute some hypotheses, experimental verification is necessary. Main conclusions that stem from my research and are further confirmed in this paper are: universes are, indeed, completely relative; Solar System is a scaled (inflated, in some interpretations) Carbon isotope with a nucleus in a condensed (bosonic) state and components in various vertically excited states; life is common everywhere, albeit extroverted complex forms are present on planetary surfaces only during planetary neurogenesis; anthropogenic climate change is only a part (trigger from one perspective) of bigger global changes; major extinction events on a surface of a planet are relative extinctions, a regular part of transformation and transfer of life in the process of planetary neurogenesis.
Taupō, New Zealand, is an active caldera volcano that in recent times has erupted on average every ~500 years, with the latest explosive eruption in 232±10 CE. Monitoring at Taupō is challenging as there has been no eruptive activity in documented history; however, Taupō does undergo periods of unrest on roughly a decadal timescale, such as in 2019. Key to identifying these unrest periods is understanding what represents 'normal' inter-unrest activity. In this study, we generate an earthquake catalogue for Taupō for 2010–2019 inclusive, consisting of 46,481 earthquakes. This shows that the Taupō region has background earthquake rates of 50–200 earthquakes per month and the 2019 unrest episode was preceded by an exponential increase in earthquake rate. We also show that when attenuation is accounted for there is no evidence for low-frequency earthquakes at Taupō, and that this is an important consideration for volcano monitoring and determining the presence of significant magma movement.
Double-Wien filter-selection-aperture and hexapole-collision-cell technologies coupled to laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS/MS) enables in situ analysis of ⁸⁷ Sr variations produced by ⁸⁷ Rb decay.
Subaerial volcaniclastic deposits are produced principally by volcanic debris avalanches, pyroclastic density currents, lahars, and tephra falls. Those deposits have widely ranging geomorphic and sedimentologic characteristics; they can mantle, modify, or create new topography, and their emplacement and subsequent reworking can have an outsized impact on the geomorphic and sedimentologic responses of watersheds surrounding, and channels draining, volcanoes. Volcaniclastic deposits provide a wealth of information about eruptive histories, volcanic processes, and landscape responses to eruptions. The volcanic processes that produce these deposits, and consequently the character and sedimentary structures of the deposits themselves, are influenced by initiation mechanism. Deposit preservation is affected by deposit magnitude, texture, and composition, depositional environment, and climate regime. Innovative analyses of deposits from several modern eruptions and advancements in physical and numerical modelling have vastly improved our understanding of volcanic processes, interpretations of eruptive histories, and recognition of the hazards posed by volcanic eruptions. This contribution highlights and summarizes major advances that have occurred in the past few decades in understanding of volcaniclastic deposits and linkages with volcanic processes.
The Lower Permian Wurzen caldera formed during a VEI 8 supereruption that tapped monotonous intermediate magmas in a continental rift. The well-preserved crystal-rich intracaldera ignimbrite (<58 vol%) and its related plutonic facies allow for compositional and textural studies on juvenile fragments and mineral phases, providing a unique opportunity to unravel the caldera's pre-eruptive evolution.
The climactic Wurzen ignimbrite contains different fiamme of fine-grained mafic, fine-grained rhyolitic (71.8 to 76.6 wt% SiO2), porphyritic (trachy-) dacitic (64.5 to 67.6 wt% SiO2), and coarse porphyritic rhyodacitic composition (64.7 to 74.3 wt% SiO2), which point to a permanent interplay between rejuvenation, differentiation, and cooling in the Wurzen magma body. The similarity of their trace element concentrations suggests derivation from a shallow magma chamber (Nb and Ta at 22 and 1.6 ppm, respectively). The picture becomes more differentiated when considering mineral composition and thermobarometric estimations. Resorption textures in anorthitic plagioclase (An >45 mol%) are presumably formed by excess heat from underplating magmas. Coarse sieve textures in plagioclase indicate a rapid ascent of magma into the shallow magma body. In contrast, oscillatory zonation and rapakivi texture in quartz, albitic plagioclase, and sanidine indicate convection during crystal growth. The spatially discrete crystallization of augite and pigeonite indicates tapping of anhydrous Ca-saturated and -undersaturated magma. The presence of pigeonite indicates eruption from a superheated anhydrous (trachy-) dacitic magma batch at temperatures of 1010 °C, whereas the occurrence of annite implies simultaneous tapping of hydrous shallow magma chambers at low temperatures (~750 °C). By applied barometers on clinopyroxene and biotite, it suggests a deep-seated magma chamber at depths of 25 to 15 km and a shallow magma chamber at ca. 11 km, respectively.
Mineral assemblage of the intracaldera Wurzen ignimbrite (quartz, sanidine, plagioclase, calcic and calcic-sodic clinopyroxene, pigeonite, annite) is more typical for crystal-poor, hot rhyolites like in the Snake River Plain (Idaho, USA) or Messum igneous complex (Namibia), than that of monotonous intermediates at active continental margins.
