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Understanding caldera structure and development: An overview of analogue models compared to natural calderas
Abstract
Understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation. Geological data, including that from analysis of deeply eroded examples, allow the overall surface setting of calderas to be defined, whereas deep drillings and geophysical investigations provide insights on their subsurface structure. Collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types (downsag, piston, funnel, piecemeal, trapdoor), being viewed as end-members. Despite its importance, such a classification does not adequately examine: (a) the structure of calderas (particularly the nature of the caldera's bounding faults); and (b) how this is achieved (including the genetic relationships among the five caldera types). Various sets of analogue models, specifically devoted to study caldera architecture and development, have been recently performed, under different conditions (apparatus, materials, scaling parameters, stress conditions).
... Two calderas have been identified on Visokoi Island, a larger basal one and a much smaller one on the island summit (Mount Hodson caldera). The absence of any obvious ellipticity of either caldera suggests a lack of a regional structural control (Acocella 2007), despite many of the South Sandwich island and seamount centres being aligned along structurally-controlled ridges (Leat et al. 2010(Leat et al. , 2014. For Visokoi Island, there is insufficient knowledge of the surface and subsurface structural features, and particularly the subsidence extent, to estimate the evolutionary stage (maturity) of either caldera (cf. ...
... For Visokoi Island, there is insufficient knowledge of the surface and subsurface structural features, and particularly the subsidence extent, to estimate the evolutionary stage (maturity) of either caldera (cf. Acocella 2007). The supposed bounding fault of the basal caldera exposed west of Mikhaylov Point (Fig. 11) has the geometry of an inwarddipping normal fault but it is a small exposure and lends little to our knowledge of the deeper structure of the caldera. ...
... The supposed bounding fault of the basal caldera exposed west of Mikhaylov Point (Fig. 11) has the geometry of an inwarddipping normal fault but it is a small exposure and lends little to our knowledge of the deeper structure of the caldera. Under-pressured caldera systems, which may be the most common (Martí et al. 1994;Acocella 2007), are characterized by outward-dipping reverse to vertical faults. However, the geometry is complicated by peripheral inward-dipping normal faults at the surface formed at a late stage (Martí et al. 1994;Roche et al. 2000;Acocella 2007). ...
Visokoi is a small volcanic island in the remote South Sandwich Islands and is unique in being dominated by the basaltic andesite products of highly explosive eruptions. Here, its geology is described in detail for the first time and can be used to characterize the construction of an active glacierized volcano in an intra-oceanic volcanic arc setting. More than 90% of the volcano is submarine and is composed of (1) a ~ 2.5 km-high mound formed of pillow lava and tuff breccia flanked by a low apron of mass flow deposits, together with (2) an overlying unit ~ 200 m thick composed of Surtseyan volcanic products representing a shoaling (and ultimately emergent) volcanic stage. The succeeding island commenced as a small volcanic shield composed of subaerial ‘a ‘ā lavas whose construction terminated in a caldera collapse that repressurized the magma chamber, presaging a major transition to highly explosive pyroclastic eruptions. They were mainly of sub-Plinian and Plinian type and their recognition on the island provides the first viable explanation for the presence of compositionally similar marine tephras sampled by drilling > 500 km from source, previously considered enigmatic. Eruptions probably took place under ice-poor conditions but evidence for quenching of juvenile clasts suggests that the magmas interacted with water high in the conduit sourced from melting of a small ice cap. The major period of high-discharge sub-Plinian and Plinian eruptions appears to have ended and any future events shall probably comprise small-volume eruptions forming Strombolian scoria cones or glaciovolcanic tuff cones.
... Similar Palaeogene rocks in this part of NW and western Scotland have been re-interpreted as volcaniclastic rocks related to caldera-forming and sector collapse processes [e.g. Bell and Emeleus 1988;Troll et al. 2000;Brown and Bell 2006;2007;Brown et al. 2009;Holohan et al. 2009;Gooday et al. 2018;Drake et al. 2022]. In this study, the first to give a detailed account of the caldera-fill, we interpret the Loch Bà felsites and agglomerates as ignimbrites, deposited during a calderaforming eruption, and breccias formed due to the collapse of caldera walls. ...
... The field relationships and structures strongly resemble piecemeal collapse observed at various calderas worldwide, including the type locality piecemeal calderas of Scafell and Glencoe [e.g. Branney and Kokelaar 1994;Lipman 1997;Moore and Kokelaar 1997;1998;Cole et al. 2005;Kokelaar and Moore 2006;Acocella 2007;Drake et al. 2022]. Following these events, the caldera then appears to have undergone further catastrophic collapse events related to oversteepening and collapse of caldera margins and fault scarps, to produce the mesobreccias and megabreccias of the Loch Bà Breccia Formation. ...
... From a review of existing field and modelling data on calderas, Acocella [2007] developed a four-stage model of caldera collapse. The four stages reflect progressively increasing subsidence and include: 1) downsag; 2) reverse ring fault subsidence; 3) peripheral downsag; and 4) peripheral normal ring fault subsidence. ...
Caldera-forming eruptions represent extremely hazardous events. The Loch Bà Caldera on the Isle of Mull, NW Scotland, preserves an ~120 m thick sequence of Palaeogene silicic pyroclastic rocks and collapse breccias. Here we present the first detailed account of the lithostratigraphy and architecture of the caldera-fill. A silicic explosive eruption generated pyroclastic density currents that deposited a range of rhyolitic ignimbrite lithofacies as the caldera collapsed. Abrupt changes in ignimbrite lithofacies and lateral thickness changes are attributed to volcano-tectonic faults and incremental collapse of the caldera. Five eruption phases have been recognised that record rapid switching between sustained high-fountaining and low-fountaining “boil-over” eruptions. The ignimbrites are unconformably overlain by mesobreccias and inward rotated megablocks of basalt lava country rock, which record catastrophic inward collapse of the caldera walls and margins. Our results provide new insights into caldera collapse and intra-caldera-fill that can be applied to other volcanoes worldwide.
... Calderas represent a catastrophic geologic event resulting from the sudden withdrawal of magma (Lipman, 1984;Acocella, 2006Acocella, , 2007Giordano et al., 2014aGiordano et al., , 2014b. They form collapse depressions, ellipsoid in shape, with diameters larger than that of craters and vents (Williams and McBirney, 1979;Cole et al., 2005;Acocella et al., 2015). ...
... Lipman, 1984;Branney, 1995). Some calderas are geometrically complex suggesting more than one formation process and multiple structural types (Cole et al., 1998;Lipman, 2000;Acocella, 2006Acocella, , 2007Geyer and Martí, 2014;Hasegawa et al., 2022). In some cases, multiple subsequent eruption episodes that gave rise to caldera formation are related to the evacuation of multiple magma reservoirs (e.g. ...
... The multiple collapse events suggested in this work are in congruence with the results of Giordano et al. (2014aGiordano et al. ( , 2014b, Fontijn et al. (2018), andHunt et al. (2019), who suggested more than one collapse event. The borehole data of the caldera floor sequence (Fig. 2C), indicate a coherent caldera floor, and together with steeply-dipping ring faults, this may suggest a piston style collapse for Gedemsa caldera (Lipman, 1997;Acocella, 2007). Vidal et al. (2022) provided a recent 40 Ar/ 39 Ar age data for upper ignimbrite caldera forming eruption as 251 ± 47 ka. ...
... Caldera collapse and associated volcanic eruptions represent a climactic stage in the evolution of magmatic systems after a prolonged growth of an underlying shallow magma chamber (e.g., Acocella, 2006Acocella, , 2007Black and Andrews, 2020;Branney, 1995;Cole et al., 2005;Giordano and Caricchi, 2022;Kennedy et al., 2018;Lipman, 1997). The chamber growth, commonly successive through multiple magma additions (e.g., Annen, 2009;Bachmann and Huber, 2016;de Saint Blanquat et al., 2011;de Silva and Gosnold, 2007;Glazner et al., 2004;Jellinek and DePaolo, 2003;Michaut and Jaupart, 2011;Miller et al., 2011;Paterson et al., 2011;Townsend et al., 2019), is inevitably coupled with oscillations in magma pressure, and thus caldera-forming eruptions are in some cases preceded by the pressure drop or build-up. ...
... Given the debate outlined above, a key tool for a better understanding of the driving mechanisms and dynamics of caldera collapse is a quantitative assessment, which may be provided by analogue and mathematical/mechanical models or a combination of both (e.g., Roche et al., 2000;Acocella, 2007;Geyer and Martí, 2014). Yet, the models have often yielded conflicting results. ...
... The number of unresolved issues stemming from various analogue and mathematical/mechanical modeling approaches to magma chamber roof fracturing and caldera collapse call for further research. In this paper, we focus on the mathematical models (the analogue modeling of caldera collapse was reviewed in great detail by Acocella (2007) and Martí et al. (2008)). To provide a basic theoretical background, we first review the existing mechanical models of caldera collapse and sort them out on the basis of the assumed rheology of the magma chamber and host rock. ...
Caldera collapse represents severe volcanic hazards for the environment, climate, and human society, but it can also be beneficial as it may contribute to the formation of ore deposits and produce fertile soils. A deeper understanding of mechanical conditions under which caldera collapse can occur is thus of great importance and interest and can be significantly advanced through mathematical modeling. Following a review of the state-of-the-art numerical modeling approaches, this contribution takes the advantage of the finite element method (FEM) to develop a general model predicting fracture development above inflating and deflating magma chambers. Dozen cases covering both underpressure and overpressure scenarios and a wide range of possible magma chamber geometries and roof aspect ratios R (roof thickness/chamber diameter), from shallow to deep-seated, mid-size and large, tabular and cylindrical, were calculated. Based on selected 11 representative cases, we demonstrate that pressure evolution inside a magma chamber is manifested by a range of fracturing processes in the host rock, including not only the growth of ring faults, but also propagation of radial and circumferential fractures, magmatic stoping, and cauldron subsidence. The modeling strategy also enabled us to describe the orientation (inward-dipping, vertical, outward-dipping), mode (shear or dilation), and direction (upwards, downwards) of a ring fault initiation and growth. The modeling shows that, regardless of magma chamber shape and caldera collapse scenario (over- or underpressure), the ring faults are reverse and always initiate at the chamber margin and propagate upwards, except for chambers with a low roof aspect ratio R < 0.25, with ring faults propagating both upwards and downwards. The ring fault orientation also changes with R, typically from moderate to steep. Faults formed above underpressurized chambers are dominantly outward-dipping or (sub)vertical, whereas those formed above overpressurized chambers are either inward-dipping or (sub)vertical. These changes in the ring fault geometry and orientation also imply a change in the dominant caldera collapse mechanism from downsag for low R through piston for moderate R to cauldron subsidence for high R, where the ring fault does not reach the surface but instead defines an arch-like roof block prone to sink into the chamber. Furthermore, our modeling approach also identifies highly fractured regions that develop within the chamber roof in some cases and potentially may represent traps for hydrothermal fluids and associated ore deposits. The presented study also confirms the FEM as an excellent tool for predicting the caldera collapse, especially when non-linear behavior and failure of host rock and nearly incompressible fluid behavior of magma are incorporated.
... Roche et al., 2000;Burchardt and Walter, 2009). Acocella (2007) has reviewed the subject. Most of the work carried out in this field seems to comply with the Anderson-type ring fault in the first stages of caldera collapse (Acocella, 2007). ...
... Acocella (2007) has reviewed the subject. Most of the work carried out in this field seems to comply with the Anderson-type ring fault in the first stages of caldera collapse (Acocella, 2007). In later collapse stages of caldera formation, the laboratory experiments show a second set of ring faults develop, with inward dip and normal sense of motion. ...
... The second major ring fault is concentric with the initial major ring fault but lies further outside, which increases the diameter of the caldera. The analogue experiments indicate that inward dipping major fault has steeper dip than the initial major ring fault at shallow depth, but the two join at greater depth (Acocella, 2007). Burchardt and Walter (2009) have shown with analogue experiments that the drop of the caldera floor in later stages of a caldera development is increasingly taken up with normal faulting on the outer lying ring fault. ...
A day and a half after the earthquake (=5.3, =5.6, =5.6) in the Bárðarbunga central volcano on Sept. 29th 1996, a volcanic eruption broke out under the Vatnajökull glacier. The eruption was located approximately 20 km SSE of the earthquake epicenter, midway between the Bárðarbunga and Grímsvötn central volcanoes. Course of events suggests a connection between earthquake and eruption and therefore a connection with a sequence of earthquakes of the same characteristics in Bárðarbunga during the years 1973–1996. The earthquakes in question are of an unusually low frequency character (corner frequency), explained by exceptionally low dynamic stress drop (\<10 bars) at shallow depth (≤5.0 km). The sequence which lasted for 22 years is characterised by ∼annual main events of magnitudes in the range of 4.5–5.7 (). It intensified in the 1990s, with some of the largest earthquakes of the whole episode occurring at that time. Moment tensor solutions of teleseismic signals and locally recorded waveforms reveal that the main events are thrust faulting earthquakes with a significant non-double couple component. Arguments are presented that the faulting occurred on a steeply inward dipping caldera fault, with reactivated motion on a weak fault. As a consequence of this hypothesis magma inflation in Bárðarbunga is the most probable cause of the 1973–1996 events. However, the loading force (the magma) may or may not have resided at a similar shallow depth as the earthquakes. Cast in the frame of the inflation model, the Bárðarbunga 1973–1996 sequence implies a resurgent caldera of at least 0.2–0.7 km³ for approximately a quarter of a century, exceeding its magma storage capacity in 1996. However, these calculations are model dependent. Bárðarbunga and neighbouring area were relatively calm during the period mid-1997 to 2004. There was a renewed activity of small earthquakes during the years 2005–2009. From the beginning of continuous seismic recording in Iceland in 1925, all eruptions in Vatnajökull on record have been accompanied with earthquake(s) of magnitude ≥4.0, within two months of the initial eruption.
... Une caldera se définit comme une dépression volcanique subcirculaire résultant d'un effondrement vertical délimité par des failles bordières concentriques (ring faults) (Figure 12.A) mises en place à la suite de la vidange de la chambre magmatique sous-jacente (Williams et McBirney, 1979;Lipman, 1997;Cole et al., 2005;Acocella, 2007). Dans ce modèle de formation où l'effondrement de la caldera est contrôlé par la vidange de la chambre magmatique, le diamètre et le taux de subsidence de celle-ci seront proportionnels à la quantité de matériel mobilisé durant une éruption. ...
... A B C 41 2008, 2013; Acocella, 2007;Maestrelli et al., 2021). C'est le cas notamment du volcan Krafla en Islande, ou de la caldera de Taupo en Nouvelle-Zélande, où la caldera s'allonge perpendiculairement à la ride et donc parallèlement à la zone d'accrétion. ...
... On remarque que l'élongation de la caldera du Piton des Neiges coïncide avec la zone d'influence et l'orientation de la rift zone N030°E (Figure 109.B et Figure 110). Ainsi, il semble que la mise en place de la caldera du Piton des Neiges soit contrôlée non pas uniquement par la plomberie 268 magmatique sous-jacente mais bien par un régime de contrainte comme cela est proposé sur d'autres édifices (e.g Krafla, Kilauea) (Holohan et al., 2005;Acocella, 2007), avec dans notre cas, la rift zone N030 comme principal acteur. ...
Le volcan Piton des Neiges (La Réunion) concentre de nombreux indices d’hydrothermalisme en surface, suggérant l’existence d’un potentiel géothermique exploitable. Les précédentes campagnes d’exploration, débutées dans les années 70, ont mis en évidence des gradients géothermiques prometteurs dans les forages d’exploration géothermiques du cirque de Salazie. Cependant, elles révèlent aussi la difficulté d’identifier les zones perméables au sein de ce volcan. Les failles de caldera pourraient présenter cette perméabilité nécessaire, or aucun consensus n’existe quant à l’existence, le nombre et l’extension des calderas du Piton des Neiges. La carte géologique actuelle ne suffit pas car elle est très peu contrainte dans la zone interne du volcan du fait de la difficulté d’accès et de ses reliefs escarpés. Ainsi, certains aspects clefs de son histoire géologique restent méconnus. Pour améliorer la compréhension de l’histoire du Piton des Neiges en vue d’une exploration géothermique, nous avons tout d’abord pallié au manque de données géochronologiques sur les formations plutoniques et explosives du volcan afin d’apporter des contraintes temporelles sur les sources de chaleur. Grâce à une étude de thermochronologie multitechnique (U-Pb sur zircon et (U-Th)/He sur apatite), nous mettons en évidence (1) un plutonisme polyphasé correspondant aux stades de reprise d’activité du volcanisme (~2 Ma, ~1,4 Ma, ~0,7 Ma et ~0,15 Ma) suivi de refroidissements rapides successifs ; (2) un épisode explosif majeur précisant l’âge de la principale caldera du volcan à 188 ± 5 Ka ; (3) une cessation de l’activité volcanique à ~27 Ka (et non 12, 22 ou 29 Ka comme proposé précédemment), ce qui précise l’âge de la dernière source de chaleur probable. Pour établir un modèle architectural du massif volcanique permettant de localiser les zones favorables à la géothermie, nous avons réalisé une nouvelle carte géologique des cirques à l’échelle 1/25000e dont la subdivision des unités s’appuie sur l’ensemble des connaissances géologiques existantes. Cette nouvelle carte est dressée à partir d’une reconnaissance exhaustive de terrain, y compris dans les zones les plus inaccessibles du massif, complétée par de la photogrammétrie aérienne. Nous identifions tout d’abord une caldera d’emprise plus restreinte que dans les propositions précédentes, de forme elliptique, mise en place à la fin du stade PN3, et ne correspondant pas aux bordures des cirques actuels. Nous mettons par ailleurs en évidence un contrôle majeur du démantèlement par les déstabilisations de flancs qui créent des discontinuités structurales et lithologiques dans le bâti de l’édifice, et ce dès les premiers stades de construction du Piton des Neiges. La présence de brèches dans la partie interne du volcan, issue de l’imbrication de plusieurs déstabilisations et de l’effondrement de la caldera, a en effet guidé l’érosion et conduit à la forme actuelle des cirques. Notre modèle architectural du Piton des Neiges apporte des contraintes nouvelles sur la caractérisation du système hydrothermal, expliquant la répartition des indices d’hydrothermalisme en surface. C’est notamment dans les formations bréchiques anciennes et à l’intersection des failles de caldera et de la rift zone et des sill-zones différenciées que se situent la majorité des sources thermales. Le système intrusif joue également un rôle de barrière hydraulique, qui entraîne une compartimentation du système hydrothermal. Les niveaux de brèches semblent, eux, constituer des réservoirs superficiels dans le système hydrothermal fonctionnant comme un réservoir fracturé.
... Calderas are subcircular volcanic depressions that occur in different tectonic (extensional, compressional, strike-slip or neutral) and magmatic (felsic to mafic) settings, both on Earth and other planets in the solar system (Cole et al., 2005;Acocella, 2007;Martí et al., 2008). Calderas result from the drainage of a subsurface magma reservoir. ...
... The magma withdrawal can either result from a large volcanic eruption (Druitt and Sparks, 1984;Cole et al., 2005) or subsurface lateral migration of magma (Geshi et al., 2002;Gudmundsson et al., 2016;Anderson et al., 2019;Fontaine et al., 2019). As magma withdraws from the chamber, the roof eventually experiences down-sag due to the loss of support, which may evolve to fault-controlled subsidence (Lipman, 1997;Roche et al., 2000;Acocella, 2007;Liu et al., 2019). ...
... Limit analysis is designed to address the conditions for initial failure, not to model the evolution of geological structures. Therefore, the models only calculate the damage distribution at the onset of caldera collapse, but does not provide information on the final structure as observed in the field (e.g., Cole et al., 2005;Kennedy et al., 2018) or in laboratory models (e.g., Acocella, 2007;Burchardt and Walter, 2010;Liu et al., 2019). It is possible to implement limit analysis to model the evolution of geological structures (Souloumiac et al., 2010;Cubas et al., 2013), but the implementation requires in-house advanced algorithms that expand beyond the use of OptumG2. ...
Calderas are subcircular volcanic depressions that can occur due to drainage of a subsurface magma reservoir. Numerous models simulating the initiation and growth of caldera collapse consider homogeneous overburden of the magma reservoir. This study describes plastic models implementing limit analysis to investigate the effects of weak layers (low cohesion and low friction) on caldera formation and structure. Our models show that the presence of weak layers within the crust favours the onset of caldera collapse, as it reduces the critical magma underpressure within the magma chamber to initiate roof failure. This effect is more pronounced with greater cumulated thickness of weak layers. In homogeneous models, the onset of caldera collapse is accommodated by a simple, localized outward dipping reverse damage band (caldera fault), whereas in layered models caldera collapse is accommodated by more complex damage structures. Weak layers confine damage underneath the layers, limiting the growth of the caldera fault toward the surface. Calculated stress trajectories are rotated across weak layers, showing that weak layers act as stress barriers. The effect of weak layers is stronger when the layer is closer to the magma reservoir, where layer-parallel damage form underneath the layer, interpreted as a potential detachment level. Multiple layers trigger more distribution of the damage and several layer-parallel damage bands. The subsurface distribution of damage due to weak layers may lead to more distributed surface subsidence, enhancing sagging before a caldera fault reaches the surface. Finally, internal detachments due to weak layers are likely important for observed episodic transient subsurface collapse episodes before collapse occurs at surface. All in all, our models that implement plastic deformation predict significant stress perturbations as a result of varying Mohr–Coulomb properties only. Our study thus shows that widely used static elastic models are not sufficient for physically relevant stress analyses of geological systems. In addition, our study shows that plastic (or elasto-plastic) models are necessary to predict the location and extent of inelastic damage accommodating volcano deformation and failure.
... the caldera, seismicity is concentrated on the northern rim, supporting the inference from geodetic observations that the collapse was highly asymmetric. Ágústsdóttir et al. (2019) report normal faulting during the eruption on steeply inward-dipping faults, striking sub-parallel to the caldera rim, interpreted as representing a combination of piecemeal and trapdoor style collapse mechanisms (Acocella, 2007;Amelung et al., 2000). ...
... They all indicate downwards motion on the inner side of the caldera ring fault. In cross-section, rear hemisphere projection focal mechanisms demonstrate this clearly, with one nodal plane consistently dipping steeply to the south, into the caldera ( Figure 1); this nodal plane is interpreted as the fault plane, fitting best with models of caldera collapse (Acocella, 2007). Section 4 provides further discussion on faulting and geometry. ...
... Previous descriptions of the caldera collapse at Bárðarbunga (Ágústsdóttir et al., 2019;Browning & Gudmundsson, 2015;Gudmundsson et al., 2016;Riel et al., 2015), similar caldera collapses and fault structures elsewhere (e.g., the 2018 Kīlauea eruption (Anderson et al., 2019); or the 2015 Axial Seamount eruption (Levy et al., 2018;Wilcock et al., 2016)), field studies, and analog and numerical models (Acocella, 2007;Branney, 1995;Geyer & Martí, 2014;Holohan et al., 2011) highlight the complex mix of collapse style endmembers often observed at individual volcanoes, and emphasize the dominant role of ring faults in facilitating deformation at mature calderas (Liu et al., 2019;Ruch et al., 2012). ...