Lake Taupō (Taupō-nui-a-Tia) infills the composite caldera above an active rhyolitic magmatic system in the central Taupō Volcanic Zone (TVZ). Ground deformation is a key unrest indicator at Taupō volcano. We present a spreadsheet tool, TaupōInflate, to calculate and plot ground deformation from magmatic inflation at depth beneath Taupō caldera. Examples show detection limits for inflating magma bodies and their ascent through the crust beneath Lake Taupō. Source locations where it is challenging to detect even substantial volumes of inflating magma bodies are as large as 20 km3, with volume changes up to 0.01 km3, owing to the restricted station placement around the lake, although a dike propagating from shallow crustal depths towards the surface is likely to be detectable. For a magma overpressure of 10 MPa, the sizes of detectable inflating bodies at depths of 5-8 km using the present monitoring system are larger than the volumes of many past eruptions, illustrating the importance of future improvements to the geodetic network. We discuss the potential for future equipment installation, including lakebed instrumentation that would require approval of local iwi Ngāti Tūwharetoa through the Tūwharetoa Māori Trust Board who oversee the health and wellbeing of Lake Taupō.
In this study, we apply a two‐dimensional, transient depth‐averaged model to simulate the inertial flow dynamics of caldera‐forming pyroclastic currents, using the available data about the Pozzolane Rosse ignimbrite (Colli Albani, Italy) eruption (460 ka, 63 km³ DRE). By performing an extensive set of numerical simulations, we test the effects of the initial parameters of the pyroclastic current (Richardson number, mass flow rate, initial flow density) on simulated deposit characteristics which can be compared with selected ignimbrite field observables, including the deposit dispersal along topography, the maximum distance from source, the deposit thickness, the grain size distribution at different distances, and the emplacement temperature. Results permit us to quantify the first‐order dependency of the flow runout on the mass flow rate, and of the deposit thickness decay pattern on the initial mixture density. By using the results of the parametric study we reconstruct the source parameters of the Pozzolane Rosse ignimbrite constrained by the ignimbrite depositional characteristics, including the mass partition into the co‐ignimbrite cloud. Despite uncertainties associated with the complex, non‐linear interplay between the flow variables, the single‐layer, depth‐averaged model demonstrates to be suitable for simulating inertial pyroclastic currents, such as those generating large‐scale caldera‐forming ignimbrites, providing a tool for reconstructing the eruption source parameters from deposits characteristics, and to assess pyroclastic currents' hazard for future eruptions.
The Ito pyroclastic flow erupted about 22,000 years ago from Aira caldera in southern Kyushu, Japan. Flow directions were determined by anisotropy of magnetic susceptibility (AMS), which measured the preferential alignment of magnetite microphenocrysts, usually ≤0.25 mm in diameter. The microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent. The grains are then deposited parallel to the flow direction with an imbrication. There is no evidence of rolling of clasts or nonflow parallel lineations, and thus AMS can be used to determine flow directions that occurred immediately before deposition. Beyond 30 km from the center of the caldera, flow directions were predominantly down paleogradient, indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system. Within 30 km, measured flow directions are random, indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system. Individual flow directions at all sites except one varied more than the analytical error, demonstrating that a variety of flow directions existed even within a small area of the flow. This implies that the depositional system was turbulent.
A plinian pumice fall deposit associated with the Campanian Ignimbrite eruption (36 ka, Phlegraean Fields caldera, Italy) occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between Benevento and the Sorrentina peninsula. In the thickest stratigraphic section at Voscone (130 cm), 45 km east of the Phlegraean caldera centre (Pozzuoli), the deposit consists of two units: the lower fall unit (LFU) is well sorted, exhibits reverse size grading and is composed of equidimensional light-grey pumice clasts with very subordinate accidental lithics; the upper fall unit (UFU) is from well to poorly sorted, crudely stratified, richer in lithics and composed of both equidimensional and prolate pumice clasts. The two fall units show slightly different dispersal axis: N90° for the LFU and N95° for the UFU. Volumes calculated with the method of Pyle (1989) are about 8 km3 for the LFU and 7 km3 for the UFU. The maximum height of the eruptive columns are estimated, using the model of the maximum lithic clasts dispersal, at 44 km for the LFU and 40 km for the UFU, classifying both fall units as ultraplinian in character. Reverse size grading within the LFU suggests an increase of the height of the column and magma discharge rate with time. Moderate sorting and crude stratification of the UFU are consistent with short-period oscillation of the column, possibly associated with repeated partial column collapses. Sharp increases in lithic content at the transition to UFU and within the UFU suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit/vent erosion. The phase of column instability preceded the emplacement of widely dispersed pyroclastic flow. The ultraplinian nature of the fall fits well with the wide dispersal of the Campanian Ignimbrite with an estimated aspect ratio of 3–4×10−4 (LARI).