Plain Language Summary
Bárðarbunga is a highly active volcano that lies beneath Iceland's Vatnajökull glacier. It last erupted for 6 months between August 2014 and February 2015—the largest eruption in Europe for 250 years—with tens of thousands of earthquakes. These earthquakes occurred in response to molten rock moving within the volcano. We analyzed the earthquakes to map active faults along the large bowl‐shaped depression that represents the volcano's caldera, and investigate the fault movements where they occur. We observed faulting on almost vertical faults around the edge of the volcano throughout 2014–2018, but in opposite directions during and after the eruption. During the eruption, the caldera floor moved downwards relative to the surrounding flanks of the volcano. After the eruption ended, the motion reversed and the caldera floor began to move back upwards. The downwards movement during 2014–2015 occurred as the molten rock—stored at a depth of about 6 km beneath the surface—moved laterally to feed the eruption over 40 km away from the volcano. We suggest that the ongoing upwards movement since the eruption ended is due to new molten rock accumulating beneath the volcano.
... The presented results are in many respects consistent with earlier analog (see review by Acocella, 2007) and numerical (Holohan et al., 2011) modeling studies. The vertical movement of the roof is primarily accommodated: (i) on inward-dipping faults when T /D is low, (ii) on sub-vertical faults when T /D is around one and (iii) on outward-dipping faults when T /D is high. ...
... The curved left model fault is a steeply inward-dipping normal fault at depth and becomes an outwarddipping reverse fault as it approaches the surface. The 'space problem' (e.g., a gap; see Lipman, 1984) that would arise due to slip on this curved outward-dipping reverse fault is solved by oppositely, i.e. inward, dipping normal faults, a kinematic relationship consistent with analog modeling (Acocella, 2007; see also Mandl, 1988). Perhaps a similar fault geometry is present along the western margin of Rabaul caldera, where hypocenter locations in section B-B' suggest a steeply outward-dipping fault, but in section A-A' an inward-dipping fault, which appears to be moderately-dipping near the surface. ...
•Distinct Element Method models of restless calderas show progressive roof weakening.
•Increasing inflation/deflation cycle numbers lead to strain localization on faults.
•Thickness to diameter roof ratio key factor in determining fault geometry.
•Roof weakening leads to decreasing ‘pre-eruption’ magma overpressure.
•Surface deformation vs. magma pressure changes define hysteresis loops.
... One mechanism that could contribute to basaltic caldera formation is caldera ring-fault reactivation and progressive deepening of the caldera floor over hundreds to thousands of years, in response to numerous dike intrusions and reservoir deflation events (Hamling et al., 2019;Shreve et al., 2019). One end-member of this caldera collapse mechanism is trapdoor faulting, when ring-faults slip asymmetrically in response to magma reservoir drainage (Acocella, 2007;Agústsdóttir et al., 2019;Galetto et al., 2019;Trasatti et al., 2019;Sandanbata et al., 2022). Activation of a trapdoor fault has been best observed in detail at Sierra Negra caldera in the Galápagos using seismicity and ground displacement measurements (Amelung et al., 2000;Bell, LaFemina et al., 2021;Chadwick Jr. et al., 2006;Jónsson, 2009;Jónsson et al., 2005). ...
... The figures show the meter-scale subsidence of the caldera floor (net 2 m when considering the entire pre-and co-eruptive cycle), while the insets show the localized net uplift on the trapdoor fault (Bell, LaFemina et al., 2021). (d) Conceptual model adapted from Acocella (2007). The dip of both the outer and inner faults are not well constrained, yet co-eruptive normal slip along the pre-existing inner trapdoor fault structure indicates that the 2018 eruption may be the initial stage of caldera collapse at Sierra Negra. ...
The 2018 Sierra Negra eruption resulted in meter‐scale subsidence due to basaltic magma extraction from a deflating reservoir. The eruption was also characterized by dike intrusions, >4 MW earthquakes, and sulfur dioxide emissions. We use a combination of Interferometric Synthetic Aperture Radar, Digital Elevation Model, Global Positioning System and seismic data to assess conditions required to trigger episodic caldera collapse at Sierra Negra. The 2018 effusive eruption was mainly sourced from a horizontal sill located at ∼2 km depth, with a minimum erupted bulk volume of 0.19 km³. The modeled reservoir is bound by a C‐shaped ring of seismicity, suggesting trapdoor fault slip. Two >4.5 MW earthquakes (5 and 22 July 2018) produced localized subsidence north of the southern trapdoor fault. After removing the modeled subsidence signal, distributed normal trapdoor fault slip explains the location of residual displacement. Furthermore, distributed fault models indicate slip occurred along the northern, central and southern segments of the trapdoor fault during the entire eruption, until 25 August 2018. Theoretical models of caldera collapse estimate that an erupted volume of 0.19 km³ is too small to trigger full‐scale caldera collapse, given the caldera aspect ratio (depth/diameter) of 0.22. Nonetheless, the spatial distribution and duration of seismicity and slip suggest trapdoor fault activation is the initial stage of caldera collapse at Sierra Negra, which may lead to full‐scale caldera collapse during larger eruptions. Alternatively, hundreds of medium‐sized eruptions, similar to that of 2018, may have triggered fault slip events over the past millennia.
... Analog modeling of these processes shows the effect of pre-existing regional normal faults on collapse geometries (Acocella, 2007;Bonini et al., 2021), causing the partial reactivation of previous structures. In particular, the interpretation of some analog models raises the possibility that the Los Humeros scarp, and part of the southwestern corridor (SWC), reactivated inherited subsurface faults . ...
... This propagation occurred from the center of the reservoir towards its periphery resulting in an outward incremental caldera growth. During this process, outward-dipping reverse ring faults are developed, which are followed by peripheral inward-dipping normal faults (Kennedy et al., 2004;Geyer et al., 2006;Acocella, 2007), which are usually the exposed parts of the main scarps, such it seems to occur in the case of Los Humeros. ...
Caldera volcanoes are complex geological systems that show, during and after their formation, a wide variability in terms of eruptive styles, magmatic and geochemical evolution, and volcanic structures. Los Humeros Volcanic Complex (LHVC) is a key area where to study these factors. It is located in the easternmost sector of the Trans-Mexican Volcanic Belt and hosts an active geothermal system. LHVC exemplifies the complex nature of calderas, recording periods of alternated effusive and explosive eruptive phases, a heterogenous magmatic source, diverse basaltic to rhyolitic products, and the overlapping of caldera collapse events. This study provides an up-to-date interpretation of the caldera framework. Our work is based on a detailed analysis of literature and novel data aimed at the morpho-structural caldera configuration, and supported by geophysical modeling, and petrology of the volcanic products. The new results confirm a more complex evolution of LHVC involving geometric configurations related to multiple caldera collapse events producing both asymmetric trapdoor, and piecemeal styles. The present configuration of the caldera framework involved the initial formation (at 164 ka) of a large caldera (Los Humeros) of ca. 15–17 km diam (shorter than previously reported), which was overlapped asymmetrically by the younger (69 ka) and smaller (ca. 8.5–10 km diam) Los Potreros caldera over the west side, as a result of multiple sequential collapsing events. The final configuration of the caldera rims (scarps) was promoted by incremental growth. A post-caldera resurgence phase was induced by the injection of multiple shallow intrusions of silicic bodies, causing a more localized deformation of the caldera floor. The Holocene volcanism records the recurrent injection of compositionally distinct magma batches uprising from different depths of the LHVC transcrustal magmatic plumbing system. This latter indicates the transition from the caldera stage magmatic system dominated by a single, large, and shallow magmatic body to a more complex, polybaric post-caldera stage plumbing system, made up of a lower crust mafic reservoir feeding smaller magma batches vertically distributed in the whole crust. The integration of all the existing data constitutes the archive to better understand how large active calderas hosting exploitable geothermal systems assemble through time.
... Analog modeling of these processes shows the effect of pre-existing regional normal faults on collapse geometries (Acocella, 2007;Bonini et al., 2021), causing the partial reactivation of previous structures. In particular, the interpretation of some analog models raises the possibility that the Los Humeros scarp, and part of the southwestern corridor (SWC), reactivated inherited subsurface faults . ...
... This propagation occurred from the center of the reservoir towards its periphery resulting in an outward incremental caldera growth. During this process, outward-dipping reverse ring faults are developed, which are followed by peripheral inward-dipping normal faults (Kennedy et al., 2004;Geyer et al., 2006;Acocella, 2007), which are usually the exposed parts of the main scarps, such it seems to occur in the case of Los Humeros. ...
Caldera volcanoes are complex geological systems involving during and after their formation a wide number of factors, such as: eruptive styles, magmatic processes, geochemical variability, and intricate volcanic structures. Los Humeros Volcanic Complex (LHVC) is a key area where to study these factors. It is located in the easternmost sector of the Trans-Mexican Volcanic Belt and, hosts an active geothermal system. LHVC exemplifies the complex nature of calderas, recording periods of alternated effusive and explosive eruptive phases, a heterogenous magmatic source, diverse basaltic to rhyolitic products, and the overlapping of caldera collapse events. This study provides an up-to-dated interpretation of the caldera framework. Our work is based on a detailed analysis of literature and novel data aimed at the morpho-structural caldera configuration, geophysical modeling, and petrology of the volcanic products. The results confirm an evolution of LHVC involving geometric configurations related to multiple caldera collapse events producing both asymmetric trapdoor, and piecemeal styles for both the older and larger (164 ka; ca. 17 km max. diam) Los Humeros caldera, and the younger and smaller (69 ka; ca. 10 km max. diam) nested Los Potreros caldera. The final configuration of the caldera rims (scarps) was promoted by an incremental growth process. A post-caldera resurgence phase was induced by the injection of multiple shallow intrusions of silicic bodies, causing a more localized deformation of the caldera floor. The following Holocene volcanism records the recurrent injection of compositionally distinct magma batches uprising from different depths of a transcrustal magmatic plumbing system. This latter indicates the transition from the caldera stage magmatic system dominated by a single, large, and shallow magmatic body to a more complex, polybaric post-caldera stage plumbing system, made up of a lower crust mafic reservoir feeding smaller magma batches vertically distributed in the whole crust. The integration of all the existing data constitutes the archive to better understand how large active calderas hosting exploitable geothermal systems assemble through time.
... Furthermore, faults may form along the volcano flanks due to slope instability induced by the processes above (McGuire, 2006). A peculiar type of fault is that develops to accommodate gravitational collapses triggered by processes such as magma migration or magma chamber withdrawal (Acocella, 2007;Branney, 1995;Branney and Acocella, 2015;Walker, 1984;Burchardt and Walter, 2010;Kennedy et al., 2004). Importantly for this article, faults accommodating gravitational collapses are often steeply dipping and can be comprised of normal and reverse fault strands acting simultaneously. ...
... As discussed in the previous paragraph, the results of this study indicate that the studied fault array displays "geometric coherence", comprising steeply-dipping normal and reverse faults that acted together to downthrow the bedding southward. As we have seen in the introduction to this article, the simultaneous occurrence of steeplydipping normal and reverse faults have been described, in volcanic areas such as that of this work, in association with gravitational collapses (Acocella, 2007;Branney, 1995;Branney and Acocella, 2015;Walker, 1984;Burchardt and Walter, 2010;Kennedy et al., 2004). These features are similar to what observed for fault arrays accommodating gravitational collapses in other, non-volcanic settings such as, for instance, those developed due to ground subsidence (e.g., sinkholes, Poppe et al., 2015), mining subsidence, or permafrost-type collapsed due to ice melting (Branney, 1995). ...
Faults in volcanic areas can have different origins and can form due to several volcano-tectonic processes. In this work, by means of detailed field observations, we analysed a fault array cutting through a recent pyroclastic succession in the Campi Flegrei caldera of southern Italy to investigate its origin. Geometric and kinematic data indicate that the fault array consists of steeply dipping faults with both normal and reverse senses of movement. Displacement data suggest that
these faults with opposed kinematics acted simultaneously, in agreement with what was previously described from analogue and numerical experiments for
faults formed in response to gravitational collapses in volcanic areas. By integrating these results with considerations of the recent volcano-tectonic evolution of the Campi Flegrei caldera, we propose a model in which the studied fault array developed to accommodate a gravitational collapse triggered by an underground magma migration predating the 1538 CE historical Monte Nuovo eruption. This work is, to our knowledge, the first thorough field description of faults developed to accommodate gravitational collapses in volcanic areas
... Caldera collapse and associated volcanic eruptions represent a climactic stage in the evolution of magmatic systems after a period of shallow magma chamber growth (e.g., [1,[12][13][14][15]). ...
... Researchers therefore investigate the direction of ring faults propagation (either starting from the surface and extending downwards until reaching the magma chamber, or starting from the magma chamber and growing upwards to the surface) and their orientation (outward-dipping, vertical, or inward-dipping, as described in Figure 1.2). It is worth mentioning that all the previously mentioned possibilities of the propagation direction and dip orientation exist and have been proven by field and geophysical evidences [15,95]. Possible approaches of the ring faults investigation follows. ...
Calderas are volcanic depressions caused by rupturing of a magma chamber roof as a consequence of pressure evolution inside the chamber. The collapse of a caldera is accommodated by ring faults, whose formation is commonly accompanied by ejection of large volumes of pyroclastic material to the Earth’s atmosphere and thus represents severe volcanic hazards for the environment, climate, and human society. However, it can also be beneficial as it may contribute to the formation of ore deposits, serve as geothermal energy resources, and produce fertile soils. A deeper understanding of geological conditions under which these events can occur is thus of great importance and interest. Geological survey and field studies can usually access just surface manifestation of immense magmatic processes taking place under the Earth’s surface. In this respect, numerical analysis has proven as a key tool in understanding the mechanical conditions of caldera collapse.
In order to advance the knowledge and bring new views on fracturing processes preceding caldera collapse through mathematical modeling, numerical simulations, and analysis, extensive research was carried out. Based on a state-of-the-art review, outstanding issues, which are still a subject of debate in the research community, were identified and provided the objectives for the thesis.
To achieve the goals, the finite element method (FEM) was employed in this work due to its capabilities and universality. To make the FE simulations realistic, yet feasible, appropriate modeling strategy, material models and parameters, and sim- plifying assumptions were selected.
Subsequently, many cases covering a wide range of possible magma chamber ge- ometries and roof aspect ratios (roof thickness/chamber diameter)—from shallow to deep-seated, mid-size and large, tabular and cylindrical—were calculated. Nev- ertheless, 12 cases, which were selected as the most representative, were in detail analyzed, interpreted, and discussed in terms of geological phenomena in the pre- sented thesis.
The modeling strategy employed in this work demonstrates that pressure evolu- tion inside a magma chamber is manifested by a range of fracturing processes in the host rock. These processes are not restricted to the formation of various ring faults alone but may also include radial and circumferential fracturing, surface tearing, magmatic stoping, and cauldron subsidence. The modeling strategy also enables capturing, orientation (inward-dipping, vertical, outward-dipping), mode (shear or extension), and direction (upwards, downwards) of a ring fault initiation and growth. Moreover, a dependence of the fracturing processes on the roof aspect ratio is identified and analyzed.
... In the context of global volcanic studies, the researcher Tilling et al. (1987a &b) provided a comprehensive overview of ring faults and their significance in volcanic structures. The mechanics and the role of these ring faults in controlling the eruptive behavior of volcanoes are well explained by Acocella et al. (2015Acocella et al. ( , 2007Acocella et al. ( , 2004Acocella et al. ( , 2003Acocella et al. ( & 2000. Importantly, understanding the interactions between magmatic processes and the structural framework provided by ring faults, shedding the light on dynamic nature of volcanic systems (Acocella et al., 2015). ...
Barren island volcano (BIV), located in the Andaman Sea, stands as a unique geological phenomenon characterized by its dynamic volcanic activity and complex morphtectonic features. In the current study, we employed advanced remote sensing techniques and detailed field surveys to elucidate the structural complexities and eruptive history of BIV. Analysis of DEM and bathymetric surveys reveals the presence of two distinct caldera formations-an older, outer caldera and a more recent, inner caldera-shaped by significant eruptive events and rapid subsidence along ring faults. The central polygenetic cone, marked by strombolian eruptions, emerges as the focal point of recent volcanic activity, surrounded by secondary cones and spatter cones indicative of ongoing magmatic processes and shallow magma chambers. Structural analysis further highlights the dynamic nature lava flow pathways, particularly the breaching of caldera walls towards the West and Northwest, influenced by pyroclastic loading and tectonic stresses from nearby faults. Integrating the satellite data and bathymetric data along with field expedition, our study maps both subaerial and submarine features of BIV, revealing submerged parasitic cones and the ruggedness of its volcanic flanks. Overall, our findings underscore the intricate interplay between magmatic processes, tectonic settings, and eruptive history in shaping the evolution of BIV. Continued monitoring and advanced remote sensing applications are imperative for predicting future volcanic behavior and mitigating potential hazards associated with BIV.
... Analog models for caldera collapse indicate that the subsidence is driven by outward-dipping ring faults (reverse faults) that propagate upward to reach the surface (e.g. Roche et al., 2000;Acocella, 2007). However, the shallowest earthquakes we relocated are 20 km beneath the seafloor and only ten of the focal mechanisms are reverse. ...
The basaltic submarine eruption offshore the island of Mayotte between July 2018 and January 2021 is one of the largest documented underwater eruptions. One of the most striking differences between this eruption and most documented eruptions is the exceptional depth of the associated seismicity, which is limited almost exclusively to the lithospheric mantle. This seismicity probably outlines magma reservoirs and dyking zones.
In order to better understand the deep processes driving the eruption, we analyze precise earthquake locations and focal mechanisms associated with this event. We present a set of 2677 accurate earthquake relocations and 300 focal mechanisms determined from data collected over the first 9 months of ocean bottom seismometer deployments, starting in February 2019.
Our relocations refine the structure of two swarms (Proximal and Distal with respect to Mayotte), and reveal well-defined mantle structures between 20 and 55 km below sea level, which we interpret as a ring-fault zone and a dyke, respectively. The Proximal swarm outlines a ring-fault zone as the locus of a large piston collapse caused by the deflation of an underlying magma reservoir. Deformation around the piston is driven by normal faulting on a set of inward dipping patches surrounding the piston. Locally, collapse of the conical shaped piston causes a radial extensional stress field with strike-slip and normal faulting ruptures accommodating the relax- ation of the damaged zone around the piston.
This piston collapse allowed the transfer of lava to the eruption site via the dyke highlighted by the Distal earthquake swarm. The link between the swarms is thus magmatic, in agreement with petrological analyses of lava from the new volcano.
This is the first time that piston collapse and localized dyking have been documented in the mantle. The pattern of deformation documented here could apply to shallower, crustal piston collapses, such as in Iceland.
... Calderas are an ubiquitous volcanic feature of both subaerial and submarine volcanoes. At subaerial volcanoes, calderas form when a large amount of magma is evacuated rapidly and the magma chamber roof collapses (Acocella, 2007;Geyer et al., 2006). At submarine volcanoes these collapse features are usually filled (partially or entirely) by both volcanic deposits such as tephras or ignimbrite, and marine sediments mostly composed of remains of marine organisms (e.g., calcareous deposits or reef debris). ...
I investigate the detectability of magma reservoirs in the vertical gravity gradient (VGG) anomalies calculated from satellite altimetry data. First, I calculate a suite of synthetic seamount models to show the expected VGG anomaly characteristic wavelength and amplitude for a simplified magmatic system, hydrothermal system, and a caldera infill, varying their dimensions for a given depth and density contrast. I find that most magmatic and hydrothermal systems create VGG anomalies with a characteristic wavelength and amplitude greater than the data uncertainty and are therefore detectable. The proposed approach consists in three main steps: (a) calculate the VGG from the two components of the deflection of the vertical, (b) calculate and remove the gravity contribution of the bathymetry interface using an independent bathymetry data set (e.g., acquired by multibeam echosounders) to obtain a VGG Bouguer gravity anomaly, (c) invert the Bouguer VGG anomaly to obtain a 3D density model. I image a 6‐by‐8‐km low density body between 3 and 9 km depth under Brothers volcano in the Kermadec arc. I hypothesize that it represents the main magmatic system, possibly with a minor fraction of hydrothermal fluids at the shallower depths. There are about 225 submarine volcanoes globally that could be studied with satellite altimetry‐derived gravity data to potentially image their magmatic system. Future altimetry data will increase the gravity data resolution and allow us to image smaller features. This is thus an invaluable data set for the study of underexplored submarine volcanoes and can help improve our volcano hazards assessment.
... This reasoning could explain why Monte Nuovo was the site of the last volcanic eruption (A.D. 1538 39,40 ), while Solfatara the site of the last phreato-magmatiic eruption (A.D. 1198 41,42 ). If this explanation was correct, and not considering any intermediate magma reservoirs, one could conclude that it is more likely to have the opening of new vents where the seismicity is deeper, while phreato-magmatic eruptions are to be expected in the north-eastern sector of the caldera, where the most shallow seismicity is observed. ...
The progressive increase of ground deformation, seismicity, and gas emission is marking a remarkable unrest at Campi Flegrei caldera. The direct involvement of magma has been invoked to explain the deformation and space/time changes of velocity anomalies at shallow crustal depths. A challenging aspect is to forecast possible scenarios for the upward migration of magmatic fluids from the source at depth. Here, we show that the most recent seismicity (period 2023–2024), derived by a machine-learning-based earthquake detection procedure, aligns on a continuous set of caldera rim faults and on top of an inflating magma source. Direct channeling of magma through such ring faults can be a way to feed future eruptions, as observed in other calderas and inferred for the Mt. Nuovo historical eruption.
... Reservoir magma can erupt through vertical conduits or ring faults, which is common in silicic eruptions (Branney & Acocella, 2015), or through lateral conduits or dikes, which occurred in most observed mafic eruptions (Gudmundsson et al., 2016;Michon et al., 2011;Stix & Kobayashi, 2008) and one observed silicic eruption (Hildreth & Fierstein, 2012) (Figure 1). Fault development during collapse can be complicated, but often ring faults eventually form that accommodate downward motion of a central block (Acocella, 2007). Variable ring fault geometries have been inferred, ranging from a single cylindrical fault to nested faults with various dips, and with slip distributions ranging from uniform (e.g., piston motion) to highly asymmetric (e.g., trapdoor motion) (Cole et al., 2005). ...
In multiple observed caldera‐forming eruptions, the rock overlying a draining magma reservoir dropped downward along ring faults in sequences of discrete collapse earthquakes. These sequences are analogous to tectonic earthquake cycles and provide opportunities to examine fault mechanics and collapse eruption dynamics over multiple events. Collapse earthquake cycles have been studied with zero‐dimensional slider‐block models, but these do not account for the complicated interplay between fluid and elastic dynamics or for factors such as the heterogeneous fault properties and non‐vertical ring fault geometries often inferred at volcanoes. We present two‐dimensional axisymmetric mafic piston‐like collapse earthquake cycle models that include rate‐and‐state friction, fully‐dynamic elasticity, and compressible viscous fluid magma flow. We demonstrate that collapse earthquake intervals and magnitudes are highly sensitive to inertial effects, evolving stress fields, fault geometry, and depth‐varying fault friction. Given the consistent earthquake cycles observed in most eruptions, this suggests that ring faults can quickly stabilize and often become nearly vertical at depth. We use the well‐monitored 2018 collapse sequence at Kı̄lauea as a case study. Our model can produce many features of Kı̄lauea seismic and geodetic observations, except for a significant amount of interseismic slip, which cannot be readily explained with simple rate‐and‐state friction parameterizations.