Analysis of 30 individual pumice blocks, together with bulk samples from the ash-flow member of the Los Chocoyos Ash within the Quezaltenango Valley, Guatemala, demonstrates that prior to its eruption, its associated magma-chamber was zoned. Eruption of a high-K (K2O/Na2O > 1), crystal-poor, biotite-bearing rhyolite with crystal equilibration temperatures of less than 800 °C produced the widespread H-tephra member and the initial phases of the ash-flow member. As the ash-flow eruption continued, a more-heterogeneous, low-K, crystal-rich, cummingtonite- and hornblende-bearing rhyolite became predominant; its phenocrysts had equilibrated at temperatures of about 950 °C. The water content of the high-K rhyolite was several percent, whereas the low-K rhyolite was much drier. Bulk samples of the ash-flow member are homogenized mixtures of matrix shards that represent either the high-K or low-K rhyolite magmas; the overall ratio for the ash-flow member is 60% high-K and 40% low-K type. The 87Sr/86Sr ratios for both high-K and low-K magma types are identical and average 0.70405 ± 0.00003. This value is nearly the same as all basaltic, all andesitic, and most rhyolitic Quaternary volcanic rocks tested in Guatemala so far. The 87Sr/86Sr ratios for bulk samples of the ash are significantly higher and more variable (0.70426 ± 0.00009), probably because of xenocrystic contamination. Detailed mixing and Rayleigh calculations using observed mineral phases in the ash show that the concentrations of 8 major and 17 minor elements in the ash are consistent with the derivation of high-K rhyolite from low-K magma by crystal fractionation at shallow depths. The time required for such fractionation is at least 104 yr. The absence of a continuum of compositions from low-K to high-K rhyolite and the differences in p H2O and temperature suggest that the two magmas were separated during fractionation. The Los Chocoyos Ash is the most silicic major Quaternary unit in the Guatemalan Highlands; the volume of magma from which it was derived is far greater than that of all other Quaternary volcanic rock units in the area.
This bulletin describes the geological aspects of a 15-year investigation of the geothermal resources of the Wairakei hydrothermal field. Since 1950, 120 drillholes (to a maximum depth of 4,550 ft) have been drilled, and a power station, steam transmission lines, and ancillary structures constructed. Power generation commenced in 1958 and present peak output is 175 mw at 85 - 90% load factor. Installed capacity is 192 mw, giving a plant factor of 70 - 90%. After an historical introduction, Chapters 2 - 4 deal with the local geology and geological history. Chapter 5 discusses the siting of drillholes, drilling procedures, performance of holes, power production and the effects of exploitation. Chapter 6 considers the location and size of the heat source. Chapter 7 compares the Wairakei hydrothermal field with other fields in New Zealand and overseas. Maps, sections, and drill logs attached.
Existing models for volcanic eruption plumes are successful in predicting sizes and growth rates, and the dispersal characteristics of the resulting fall deposits given an estimate of the magma-output rate. These models work well for "dry' eruptions, where the gases involved were originally dissolved in magma. The models predict that, in "wet' activity where abundant external water mixes with the magma during eruption and flashes to steam, the eruption plume should be lower and the resulting fall deposits less widely dispersed for a given magma-output rate. However, parts fo the fall deposits from the very large, wet, 22 500-year-old Oruanui eruption (New Zealand) show the opposite behavior and represent the most powerfully dispersed fall deposits yet documented. -Author
Upper Pleistocene rhyolitic ash-flow and air-fall tuffs, erupted from several centers, were sampled in 23 pumice-filled basins over an area of 16,000 km2. Fifty pumice-matrix samples were analyzed for as many as 20 trace elements. Ba, Fe, Hf, Rb, Sm, Sr, Th, Ti, and Zr were particularly useful in “fingerprinting” correlations between basins and in corroborating the stratigraphy previously established within individual basins. On the basis of similar trace elements, a tephra and an overlying ash-flow sheet (together, a unit here named the Los Chocoyos Ash) appear to have formed from a multiphase eruption. The tephra, whose volume exceeded 100 km3, blanketed an area greater than 1 × 106 km2. The second phase of the eruption produced an ash flow of greater than 200 km3. Areal geochemical patterns within the ash-flow sheet are probably related to sequentially less explosive eruptions of progressively more mafic ash flows. Changes in chemical composition, size of pumice and lithic fragments, thickness, and elevation all suggest a source for the Los Chocoyos Ash in the Lake Atitlán cauldron. Chemical data suggest correlation of the H-tephra member of the Los Chocoyos Ash with the most prominent D layer of the Worzel ash of the equatorial Pacific.