... Calderas are volcano-tectonic collapse structures between ca. 2 and 100 km in diameter resulting from during paroxysmal explosions and effusive of highlevel magma chambers evacuation (Smith and Bailey, 1968;Bailey et al., 1976, Tilling andDvorak, 1993;Lipman, 2000;Acocella, 2007), representing one of the most enigmatic geological structures recognized on Earth and other terrestrial planets (Francis, 2003;Lipman, 2000). Caldera-forming eruptions represent the culmination of a long-lived geological process involving the generation of magma at depth, its ascent and differentiation, and finally its eruption on the Earth surface producing the most voluminous (up to 5,000 km 3 ) explosive eruptions on Earth (e.g., Lipman, 2000;Francis and Oppenheimer, 2003). ...
The caldera-forming eruption of Mt. Masurai produced a 7 km-wide caldera, not listed in world caldera list because of lack or no previous research about Masurai eruptions history. The 33,712 yr calBP based on age determination using 14C dating acquired from charcoal within ignimbrite pyroclastic density currents (PDC's) deposit related to the first stage of caldera formation produced 15-20 km 3 DRE total volume of PDCs deposited in the northeastern-southwestern part of Masurai. Understanding early stage of caldera formation is necessary through measuring lithostratigraphic unit sections of northeastern-western area that divided into two main lithostratigraphic unit, (1) Ngeruyung Unit; consist of Jura and Neogene basement layers, andesitic lava flow, and 38 m thick of PDCs product with no or not observed evidence base surge deposit, (2) Sengaje Unit; complete PDCs deposit sequence with considerable thickness up to 100 m consist of pumice ignimbrite and ash fall layers at the bottom, base surge with scouring-convolute structure, pyroclastic flow with welded pumice ignimbrite layer observed in the middle part and locally columnar joint structure, and pyroclastic air fall deposit. The second large eruption in Masurai history based on 14 C numerical dating of charcoal found within PDC's deposit in the southern part of Masurai occured 21,420 yr calBP. The latter eruption produced PDCs deposit limited to the south-southeastern region of Masurai with 50-70 m complete lithostratigraphic unit in Renah Pelaan area described as Sei Hitam unit consist of (a) Jurasic basement layer, (b) PDCs sequence deposit with base surge layer, pyroclastic flow with welded pumice ignimbrite part in the middle, (c) PDCs deposit pumice ignimbrite with black pumice fragment. The recent eruption data of Late Pleistocene ultraplinian Mt. Masurai, hopefully, can provide latest information and data about caldera in the Sumatra.
... An alternative model for thrust earthquakes could involve the failure of outward dipping faulting on top of a depleting shallow reservoir, similar as observed during the first stage of calderas formation (Acocella, 2007;Levy et al., 2018). There are a few observations which make such a scenario less likely. ...
Plain Language Summary
The largest plate boundary systems on Earth are Mid‐ocean ridges (MOR), where the plates continuously drift apart and new lithosphere is constantly being formed. Although the process is well understood, we rarely detect spreading events at MOR, mainly because these regions are remote and local monitoring is rarely possible. In September–November 2022 a large, unusual seismic swarm occurred along a spreading center ridge segment of the North Mid‐Atlantic Ridge. Despite the remoteness of the region, we managed to model regional and teleseismic data to perform earthquake relocation, depth estimation and moment tensor inversion. In this way, we could reconstruct the geometry and the evolution of the seismicity. We found that in the early days of the swarm, seismicity migrated unilaterally over ∼60 km along the ridge axis, from North to South, triggering normal faulting earthquakes, which are typical at MOR. Later, large thrust mechanisms, anomalous in an extensional environment, appeared and quickly became predominant. We explain seismological observations by a magmatic intrusion, which first propagated southward, producing shallow normal faulting earthquakes above the vertical magma dike, and later thickened, increasing compressional stresses on its sides, and triggering large thrust earthquakes.
... Similar parameters apply to the shape of a collapse caldera, which results from the regional/local tectonic stress field and usually traces the underlying magma plumbing architecture and pre-existing host-rock structures (Lipman 1997;Girard and van Wyk de Vries 2005;Acocella 2007; Kennedy et al. 2017). A circular caldera develops in regional stress-free areas or where the rate of caldera formation is too fast to allow interaction with the regional stress field (de Saint Blanquat et al. 2011). ...
The evolution of eruptive vents related to calderas is not fully understood. We focus on a structural, rock-magnetic, and geochemical investigation of a ∼314 Ma rhyolite dyke swarm associated with the late-orogenic Altenberg–Teplice Caldera, Bohemian Massif, eastern Variscan belt. The whole-rock major element, trace element, and Nd–Pb isotope geochemistry along with the published U-Pb zircon geochronology link the extra-caldera dyke swarm with intra-caldera ignimbrites. The magnetic fabrics determined using the anisotropy of magnetic susceptibility are interpreted to record a continuum from magma ascent, emplacement, and eruption during sinistral shearing. The latter evidences an interplay with regional tectonics associated with the activity of crustal-scale shear zones. The sinistral kinematics and strike of the dyke swarm, the elongation of caldera intrusive units, and the kinematics of major caldera faults are consistent with the dextral Riedel shear system, where the dykes correspond to antithetic Ŕ/X-shears. Such a kinematic configuration implies that the maximum and minimum principal stresses were oriented roughly north-south and east-west, respectively. The relation between the stress field with respect to the caldera elongation and orientation is not typical. We suggest that a pre-existing mutually perpendicular set of cross-cutting structural lineaments largely controlled the magma chamber and caldera formation.
Supplementary material: The whole-rock major, trace element and isotope geochemical tables, magnetic fabrics source data, and methodology details are available at https://doi.org/10.6084/m9.figshare.c.6715893 .
... In addition, we found a corn-shaped hypocenter distribution in the deeper part of the S cluster (Figures 1c, 1d, and Movie S2). This characteristic distribution is often present beneath volcanoes and is commonly interpreted as a circular dyke or the collapse of the chamber roof (Acocella, 2007). Although no volcanism has occurred around the swarm area since the Neogene (Ishiyama et al., 2017), there are hot springs with high geothermal gradients (Tanaka et al., 2004) and one with a high 3 He/ 4 He ratio (Umeda et al., 2009) near the swarm area ( Figure S4 in Supporting Information S1). ...
Plain Language Summary
Earthquake swarms are sequences of several earthquakes occurring in a concentrated area over a given period. Unlike other major earthquakes, which have one main shock and several subsequent aftershocks, swarms lack a clear mainshock event. The causes of long‐lasting earthquake swarms are not sufficiently understood. In the northeastern tip of the Noto Peninsula in Japan, more than 20,000 earthquakes occurred between May 2018 and June 2022, including 10 events over magnitude 4.0. To understand the controlling factors of this long‐living earthquake swarm, we investigated the spatiotemporal characteristics of the swarm using high‐resolution relocated hypocenter locations. The hypocenters of the swarm are spatially separated in four clusters and initiated from the southern cluster. We also observed a diffusive pattern in hypocenter distribution, which is typical of earthquake swarms surrounding volcanoes or fluid injection wells, implying the existence of fluid as a driving factor of the swarm. In the southern cluster specifically, we found several intermittent seismic activities with rapid diffusive changes in hypocenter distribution, suggesting the presence of a highly pressurized, deep‐source fluid supply. The intermittent fluid supply from the southern cluster toward the others and the relatively low‐permeability environment are key factors in the longevity of this earthquake swarm.
... We elaborate further on this point in Section 4. We note that when we use the term "caldera," we are referring to the general surface depression that is associated with all calderas. Differences in the origin, structure and setting of calderas (e.g., Acocella, 2007;Cole et al., 2005) are neglected. ...
Simulating magma propagation pathways requires both a well‐calibrated model for the stress state of the volcano and models for dike advance within such a stress field. Here, we establish a framework for calculating computationally efficient and flexible magma propagation scenarios in the presence of caldera structures. We first develop a three‐dimensional (3D) numerical model for the stress state at volcanoes with mild topography, including the stress induced by surface loads and unloading due to the formation of caldera depressions. Then, we introduce a new, simplified 3D model of dike propagation. Such a model captures the complexity of 3D magma trajectories with low running time, and can backtrack dikes from a vent to the magma storage region. We compare the new dike propagation model to a previously published 3D model. Finally, we employ the simplified model to produce shallow dike propagation scenarios for a set of synthetic caldera settings with increasingly complex topographies. The resulting synthetic magma pathways and eruptive vent locations broadly reproduce the variability observed in natural calderas.
... Despite this, regional tectonics may still affect some features, such as the caldera shape in map view, generating an elliptical collapse with a major axis parallel to the minimum horizontal principal stress at the rim. These elliptical calderas may develop from elliptical or even circular (in map view) chambers at depth due to a regional tectonic contribution (Acocella, 2007). ...
The late Neoproterozoic Dokhan Volcanics (DV) of the Egyptian Eastern Desert (northern Arabian-Nubian Shield), were investigated at Gebel Nuqara, near the boundary between the Northern and Central Eastern Deserts. In particular, the nature of the peculiar circular and annular regional structures at Nuqara, were targeted for study. This included exploring the relationships of DV and intrusive units to each other and to regional dikes and faults. The central part of the Nuqara DV exposures consists of two volcanic units: 1) “Older DV” (intermediate varieties, dominantly andesites; 659 ± 16 Ma), with associated arc granitoids (663.2 ± 8.4 Ma), followed by andesite dikes (636.9 ± 7.2 Ma); and 2) “Younger DV” (acidic varieties; 602.3 ± 4.4 Ma for rhyolites, and 589.4 ± 6.1 Ma for rhyolite porphyry). This central area of DV units is enclosed within an annular intrusion of granite. The geochemical data for the Nuqara DV reveal intermediate low-silica adakitic (basaltic-andesite and andesite) and acidic high-silica adakitic (rhyolite and rhyolite porphyry). Both adakite groups share a common geochemical trend, and show evidence for magma diversification via fractional crystallization. The Nuqara DV is identified as a caldera sequence, with resurgent uplift, based on a positive comparison of the volcanic morphology, lithology, structure and mechanism of DV eruptions in the study area with those of other resurgent calderas. The geodynamic setting, volcanic architecture, and geochemistry of the DV, as well as its in-situ zircon LA–ICP–MS estimated ages, allow recognition of a series of eruptive stages, leading to collapse to form the Nuqara caldera, followed by resurgent uplift of the caldera floor during intrusion of rhyolite porphyry domes. The resurgent caldera model assigns a greater significance for the rhyolite porphyry as the exposed top of a sub-volcanic magma chamber. Resurgence is viewed as resulting from increased pressure or volume changes in the magma reservoir. A consequence of resurgence was uplift and erosion of the previous collapse-related topography.
... The tridimensional complexity of the recognized fault patterns is likely influenced by a combination of factors such as the intricate manner in which a stress field might be perturbed by the rheological anisotropies of the basement, the strong mechanical transition between the basement and the volcano-sedimentary fill, and certainly the geometry of the magma-chamber at depth. Some of the interpreted structures, such as normal planar fault segments and reverse bell-shaped reverse faults, semicircular to rectilinear in map view, are consistent with other published works (Acocella, 2007;Kennedy et al., 2004;Bonini et al., 2021). ...
Over the past three decades, studies have provided important information on the current volcanic activity and the associated thermal anomalies spread over the southern Puna in Argentina, as in the Cerro Blanco Volcanic Complex case. The volcanic-hosted geothermal systems have received critical attention since they are low-carbon energy sources and are frequently linked to geothermal lithium deposits. A key issue in developing productive projects is quantifying the thermal resource because this value determines investor interest. Our research aims to estimate the power capacity of the Cerro Blanco geothermal system in terms of MWe. Two methods were employed: a conductive cooling of the magmatic chamber and the classical volumetric method, coupled with Montecarlo simulations. After a rigorous analysis of recently published data, a 3D structural mapping supported by a series of analogue models, combined with a deep fluid compositional reconstruction, was employed to estimate the reservoir temperature and to support the resource assessment. Our experimental calderas show that the collapse onset is controlled by a combination of internal reverse faults and peripheral ring faults; concentric inner faults propagate outward from the caldera structure's central part. The 3D structural mapping highlights that the main structures of the Cerro Blanco geothermal system constrain the reservoir area. The estimation obtained by the magmatic heat transfer method indicates that Cerro Blanco might produce 6 MWe per km² at 50% confidence and that it hosts a total power capacity of 127 MWe. Also, it was calculated that there are >18 km² above 200 °C at 2 km depth. By the volumetric method, a power capacity of 6 MWe and 32 MWe at 90% and 50% confidence were estimated, respectively. These results should be interpreted cautiously considering that, for example, 1% of the unknown recovery factor correlates with 4.1 MWe in the power capacity. These estimations do not confirm the existence of the geothermal resource but raise encouraging results. Moreover, they become relevant because the Cerro Blanco geothermal system has few thermal manifestations and might be considered a blind geothermal system.
... Seismicity bursts (Fig. 3a-d) were shortly followed by increased lava emission and/or opening of new vents 25,34 , which may reflect structural instabilities resulting from increased magma withdrawal from the deep reservoir. An increase of the shallow seismicity rate by late November 2021 (Fig. 3b), reaching 4-8 km depth, may indicate fracturing processes at the roof of the depleting reservoir [35][36][37] . Stage 4 comprises from the eruption end and the drop of tremor (December 13) to the end of the deep seismicity (December 21). ...
The 2021 volcanic eruption at La Palma, Canary Islands, was the island’s most voluminous historical eruption. Little is known about this volcano’s feeding system. During the eruption, seismicity was distributed in two clusters at ~10-14 km and ~33-39 km depth, separated by an aseismic zone. This gap coincides with the location of weak seismic swarms in 2017-2021 and where petrological data have implied pre-eruptive magma storage. Here we use seismological methods to understand the seismic response to magma transfer, with 8,488 hypocentral relocations resolving small-scale seismogenic structures, and 156 moment tensors identifying stress heterogeneities and principal axes flips. Results suggest a long-lasting preparatory stage with the progressive destabilisation of an intermediate, mushy reservoir, and a co-eruptive stage with seismicity controlled by the drainage and interplay of two localised reservoirs. Our study provides new insights into the plumbing system that will improve the monitoring of future eruptions in the island. In a new study, the authors use seismological methods to understand the eruption of La Palma 2021. Results suggest a preparatory phase of de-stabilisation of a mushy reservoir, and a co-eruptive phase with seismicity controlled by the drainage and interplay of two reservoirs.
... Geometrically comparable structures, albeit on a much smaller scale and with substantially steeper fault dip angles, have been the subject of studies focused on the dynamics of caldera collapses (Acocella, 2006(Acocella, , 2007Burchardt & Walter, 2010;Geyer & Martí, 2014;Levy et al., 2018). The formation of outward-dipping reverse ring faults is a kinematic consequence of caldera subsidence and collapse. ...
The lunar maria are large expanses of basalt that infill antecedent impact basins and show evidence for postemplacement deformation. Landforms within many of these basins suggest a period of compressive tectonics, although their formation mechanism has yet to be established. Previous work for Mare Crisium demonstrated that basin‐circumferential wrinkle ridges, which typically demarcate the inner edge of an annulus of elevated terrain, are the result of deep‐seated thrust faults that preferentially form along the boundary of an elevated superisostatic portion of mantle and a thick, subisostatic collar of crustal material. Here, we show that a similar fault architecture exists for several other mascon‐bearing basins, including Maria Serenitatis, Nectaris, Moscoviense, and, to a lesser degree, Humorum and Imbrium. These deeply penetrating basin‐circumferential thrust faults, as for Mare Crisium, form a (partial) outward‐dipping ring‐fault system that bounds the elevated mantle plug beneath each basin as a geometric consequence of mascon evolution. If this geometric arrangement is unique to the Moon, then some characteristic(s) of lunar mascon evolution enables the formation of such mascon‐bounding faults. Despite the ubiquitous nature of mascon‐bound thrust ring faults at several lunar basins, whether such structures exist at mascon basins on other terrestrial worlds remains an open question.
... On the other hand, the Hiddekel Cavus shows significant crustal collapse and gravitational slumping within a narrow cone-shaped depression, which resembles the funnel-like caldera described in Acocella (2006). Experimental models also indicated that the formation of funnel-like calderas is accompanied by minor explosive activity (Acocella, 2007), coinciding with limited local volcanic deposits at the significantly deep depression of Hiddekel. ...
Plain Language Summary
Perhaps the most recognizable image of a volcano is a topographic rise, such as a cone or mountain, flanked by lava flows. While this image fits many volcanic settings on Earth and Mars, many known volcanic settings on Earth do not fit this iconic description at all and the question remains open as to whether that exception might also be true for Mars, where observations are limited to remote sensing interpretations. Several volcanoes in Arabia Terra on Mars lack recognizable topographic features, suggesting an important form of explosive volcanism associated with crustal collapse. These source vents could have produced huge amounts of ash airfall deposits, now widely observed throughout the region as layered deposits. These features are likely more common on Mars than is currently recognized and are likely a critical component of Mars' geologic puzzle. This work describes in detail the geological characteristics of Eden Patera (a large caldera with diameter over 60 km), located within Arabia Terra on Mars. This site is a “type‐locality” to which other collapsed calderas on Mars can be compared. Mapping results from Eden Patera illuminate a complex geologic history driven by both explosive (ash‐dominated) and effusive (lava‐dominated) eruptions.
... We also underline that the surficial structures (<300 m) of the caldera rims in the offshore show moderate dip angles (~65 • ) with normal kinematics (Fig. 5k), differently from the previous literature that implied vertical faults (e.g., Orsi et al., 1996), or outward-dipping reverse faults such as predicted by analogue modelling at shallow levels (Acocella, 2008). However, the reverse faults may occur at deeper levels (below seismic penetration limit) to accommodate the horizontal movement triggered by the lithostatic collapse (e.g., Acocella, 2007) unless a significant extensional field occurs (Geyer and Martí, 2014). More importantly, despite the long-term caldera resurgence exceeding 100 m of in the last 15 kyr, no evidence of positive inversion (from normal to reverse kinematics) is found along the offshore ring faults, unlike the results of analogue models (e.g., Acocella et al., 2000), and seismic profiles interpretation (e.g., Corradino et al., 2021). ...
The structure of a caldera may influence its activity, making its understanding crucial for hazard assessment. Here, we analysed high-resolution seismic profiles in the Campi Flegrei (southern Italy) offshore sector. We recognized two main fault systems, including those associated with the formation of the caldera and those affecting the resurgent dome. The former system comprises three broadly concentric fault zones (inner, medial and outer ring fault zones) depicting a nested caldera geometry. Considering the relations between faults and seismic units that represent the marine and volcaniclastic successions filling the caldera, all ring faults were formed during the Campanian Ignimbrite eruption (40 ka) and subsequently reactivated during the Neapolitan Yellow Tuff eruption (15 ka). In this last caldera-forming event, the inner and medial fault zones accommodated most of the collapse and were episodically reactivated during the younger volcano-tectonic activity. The second fault system occurs in the apical zone of the resurgent dome and comprises dominantly high-angle normal faults that are mainly related to the volcano-tectonic collapse that followed the Agnano-Monte Spina Plinian eruption (4.55 ka). Finally, we provide a volcano-tectonic evolutionary model of the last 40 kyr, considering the interplay among ring and dome faults activity, volcaniclastic sedimentation, ground deformation and sea-level changes.
... However, more recent studies suggest that these distinct modes of formation may be too simple in some instances. First, some calderas are geometrically complex, containing elements of more than a single structural type and formation process (Cole et al., 1998;Lipman, 2000;Acocella, 2006Acocella, , 2007Geyer and Marti, 2014). Second, the classification is generally based on the assumption that calderas are formed by eruptions from a single large magma body, but recent studies have suggested that caldera-forming eruptions may be more commonly related to multiple magma bodies (Wilson et al., 2006;Deering et al., 2007;Nakagawa et al., 2018). ...
Some calderas are geometrically complex that may be related not to a single eruption, magma body, or structure. In order to reveal their forming processes, multidisciplinary methods should be applied. Akan volcano has E-W elongated and irregular-shaped caldera (24 × 13 km), implying a complex mechanism of formation. Akan caldera results from successive explosive eruptions from 1.4 to 0.1 Ma. On the basis of duration of dormancy and petrological features (mainly whole-rock major element compositions) of juvenile materials, these eruptions have been grouped into 17 eruptive groups (Ak1–Ak17), each of which consists of a single or a sequential phase. In order to investigate the processes of caldera formation, we focus on the younger eruptive groups (Ak1–Ak7: 0.8 to 0.2 Ma) that have relatively large magnitudes (>10 km³) and likely control the present caldera shape. We performed component analysis of lithic fragments from pyroclastic fallout deposits, whole-rock trace element analysis of juveniles, and gravitational survey of the caldera. We grouped Ak1–Ak7 into three types, namely, type A (Ak1, Ak2), type B (Ak3–Ak5), and type C (Ak6, Ak7), based on the lithic componentry, most of which are accessary and accidental fragments from vent and conduit areas. The characteristic lithic component in each type is as follows: altered rock in type A, aphyric dacite in type B, and pyroxene andesite in type C. These data suggest that explosive eruptions of each type are derived from distinct sources. The whole-rock composition of juvenile materials of each type also shows distinct trends on Harker diagrams. These three chemical trends are nearly parallel, suggesting that three different magma systems were active. This is consistent with the lithic componentry showing three spatially distinct vent sources. The geological and petrological evidence is supported by a Bouguer anomaly map. Akan caldera is characterized by three circular closed minima, indicative of three depressed segments that correspond to the source regions, each of which possibly discharged the three types of magma. Caldera-forming eruptions of Akan caldera occurred from at least three distinct sources with distinct magma systems. In conclusion, Akan caldera is a composite caldera, and its shape reflects the distribution of multiple source regions. The case study of Akan caldera shows a possible time-space evolutionary pathway for a caldera complex where several smaller calderas are nested.
The Pire Mahuida Volcanic Complex (PMVC), located in the extra-Andean region of southern Argentina (68◦ to
68◦ 40′W and 41◦ 51′ to 42◦ 28′ S), is a bimodal volcanic field that developed during the Miocene (17Ma-14Ma,
Langhian-Burdigalian Stages). The complex partially surrounds the southern boundary of the Somún Cur´a
basaltic plateau. The PMVC is mainly composed of acidic pyroclastic and lava facies (rhyolite/dacite flows, lava
domes and coulees) and stands out as the one with the largest volume of acid rocks in the Somún Cur`a province,
involving two evolutionary trends (alkaline and subalkaline). Subordinate in volume, basaltic flows overlie this
extensive sequence of acidic rocks. Relationships between some units are difficult to establish because they are
the result of different eruptive centres. However, stratigraphy, morphology and petrography allow two acid
phases to be distinguished, one a mainly lavic phase and the other a mainly pyroclastic phase. U-Pb ages allow
precise dating and temporal placement of the acid sequence: 1- lava flows and 2- lava domes and related pyroclastic
phases. Two calderas and fissures are responsible for the emission of the felsic rocks. The basaltic facies
shows a wide range of characteristics, which also allow two different groups to be distinguished. 1. The basalts of
the main plateau are associated with a main fault of NW-SE pattern and 2.The younger basalts are associated
with small volcanic edifices. The effusion of the complex was developed in a relatively short time with the basic
episode being more prolonged than the acid episode.