Pre-330 000 yr volcanic rocks in this area are poorly preserved due to deep burial. Eruption of the voluminous Whakamaru-group welded ignimbrites and an associated widespread airfall deposit occurred between 330 000 and 230 000 yr, inferred to be associated with the formation of a large caldera situated in the northern-Taupo-Maroa area. Subsequent activity developed a northwestern dome complex, a western dome complex, Maroa volcano which is a caldera structure associated with modest-volume ignimbrite eruptions infilled by ignimbrites and numerous rhyolite domes, and Taupo volcano. Taupo erupted lavas plus subordinate pyroclastics up to 50 000-20 000 yr, but since then explosive activity has generated a young, complex caldera. Volcanism has been dominantly rhyolitic and explosive, but scattered outcrops of basalt and andesite also occur, and andesites are found in geothermal drillholes. Structural influences on vent positions and caldera collapse are controlled by dominant northeasterly and subordinate N-S oriented structures. Maroa has erupted a high proportion of lavas, accompanied by subordinate, weakly explosive activity, while Taupo has erupted almost all of its products explosively. Maroa is only probably feebly active whereas Taupo is one of the most active rhyolite volcanoes known.-R.M.B.
Drag resistance developed between a pyroclastic flow and the ground results in a transitional zone of low velocity between the maximum velocity of the flow and the stationary ground. Fragments of all sizes within the turbulent flow travel irregular paths and therefore enter the reduced velocity zone at random and are deposited together irrespective of size. This process results in poorly sorted deposits with systematic vertical variations in mineralogy which are dependent upon variations in the original magma chamber and with lateral variation which are dependent upon the gradual energy losses of the main flow. Mineral variations in one unit of a compound cooling unit, the Picture Gorge ignimbrite, from the John Day Formation in eastern Oregon are explained on the basis of the above model.
The pyroclastic deposits of many basaltic volcanic centres show abrupt
transitions between contrasting eruptive styles, e.g., Hawaiian versus
Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These
transitions are controlled dominantly by variations in degassing
patterns, magma ascent rates and degrees of interaction with external
water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre
in the Auckland volcanic field, New Zealand, as a case study of the
transitions between these end-member eruptive styles. The Crater Hill
eruption took place from at least 4 vents spaced along a NNE-trending,
600-m-long fissure that is contained entirely within a tuff ring
generated during the earliest eruption phases. Early explosive phases at
Crater Hill were characterised by eruption from multiple unstable and
short-lived vents; later, dominantly extrusive, volcanism took place
from a more stable point source. Most of the Crater Hill pyroclastic
deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M)
episodes, forming in turn the outer tuff ring and maar crater (P1, M1,
P2) and scoria cone 1 (M2-M4). This activity was followed by formation
of a lava shield and scoria cone 2. Purely `wet' activity is represented
by the bulk of P1 and P2, and purely `dry' activity by much of M2-M4.
However, M1 and parts of M2 and M4 show evidence for simultaneous
eruptions of differing style from adjacent vents and rapid variations in
the extent and timing of magma:water interaction at each vent. The
nature of the wall-rock lithics, and these rapid variations in inferred
water/magma ratios imply interaction was occurring mostly at depths of
≤80 m, and the vesicularity patterns in juvenile clasts from these
and the P beds imply that rapid degassing occurred at these shallow
levels. We suggest that abrupt transitions between eruptive styles, in
time and space, at Crater Hill were linked to changes in the local magma
supply rate and patterns and vigour of degassing during the final metres
of ascent.
The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield and scoria cone 2. Purely `wet' activity is represented by the bulk of P1 and P2, and purely `dry' activity by much of M2–M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent.
A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.
Late Pleistocene rhyolitic tephras erupted in the period c. 20000–42000 years B.P. and preserved in the Taupo district have been mapped and described with reference to their field appearance and stratigraphic sequence. Five new formations of tephras erupted from the Taupo Volcanic Centre are defined; they are, from youngest to oldest: Poihipi, Okaia, Tihoi, Waihora, and Otake Tephra Formations. Their relationship is established with tephras of similar age erupted from the Okataina Volcanic Centre.
Sediments sampled by piston and gravity corers from the continental slope off Hawkes Bay Land District, New Zealand, consist of mud with ash and turbidite layers.The ash layers can be correlated by mineralogical and geochemical methods with dated ash layers on the adjacent land. The 3400-year-old Waimihia ash is identified in all cores. In some cores a band of pumice in mud above the Waimihia ash is considered to represent the Taupo eruption of 1800 years ago, and deeply buried ashes are correlated with Oruanui Ash, Mangaone Lapilli Formation member (c), and Rotoehu Ash, which were erupted about 20 500, 30 000 and 43 000 years ago respectively.Rates of sedimentation during the last 3400 years are estimated (from the thickness of sediment overlying the Waimihia ash) to range from 0 to 0·36m per thousand years.In synclinal depressions that are downslope from submarine channels, the sediment above and below the Waimihia ash consists of thick layers of mud separated by thin sandy layers, some of which contain shallow water foraminifera. The sandy layers are considered to have been deposited by turbidity currents, and a maximum of nine of them overlie the Waimihia ash.