The Nari Caldera on Ulleung Island, an oceanic intraplate volcano, is a significant repository of information on the latest volcanic activity. To interpret the characteristics of the latest volcanic activity, it is essential to understand the caldera structures formed by the most explosive eruption of the Ulleung volcanic edifice. This study presents a three-dimensional (3-D) resistivity model based on audio-magnetotelluric (AMT) data and interprets the caldera structure of Ulleung Island. New land and ocean terrain models were used for the 3-D inversion of the AMT data, and a finer, nonuniform grid was generated for the caldera area. Subsequently, 3-D inversion and imaging were conducted on the AMT data at 25 stations. In the caldera area of Ulleung Island, the 3-D resistivity model is divided into two bodies: a high resistivity body located in the south and a low resistivity body located in the north. This structure is consistent with the location of the two calderas, Seongin Caldera and Nari Caldera, as inferred from geological studies. Furthermore, the high resistivity body located in the south exhibited a bowl shape in the 3-D space. Therefore, we suggest that the high resistivity body located south of the caldera on Ulleung Island is a structure of the Seongin Caldera. The Seongin Caldera has a diameter of approximately 1.5 km and a caldera fill height of approximately 0.8 km, as measured from the resistivity model. Based on the stratigraphy of Ulleung Island and the physical properties of the rock types, the interior of the Seongin Caldera was considered to have been filled with trachytic lavas of the Seonginbong Group. From the high geothermal gradient of Ulleung Island and the stratigraphy of the GH-4 borehole, the low resistivity body extending from the shallow depths of the Nari Caldera to the lower part of the Seongin Caldera could be interpreted as trachytic rocks that underwent hydrothermal alteration. In addition, a low resistivity body contains highly porous and/or weathered rock. This study presents information on the calderas of Ulleung Island that can aid in interpreting the characteristics of the latest volcanic activity. We expect this information to contribute to the preparation for potential volcanic hazards.
Plain Language Summary
On 8 October 2023, a tsunami was widely observed along the Japanese coast without any major tsunamigenic earthquake, while a series of 14 small earthquakes occurred near Sofugan, located in the Izu‐Bonin Islands. Two possible candidates for this tsunami have been proposed, involving submarine volcanic processes or submarine landslides, but the exact cause remains unclarified. Using the tsunami data observed by the seafloor pressure gauges located more than 600 km from the tsunami source region, we analyzed sea height movements to obtain insights into the origin of this enigmatic tsunami. Our analysis showed that the tsunami source consisted of the seafloor uplift that repetitively occurred at a submarine volcanic caldera. Our results also showed an accelerating tsunami excitation, such that the amount of the seafloor uplift movement increased over time and the time intervals of the earthquakes gradually shortened. These results are consistent with the acceleration process of volcanic activity, suggesting the tsunami originated from the multiple sudden uplifts of the submarine caldera.
Modern volcanoes and volcanic centres encompass a wide variety of morphological, physical, facies and stratigraphic characteristics, and duration of primary volcanic versus sedimentary or epiclastic processes. This leads to the development of general facies models, which are relevant for modern settings, and valuable guides for making meaningful geological reconstructions of ancient volcanic successions. Our discussion covers the full scale of volcanic landforms, ranging from small individual monogenetic scoria and pumice cones and phreatomagmatic maar-type volcanoes, to larger polygenetic volcanoes. These include marine basaltic shields that from their base on the seafloor represent some of the largest volcanoes on the planet, as well as eruptive vents for flood plateau and plains basalt provinces, stratovolcanoes both found in both continental and marine settings, and the largest silicic explosive caldera volcanoes. The latter are sometimes called “supervolcanoes” which are responsible for some of the biggest explosive eruptions recorded in geological history, resulting from structural caldera collapse and in some settings contributing to regional ignimbrite “flare-up” events. Moreover, the special case of marine silicic calderas is also considered. We also now recognise a number of more complex intermediate to silicic volcanic systems; these include “multiplex volcanoes” which have changed in character through their evolution, and other multi-vent centres that appear to occur in the absence of a large central cone structure, or conversely are not controlled by caldera collapse depressions. The remaining two sections are grouped into intra- or subglacial volcanoes (formed under ice and meltwater lakes), and mafic oceanic volcanic centres, namely, spreading ridges, plateaus, seamounts, and surtseyan tuff cones. We also introduce the reader to the economic significance of potentially prospective volcano types and their successions prior to their detailed description in Chap. 18, as well as concluding with a summary of the potential hazards posed by different types of volcanoes.
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.
Caldera collapses are paramount volcano-tectonic features because they form during hazardous explosive volcanic
eruptions, they are ideal sites for geothermal development and mineral resources exploitation, and also
because they preserve the evidence of the interaction between caldera magmatism and the regional tectonic
processes. Despite this, many aspects of the caldera collapse process remain unclear, particularly concerning the
interaction between caldera and tectonic related fault systems. We therefore used analogue models 1) to quantify
the effect of regional strain on caldera elongation in extensional settings, such as the Main Ethiopian and the
Kenya rifts, 2) to describe the effect of regional strain on caldera structures and, vice-versa 3) to document, for
the first time, “the other side of the coin”, that is how caldera structures affect the formation of newly forming
regional extensional faults. Our models showed that tectonic extension only explains a small proportion (e.g.
13% for the Main Ethiopian Rift) of the elongation of most rift calderas. Furthermore, we showed how specific
segments of caldera faults may accommodate regional extension by reactivating, therefore precluding caldera
elongation. Finally, we showed how the presence of caldera structures may influence the geometry of newly
forming regional normal faults, that display a marked curvature, “faking” caldera ring faults. We have suggested
that these “fake” curved caldera ring faults may lead to incorrect estimations of caldera elongation in nature. In
addition, such faults may also mislead geothermal fluid exploration, as they are likely disconnected from the
caldera structures or the caldera plumbing system, and less likely the locus of hydrothermal fluid flow.
Noctis Labyrinthus has been a region of disputed origin due to its complexity and poor understanding of how
various processes and mechanisms may have combined to form it. The surface is an integrated record of intensive
tectonic activity expressed by a multiple extended sets of dip-slip faults oriented in different directions, and
thought to have acted on this region over its history. These faults are always coalescent to pits and pit chains
displaying a complicated geological history in the region. To understand this geological history, we mapped the
surface features in Noctis Labyrinthus using the High-Resolution Stereo Camera (HRSC) onboard Mars Express
ND2 nadir channel basemaps, and we adapted the Digital Terrain Map (DTM) from the Mission Experiment
Gridded Data Record (MEGDR) of Mars Orbiter Laser Altimeter (MOLA) onboard Mars Global Surveyor (MGS)
for the topography. We have investigated the spatial distribution and trend of fault systems, the pit chains’
morphology, and the correlation between these two types of features. Our results show three fault systems: i) NS
and NNE-SSW, ii) EW and ENE-WSW, and iii) NNW-SSE and NW. The analysis of the faults trending, cross-cutting
correlation and the superimposition led to identify multiple intersections between these faults that have been
alongside with the reactivations of some inherited faults. We interpreted the first system of fault to be related to
coeval lateral extension, generated by regional stress tensor, which is probably related to the slight bending of
Valles Marineris within two phases of bidirectional extension. The second system of faults has been generated by
the radial oblate stress tensor related to the formation of the small shield volcanoes in Syria Planum. However,
the third system is likely related to the external driving process, probably in the Tharsis province. We classified
pits in four evolutionary stages based on their morphometric attributes. We believe that the formation of the pit
chains in Noctis Labyrinthus is related to a surface collapse after a pressure drop related to the magma chamber
deflation associated with Syria Planum volcanic province. We propose a deformational model based on early
extension and magmatic plumbing as driving processes for the formation of Noctis Labyrinthus
Measuring ~50 × 30 km, the Cerro Guacha Caldera Complex (CGCC) is a polycyclic, nested caldera complex within the Altiplano Puna Volcanic Complex (APVC) of the Central Andes. Previous work had established that CGCC was built in three stages. Catastrophic supereruptions of the 1300 km 3 (DRE-Dense Rock Equivalent) Guacha ignimbrite at ca. 5.65 Ma and the 800 km 3 (DRE) Tara ignimbrite at ca. 3.49 Ma resulted in two nested caldera collapses that define the first two stages (G1C and G2C) of the CGCC. The third stage, G3C, initiated ~1.8 Ma with the eruption of the Puripica Chico ignimbrite and associated lavas. The only previous detailed systematic study of the CGCC was of the G2C (Grocke et al., 2017a, 2017b). In this work we provide new field, petrochemical and geochronological data on the first and third cycles that now allow us to present the first comprehensive account of the eruption history, as well as the magmatic and volcanological evolution of the CGCC as a long-lived caldera complex. Erupted compositions during the G1C and G3C cycles are dominantly high-K, calc-alkaline dacites and minor andesites and these fall within the andesite to rhyolite compositional array previously defined for the G2C. In the CGCC as a whole, dacitic compositions account for over 90% of the ~2200 km 3 of magma erupted. These data reveal a consistency of magma composition, storage and evolution throughout the >4 Ma history of the CGCC. New U-Pb in zircon ages reveal distinct zircon age distributions that indicate that each caldera cycle represented new magmatic cycles that accumulated 10's to 100's of thousands of years prior to eruption. We find zircon antecrysts in all eruptive stages which we interpret as assimilation of non-erupted progenitors of earlier magmatic cycles. Occassional zircon xenocrysts and cores are interpreted as assimilation of upper crustal basement lithologies, supporting previous interpretations of upper crustal open system magmatic evolution for the G2C to the entire CGCC. Both the G1C and G2C calderas are west-hinged trap-door style collapses. Of these, the 5.65 Ma G1C caldera measures ~50 km × 30 km, whereas the 3.49 Ma G2C is 30 × 20 km and is nested within the G1C. Resurgent uplift in both the calderas reveals at least a kilometer thickness of intracaldera ignimbrite. Outflow ignimbrites from the CGCC extend 100 km N-S and 40 km E-W around CGCC and were topographically limited. Intracaldera ignimbrite volume dominates outflow volumes by a ratio of ~5:1. Repose periods between successive stages are now calculated to be 2.1 (G1C to G2C) and 1.8 (G2C to G3C) Myr respectively. The evolution of the CGCC parallels other APVC calderas and is closely linked to the history of the APVC flare-up. The two supereruptions at 5.65 Ma (Guacha Ignimbrite) and 3.49 Ma (Tara Ignimbrite) coincide with the peak of the flare-up and its eruptive and magmatic tempo correlate with the regional tempo. The eruptive and magmatic histories connote a composite granodioritic subcaldera pluton of significant extent accumulated in the upper crust beneath the CGCC.
Calderas are kilometer-scale basins formed when magma is rapidly removed from shallow magma storage zones. Despite extensive previous research, many questions remain about how host rock material properties influence the development of caldera structures. We employ a mesh-free, continuum numerical method, Smoothed Particle Hydrodynamics (SPH) to study caldera formation, with a focus on the role of host rock material properties. SPH provides several advantages over previous numerical approaches (finite element or discrete element methods), naturally accommodating strain localization and large deformations while employing well-known constitutive models. A continuum elastoplastic constitutive model with a simple Drucker–Prager yield condition can explain many observations from analogue sandbox models of caldera development. For this loading configuration, shear band orientation is primarily controlled by the angle of dilation. Evolving shear band orientation, as commonly observed in analogue experiments, requires a constitutive model where frictional strength and dilatancy decrease with strain, approaching a state of zero volumetric strain rate. This constitutive model also explains recorded loads on the down-going trapdoor in analogue experiments. Our results, combined with theoretical scaling arguments, raise questions about the use of analogue models to study caldera formation. Finally, we apply the model to the 2018 caldera collapse at Kīlauea volcano and conclude that the host rock at Kīlauea must exhibit relatively low dilatancy to explain the inferred near-vertical ring faults.