Pyroclastic fall and flow deposits occupy two distinct fields on an plot (Inman parameters), and a contoured diagram is given based on 1,600 samples to facilitate comparison of mechanical analyses. Analyses which plot where the fields overlap include rain-flushed ashes and thin flow deposits. Among factors influencing of fall deposits is the wind: a strong wind will reduce its value. Another is the characteristics of the initial population-the entire assemblage of fragments coming from the vent-which is quite different for crystals than for pumice or lithic components. Each component in a polycomponent deposit has a different grain-size distribution due to this and subsequent air sorting. Histograms or cumulative curves where the weight percentages are plotted against the fall velocity are shown to be more meaningful than those against the grain size, and a quantity V is defined analogous to . Ignimbrites are remarkably homogeneous, but two departures are he...
One of Earth's largest known eruptions, the Toba eruption of 75 ka, erupted a minimum of 2800 km3 of magma, of which at least 800 km3 was deposited as ash fall. This ash may be entirely of coignimbrite origin and dispersed widely because of high drag coefficients on the predominantly bubble-wall shards. Shards of this shape are broken from the walls of spherical vesicles, which formed in high abundance in isotropic strain shadows near phenocrysts in this crystal-rich magma.
Thick silicic volcanic ash layers commonly observed hundreds of
kilometres from potential source areas have resulted from
large-magnitude explosive eruptions that have no historical equivalents.
We have developed a model that predicts the duration of these eruptions
from the vertical size grading of feldspar phenocrysts near the base of
deep-sea tephra layers. The size grading is a function of the release
time of the particles, their settling velocity, the water depth at the
site of the ash layer, and the duration of the eruption. The model has
been tested on two layers of the Worzel D ash in the eastern equatorial
Pacific. This ash layer is the distal counterpart of the rhyolitic Los
Chocoyos ash-flow tuff and H-tephra layer associated with the formation
of the Lake Atitlán caldera in Guatemala. The estimated duration
of the eruption is 20 to 27 d. Calculations using published estimates of
the volume of erupted material yield an average magma-discharge rate of
about 240,000 m3/s. This rate is approximately equivalent to
that recorded in the 1956 eruption of Bezymianny and the 1912 eruption
of Katmai. *Present address: Department of Mineralogy and Petrology,
University of Cambridge, Downing Place, Cambridge, Great Britain
The formation of widespread volcanic ash-fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra (20 180 mum diameter) as it travels from the atmosphere into water. Using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of Mount St. Helens (0.2 g/cm2/hr), the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents. The currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large. At the air-water interface, the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface. The implication of the experiments is that deposition of distal tephra fall layers in deep-sea sediments may be dominantly controlled by diffuse vertical gravity currents, as opposed to passive settling of individual particles through the water column. This process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers. Support for this mechanism comes from observations of greatly accelerated tephra settling rates in the South China Sea following the 1991 eruption of Mount Pinatubo in the Philippines.
The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1-F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1-F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2-F8), lack phenocrystic pyroxenes, and contain few or no Glass Mountain-derived rhyolite lithic fragments. Later packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain-derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less-welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to ≥1 mm/s of densewelded massive ignimbrite, equivalent to ≥2.5 mm/s of non-welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in ≥35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south-central part of the caldera (F1-8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre-eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.
When magma vents into the sea or a crater lake, the ensuing magma-water interaction can affect the style of eruption dramatically. If the mass of surface water incorporated into the erupting material is small, (< 15% of the total mass), then typically, this water vaporizes, and the density and temperature of the erupting mixture decreases. As a result, the minimum eruption velocity for which a Plinian-style eruption column may develop decreases. If a larger mass of cold surface water is added to the mixture, then part of this water may not vaporize, the initial mixture has the saturation temperature, and the initial density increases again. For sufficiently large masses of surface water mixed into the erupting magma, the ascending mixture cannot become buoyant. Instead, relatively cold, wet and dense ash hows spread laterally from a collapsing fountain. With a small or moderate eruption rate, < 10(8) kg/s, the height of rise of a buoyant column does not vary significantly with the surface water content. However, for very large eruption rates, > 10(8) kg/s, the height progressively decreases with surface water content. This occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column, so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air. The transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of Taal in 1965 and Miyake-jima in 1983 and for the changes in the eruptive activity at Surtsey in 1963-1964 as the access of seawater to the vent became more restricted. We also present calculations which suggest that the accretionary lapilli, which are often found in wet how deposits, may result from condensation of vapor in both the cold, wet collapsing fountains and in the flows themselves.
About 22,000 years ago a series of large-scale pyroclastic eruptions produced the Aira caldera (20 km×20 km wide at the northern end of Kagoshima Bay in southern Kyushu. It started with a Plinian pumice erution (Osumi pumice fall, 98 km3) followed by oxidized, fine-grained Tsumaya pyroclastic flow (13 km3), both erupted from a vent located at the present site of Sakuraijima volano, 8 km south of the caldera center. After a very short pause, violent explosive ejection of the basement rock fragments and pumiceous materials occurred at the central vent, gradually changing itself to a huge eruption column rapidly collapsing to form the Ito pyroclastic flow about 300 km3 in volume. The earliest phase produced up to 30-m-thick Kamewarizaka breccia developed along the caldera rim and charged with basement (lithic) fragments up to 2 m across. The breccia is a near-vent variety of the bottom concentration zone of lithics in the Ito deposit. Various textural features and monotonous petrologic character indicate that the main part of the Ito pyroclastic flow was emplaced by a simple, short-lived eruptive mechanism. The Aira-Tn ash, a fine-grained counterpart of the Ito pyroclastic flow, covered a wide area more than 1000 km from the vent. Evacuation of more than 110 km3 of rhyolitic magma produced a funnel-shaped collapse structure with the center of the magma chamber about 10 km deep. Like many other Japanese Quaternary calderas, the Aira caldera is considered to have formed not by a piston cylinder-type subsidence utilizing a ring fracture but by coring and high-angle slumping of the wall rocks into a funnel-shaped central vent. The outline of the caldera was strongly controlled by the faults bounding the volcano-tectonic graben forming Kagoshima Bay.
The Taupo volcanic zone (TVZ) has been derived since 2 Ma and has erupted greater than 10**4 km**3 of dominantly rhyolitic magma during the last 1 m. y. Most of the volcanism is concentrated in a 125 multiplied by 60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km**3 of magma. All centers except one are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Volcanism from the centers has been overwhelmingly rhyolitic ( greater than 97%; SiO//2 69-77 wt %) with minor high-A1 basalt and dacite and traces of andesite, mostly as lithic fragments. The current average rhyolite magma eruption rate from the central TVZ is approx 0. 27 m**3 s** minus **1. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant, and this is correlated with lower phenocryst equilibration temperatures in the eruptives.
The May 18, 1980, eruption of Mount St. Helens (MSH) produced an extensive ashfall deposit in Washington, Idaho, and Montana with a minimum volume of 0.55 km3 (tephra). An unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ENE of MSH near Ritzville, Washington. Grain size and component abundance analysis of samples along the main is very fine grained (mean size, 2 mum), poorly sorted, polymodal, and rich in glass shards and pumice fragments. A computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics, particularly the thickness versus distance relationship. The model was constrained by observations of the eruption column height, elevation of major ash transport, lateral spreading of the eruption plume, and atmospheric wind structure in the vicinity of MSH. Results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central Washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause. For the model to fit the observed characteristics of the deposit, significant particle aggregation of ash finer than 63 mum must be invoked. The best fit occurs when ash less than 63 mum is aggregated into particles several hundred microns in diameter with a settling velocity of 0.35 m/s. Support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at Pullman, Washington, during the May 18 eruption (Sorem, 1982). The premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics, variations in component abundances, and thickness versus distance relationship of the May 18 MSH ash fall deposit.
Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.
A spectrum of ignimbrite emplacement types exists, ranging from the “conventional” high-aspect ratio (H.A.R.I.) type, emplaced relatively quietly and passively in valleys, to the low-aspect ratio (L.A.R.I.) type, emplaced cataclysmically. Features of the L.A.R.I., such as a remarkable ability to scale mountains and cross open water and a strong fines-depletion of part of their deposits, stem from a high flow velocity which may result from an extremely high magma discharge rate. Being less rare than large-volume H.A.R.I. eruptions covering the same area, L.A.R.I. eruptions are a much more immediate volcanic hazard. Being thin and inconspicuous, a L.A.R.I. may easily be overlooked when determining the past record of a volcano.Another ignimbrite spectrum depends on variations in particle viscosity during emplacement and extends from the low-grade (water-cooled?) ignimbrite which is totally non-welded even if >50 m thick, to the high-grade (superheated?) one which is densely welded even if <50 m thick. Problems of air-cooling and water-cooling of ash flows need to be tackled, and it may be necessary to recognize strongly cooled ash flows which were emplaced in part at <100°C.One problem of ignimbrite eruptions is the origin of the extensive associated ash fall, comparable in volume to the ignimbrite. This ash may be: (a) pre-ignimbrite Plinian pumice; (b) co-ignimbrite ash, containing material lost from both the eruptive column and ash flow; (c) phreatoplinian, due to the entry of significant amounts of water into the vent; (d) phreatoplinian co-ignimbrite, due to explosions at rootless vents where ash flows enter water from land.Another problem is the origin of associated well-sorted and sometimes wavy-bedded deposits. These deposits may be from: (a) base surges, related to either the collapsing column or entry of water to the vent; (b) base surges, due to explosions at rootless vents where ash flows enter water from land; (c) fines depletion in, and deposition from, the strongly fluidized head of the ash flow; (d) standing waves in a high-velocity ash flow; (e) pyroclastic surges springing from the ash flow; (f) superficial turbulence in the topmost fractions of the ash flow as it comes to rest.Major problems concern the relationship between pyroclastic surges and flows, the ability of one to change into the other, and the distinction between their deposits. Thus, the May 18th 1980 “directed blast” of Mount St. Helens is widely regarded as a surge, yet produced deposits having many characteristics of a L.A.R.I.. Understanding the behaviour of the fine ash and dust fraction is thought to be critical to the solution of these problems.
Much of the volcanic ash deposited in the vicinity of Pullman, Washington, from the May 18th eruption of Mt. St. Helens in 1980 fell in the form of composite ash clusters, 0.25 to 0.5 mm in diameter. Particles in the clusters ranged from sub-micron to more than 40 microns in size. The porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward. The result is poorly sorted ash deposits as far as 644 km east (Missoula).Within individual clusters, textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation. The clusters are very fragile and may never be preserved in the stratigraphic record, but it is suggested that this kind of ash transport may well have occurred in the geologic past. Documentation of this ash-cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine-grained volcanic ash. The poor size-sorting in some ancient ash deposits may thus be explained.
The whole-deposit grain-size populations have been determined for two phreatoplinian ashes of the rhyolitic Taupo volcano (New Zealand). One is much finer than plinian pumice deposits studied from the same volcano, and the other differs in having a lower content of coarse pumice. The poor sorting of the two ashes is attributed mainly to water-scavenging of the ash cloud. The very limited fractionation with distance from source, combined with the very regular exponential thickness decrease, indicates that the scavenging water was an integral part of the ash cloud and was derived from Lake Taupo. At times when the proportion of erupted water to magma became particularly high, erosion of the underlying deposits took place, and to this is attributed the striking gullying of the surface which separates the two ashes.
The Kos Plateau Tuff consists of pyroclastic deposits from a major Quaternary explosive rhyolitic eruption, centred about 10 km south of the island of Kos in the eastern Aegean, Greece. Five main units are present, the first two (units A and B) were the product of a phreatoplinian eruption. The eruption style then changed to `dry' explosive style as the eruption intensity increased forming a sequence of ignimbrites and initiating caldera collapse. The final waning phase returned to phreatomagmatic eruptive conditions (unit F). The phreatomagmatic units are fine grained, poorly sorted, and dominated by blocky vitric ash, thickly ash-coated lapilli and accretionary lapilli. They are non-welded and were probably deposited at temperatures below 100°C. All existing exposures occur at distances between 10 km and 40 km from the inferred source. Unit A is a widespread (>42 km from source), thin (upwind on Kos) to very thick (downwind), internally laminated, dominantly ash bed with mantling, sheet-like form. Upwind unit A and the lower and middle part of downwind unit A are ash-rich (ash-rich facies) whereas the upper part of downwind unit A includes thin beds of well sorted fine pumice lapilli (pumice-rich facies). Unit A is interpreted to be a phreatoplinian fall deposit. Although locally the bedforms were influenced by wind, surface water and topography. The nature and position of the pumice-rich facies suggests that the eruption style alternated between `wet' phreatoplinian and `dry' plinian during the final stages of unit A deposition.
Taupo volcano is the southerly of two dormant caldera volcanoes in the rhyolite-dominated central portion of the Taupo Volcanic Zone in the North Island of New Zealand. Taupo has an average magma output rate of 0.2 m3 s-1 over the past 65 000 years, and is one of the most frequently active and productive rhyolite volcanoes known. The structure of the modern "inverse' volcano was formed largely by caldera collapse associated with the voluminous 22 600 14C years BP Oruanui eruption, and has been little modified since except for collapse following the 1850 14C years BP eruption. The products of 28 eruptions all of which post-date the Oruanui eruption, are defined and described here. The post-Oruanui activity at Taupo represents "noise' superimposed on the more uniform, longer term activity in the central Taupo Volcanic Zone, where large (greater than 100 km3) eruptions, such as the Oruanui, occur at more evenly spaced intervals of one per 40-60 000 years. -from Author
The ca. 30 km³ Taupo ignimbrite was erupted as a climax to the ca. AD 186 Taupo eruption in the central North Island of New Zealand. It was erupted as a single vent-generated flow unit over a time period of ca. 400s and was emplaced very rapidly (locally at more than 250-300ms⁻¹) and violently. The parent flow reached 80 ± 10 km from source in all directions, crossed all but one of the mountains within its range and only stopped when it ran out of material. The ignimbrite is divisible into layers 1 and 2, and a distant facies which combines features of both layers. Layer 1 was generated as a result of strong fluidization in the flow head, caused by air ingestion, and consists of two main facies. Layer 1(P) is a pumiceous, mildly to strongly fines-depleted unit, generated by the expulsion of material from the flow front, and termed the jetted deposits. The overlying layer 1 (H) is a thinner, crystal- and lithic-rich, fines-depleted unit, generated by the sedimentation of coarse/dense constituents segregated out by strong fluidization within the flow head and termed the ground layer. Layer 2 consists of two facies with similar compositions but contrasting morphologies; during emplacement, material left behind by the flow body partially drained into depressions to form the valley-ponded ignimbrite, leaving the veneer deposit as a thin, landscape mantling layer on interfluves. The distant facies occurs in some outermost hilly areas of the ignimbrite where the flow velocity remained high but its volume had shrunk through deposition so that air ingestion fluidization affected the whole flow. The ignimbrite shows great lateral variations. Each facies, or variants therein, exhibits systematic degrees of development with varying distances from vent. Near vent, the flow consisted of a series of batches of material which by ca. 25 km had coalesced into a single wavy flow and by ca. 40 km into a single wave. Out to ca. 13 km, the flow was rather dilute and highly turbulent as it deflated from the collapsing eruption column. Beyond this distance it was fairly concentrated, being less than 100% expanded over its non-fluidized compacted state, and had acquired a fluidization-induced stable density stratification, which strongly suppressed turbulence in the flow body. Deflation from the eruption column was largely complete by ca. 13 km but influenced the flow as far as 20-25 km from vent. Grainsize and compositional parameters measured in the ignimbrite show lateral variations which equal or exceed the entire spectrum of published ignimbrite data. The flow had deflated and coalesced from the eruption column by ca. 20 km from vent. Beyond this distance most lateral variations are modelled by considering the flow to be a giant fluidized bed. As the flow moved, material was deposited from its base, and hence predictable vertical variations in the model fluidized bed are comparable with lateral variations in the ignimbrite. The agreement is excellent, and, in particular, discontinuities in the nature of the ignimbrite at 55-60 km from vent suggest that the more distal ignimbrite represents a vast segregation layer generated above the moving flow. Differences between the model and variations of some parameters reflect the influence of kinetic processes, such as shearing and local fluidization, that operated regardless of the bulk flow composition. The strong fluidization in the flow is a result of the high flow velocities (promoting air ingestion), not vice versa as is often accepted. Contrasts in the natures of layers 1 and 2 imply that the first material erupted contained significantly coarser, and a higher content of, lithics than the bulk of the flow. During emplacement, this earlier material was depleted by deposition and diluted by material introduced from the flow body. Systematic regional variations also occur in the ignimbrite: for example, it contains lower crystal: lithic ratios and higher density pumice in a northeasterly sector, and vice versa to the southwest. Ignimbrite found in mountainous areas shows changes consistent with its derivation from the upper, more mobile and pumiceous top of the flow. Fluidization processes generated structures and facies in the ignimbrite on various scales. Individual segregation bodies found at any exposure show features mimicking those of the ground layer, i.e. fines depletion and crystal- and lithic-enrichment. Fluidization-induced grading visible at individual exposures accounts for the great range of grading styles seen in the valley-ponded ignimbrite, and strong fluidization has locally generated an upper fines- and pumice-rich segregation layer (here termed layer 2c). On the largest scale, fluidization was primarily responsible for the generation of the layer 1 deposits, and for the grainsize and compositional zonation within the flow that produced the lateral variations in the ignimbrite. Ingested and heated air is inferred to have been the most important gas source for fluidization within the flow, although several other gas sources were locally dominant. It is clear that the thickness, grainsize and composition of the ignimbrite at any point are not simply related to values of these parameters in either the originally erupted material or the parent flow, and that, except for its density, the dimensions and composition of the parent flow cannot be directly inferred from the ignimbrite.
The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 × 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass% lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40% is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in sigma phi with distance from source. Layer C shows no change in median grain diameter, but a decrease in sigma phi with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Mdphi against sigma phi diagram, on a sigma phi against alpha phi (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.
We propose that various rhyolitic pumice deposits in the Taupo Volcanic Zone and beyond, that were previously called Wairakei Breccia, Waitahanui Breccia (in part), Oruanui (Pumice) Formation, and Kawakawa Tephra Formation, are the products of one large eruption at 20 000 years B.P. On grounds of precedence we name these voluminous deposits Wairakei Formation and suggest that other names be made redundant. Correlations across New Zealand and into the Southwest Pacific basin are presented, together with data from a stratigraphic drillhole near Taupo. The large volume of water inferred to have taken part in magma-water explosive interaction came from proto-Lake Taupo. The correlation of these 20 000 year old deposits, previously thought to be different ages, has a bearing on the age of the Huka Falls Formation and the late Quaternary history of the Taupo district and the Waikato River.