Se presentan las pautas para unificar la representación de los mapas geológicos de volcanes compuestos en Colombia como resultado de los avances del Servicio Geológico Colombiano (SGC) en la representación efectiva para el mapeo de unidades volcanogénicas en los Andes del norte, a partir de la búsqueda y análisis de diversas estrategias cartográficas implementadas en otros lugares del mundo, e identificadas en la literatura científica y en sitios web de otros servicios geológicos. El principal objetivo de este estándar es brindar una guía para la presentación digital de los mapas geológicos de volcanes compuestos del Plioceno-Holoceno, que proporcione una exposición integral de la cartografía y la estratigrafía, con base en una geodatabase dinámica, y que integre mejores prácticas en la representación y organización digital sistemática de mapas temáticos de volcanes compuestos para establecer un lenguaje común que facilite la comunicación y el entendimiento entre diferentes usuarios. En esta publicación se presentan los lineamientos para el manejo de la información geográfica asociada a la cartografía geológica de volcanes colombianos, como parte de las políticas y lineamientos de la Infraestructura Colombiana de Datos Espaciales del SGC, que busca optimizar la interoperabilidad y la normalización de la información geoespacial, con procesos de generación y actualización de los datos y herramientas para la evaluación de calidad, que faciliten el intercambio de información.
Despite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei Caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano-tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 150 m depth. Two main NE–SW and NW–SE strike-fault sets were imaged using electrical-resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root-conduit-like structures and shallower dome-like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid-emission area along the border of the younger volcanic structure exhibits a +40 °C maximum soil temperature anomaly. The proposed volcano-tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei Caldera sector and the monitoring strategies for the Astroni Volcano.
The structural setting of the Ischia resurgent caldera and its magmatic system has been investigated by a joined interpretation of a 3D inversion of previously collected gravimetric data and all the available geological, geophysical and petrological data.
Starting from the available Bouguer gravity map of the Neapolitan volcanic area and a previous 2.5D modelling, a selection of on-land and off-shore gravity data has been used to perform a 3D inversion, adapting and merging the basic ideas of two already tested methods, used to detect isolated bodies and layered discontinuities respectively.
The base of the map is a set of gravity values, covering the whole Neapolitan volcanic area and the Gulf of Naples, which results from the union of 862 offshore and about 2000 on land already existing gravity data, uniformed and re-analyzed.
The final model proposed here allow to outline a very detailed and well constrained structural setting of the crustal sector beneath Ischia. In particular, the 3D gravity inversion allowed to outline a body with negative density contrast under the Mt. Epomeo, interpreted as the resurgent block, and to describe the magmatic system underneath it as a complex system of intrusions, rather than an uniformly distributed laccolithic body.
We identified four pyroclastic flow deposits in central Hokkaido as
belonging to the same flow deposit which erupted from the Tokachimitsumata
basin with a circular topographic moat（10×14 km）. A dense
network survey of gravimetric data revealed a negative depression profile
of a Bouguer anomaly in the basin. The surface elevation and thickness
of the Muka pyroclastic flow deposit adjacent to the basin gradually
increase toward the basin moat. These findings suggest that the
Muka deposit was derived from the basin, even though the volume of
this deposit is substantially less than that estimated for the caldera.
Field surveys, petrographic analyses, and K-Ar dating were also conducted
on the other three deposits（i.e. the Meto tuff bed, Kuttari
pyroclastic flow deposit, and Biotite dacite tuff-breccia）, which had
similar petrographic characteristics to those of the Muka deposit. The
strong correlation between the four deposits can be inferred by the following:
the K-Ar ages of feldspar minerals from pumices of the four
deposits are identical（1 Ma）, the pumices of the four deposits exhibit
very similar mineral assemblages and volcanic-glass and mineral chemistry,
and, like the Muka deposit, the surface elevation, thickness and
welding degree of the three deposits appear to increase toward the
basin. The Muka, Meto, and Kuttari deposits began forming a pumice
fall deposit, indicating a plinian phase for the first eruption. It can
thus be inferred that these four deposits have arisen from a large-scale
eruption that occurred 1 Ma, which formed the caldera. The total volume
of ejecta was approximately ＞＿ 130 km3. We have named the fall
deposit, flow deposit, and basin the Tokachimitsumata pyroclastic fall
deposit（Tkm-pfa）, Tokachimitsumata pyroclastic flow deposit（Tkmpfl）,
and Tokachimitsumata caldera, respectively.
Increasing evidence suggests that non-planar faults have been activated during caldera collapse events linked to basaltic eruptions around the world. Despite advances in the description of the source location and processes, there is inconclusive evidence regarding the quality of the locations, both hypocentres and centroids, and their relationship with the geology and laboratory experiments. For example, trapdoor and piston collapses are not expected to activate faults within the caldera boundaries, unlike piecemeal or funnel collapses. Small earthquakes at ring faults present small rupture areas, hence, the curvature does not affect the location of the events and the source can be approximated as a planar fault. For larger events, the curvature becomes critical and the centroid migrates towards the centre of the ring while the arc rupture increases. Consequently, centroid locations within the caldera rims might be either real events in the corresponding location or an artefact due to the curvature of the source. In this work, I test the well-known equation of the centroid location at a ring fault with uniform slip and contrast it with a Gaussian-like distribution. Then, I perform numerical simulations assuming three cases of rupture velocity to compare hypocentre estimations with the analytical expressions for centroid location. For instance, hypocentres of an instantaneous rupture behave as the centroid of a uniform slip distribution, a 6 km/s rupture shows hypocentres congruent with the centroid of a Gaussian-like slip distribution, and finally, a 0.9vS rupture velocity shows the most likely locations at the rims but showing large uncertainty. I applied these results to two cases; hypocentres calculated at Bárðarbunga and centroids calculated for Sierra Negra caldera collapse earthquakes. By using this methodology, both cases give a first order approximation on the quantification of the artefact affecting locations due to curvature.
Recent geological and petrological studies for late Cenozoic volcanic rocks from the NE Honshu arc, Japan, revealed the presence of clear secular change of mode of magmatism. Three prominent periods of volcanic activity: continental margin (66-21 Ma), back-arc basin (21-13 Ma), and island-arc stage (13-0 Ma), are identical there. Each period has its unique pattern of lateral alkali variation that is closely related with the thermal structure of the uppermost wedge mantle. The island-arc stage can be divided into four phases: submarine volcanism (13-8 Ma), late Miocene caldera-forming phase (8-5 Ma), Pliocene caldera-forming phase (5-1.7 Ma) and compressional volcanic arc (1.7-0 Ma). These changes in the mode of igneous activity correlated with the stress regime mainly controlled by plate motion, and with the evolutional stage of arc magmatism.
Combined geological, petrological, and geophysical studies have become a valuable tool in revealing intra-crustal structures. The thermal structures seen in the crust of the present NE Honshu arc are closely related to the distribution of the late Cenozoic collapse calderas. High temperature plutons under cooling from the magma reservoir must be exist now at middle depths under the calderas, and probably observed as low-velocity bodies with many S wave reflectors. To clarify the close relations between the late Cenozoic calderas with unexposed plutons and the seismicity will become one of the main target for predicting hazardous events at volcanic arcs.
The summits of many of the Earth’s and other planets’ larger volcanoes are occupied by calderas that formed by collapse into an evacuating, underlying magma chamber. These collapse calderas are typically several tens of square kilometers in area and are commonly elliptical in shape. We show that the long axes of late Quaternary collapse calderas in the Kenya rift valley, the western Basin and Range province, the Snake River-Yellowstone Plateau, and the Iceland rift zone are parallel to the upper crustal minimum horizontal stress direction (Sh) as determined by independent criteria. We suggest that circular magma chambers beneath these volcanoes became elliptical by stress-induced spalling of their chamber walls, by a mechanism that is analogous to the formation of breakouts in boreholes and tunnels. In breakouts, the hole becomes elongate parallel to the far-field minimum stress. In the Kenya rift, Late Pleistocene caldera collapse was accompanied by a 45° rotation of Sh and an increase in the magnitude of the maximum horizontal stress (SH). The breakout model predicts increasingly unstable caldera walls under these conditions, a possible explanation for the sudden appearance of so many collapse events in a volcanic setting that had never experienced them before. This mechanism of stress change-induced collapse may have played a role in other caldera settings.
We conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas. Dry sand contained in a 1-m-diameter cylinder served as a crustal rock analogue, and a water-filled 0.6-m-diameter rubber bladder served as an analogue magma chamber. Scaling parameters included a length ratio (L*) of 2.5 x 10(-5) and a stress ratio (sigma*) of 1.8-2.4 x 10(-5). In contrast to some previous analogue models, the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled. Generally, deformation began with broad sagging, followed by an arcuate or linear outward-dipping fault that formed on one side of the caldera. This fault propagated laterally around the caldera in both directions, sometimes joining other faults, and typically forming an overall polygonal structure. As subsidence continued, the caldera grew incrementally outward and progressively formed a series of concentric outward-dipping faults. Lastly, a peripheral zone of extension and pronounced sagging, and commonly an inward-dipping outer fault related to extension, developed at the surface. As the depth of the chamber increased, (1) the area of faulting decreased, (2) the symmetry of the caldera was affected, and (3) the coherence of the subsiding block decreased. Tilting the chamber caused highly asymmetric subsidence to occur. In this case, faults formed first where the bladder was shallowest. Subsidence then shifted rapidly to where the bladder was deepest, producing an elongate trapdoor caldera that was deepest where the bladder was deepest. Our experiments highlight the roles of sagging and faulting during caldera subsidence. Surface fault patterns both in our experiments and at natural calderas are frequently not circular. The aspect ratio of the block above the magma chamber controls the shape of the caldera, which is frequently polygonal. The faults at natural calderas determine locations and migration of eruptive vents, the degree of subsidence, the style of postcaldera resurgent magmatism, and the extent of hydrothermal circulation. Our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas.
Scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome. As the magma analogue is injected vertically into a sand cone, asymmetric deformation is caused by the formation of a curved major shear fault, which dips inward from one side of the cone to the opposite edge of the intrusion. The path of the ascending silicone deviates to follow the trajectory of the fault, and a lateral bulge grows slowly from the footwall of the fault. The oblique push makes the bulge migrate outward, causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault. We suggest that the pattern of internal deformation within Mount St. Helens prior to the May 18, 1980, eruption was similar to that observed in our scaled models.
Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2-D) or cones (3-D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.
Detailed surface mapping, subsurface drill hole data, and geophysical
modeling are the basis of a structural and hydrothermal model for the
western part of Long Valley caldera. Six fault zones are recognized in
the western caldera with dominant orientations of north, northwest, and
northeast, sub-parallel to regional fault trends in the surrounding
Sierran basement. The internal structural geometry of the cores of 12
exogenous domes inside and outside the caldera suggests that the domes
erupted at the intersections of these principal fault trends rather than
along the axis of a single dike. Gravity modeling and subsurface data
from deep geothermal wells indicate that the floor of the caldera is
segmented into a number of discrete fault blocks with varying offsets.
One of the northeast trending fault zones, designated Discovery fault in
this paper, appears to be part of the original Sierran embayment that
existed before caldera collapse. Recent hydrothermal alteration occurs
along Discovery fault strands and composite vertical offset of
intracaldera volcanic units across the entire fault zone may be as much
as 400 m. Field relationships, geophysical interpretations, and
interpretaions of drill hole data suggest that this fault is a
fundamental flaw in the western caldera. The preexisting tectonic
framework of the basement rocks controlled the configuration of the
western caldera floor and, through it, the location of postcollapse
eruptive centers. These deep basement structures may also provide the
high fracture density which controls circulation in the present
geothermal system of Long Valley.
Reinterpretation of existing geologic and geophysical work combined with new field work and extensive laboratory studies has resulted in the recognition of a 14 multiplied by 16 km collapse caldera located at the southern end of the Guatemala City graben, south of Guatemala City. The caldera has produced at least nine significant pyroclastic eruptions over a time period of about 300,000 years. It must be considered active and likely to erupt again. Extrapolating from the most recent past record, the most likely type of activity may be plinian or phreatoplinian airfall eruption or dome extrusion in the vicinity of Lake Amatitlan or in the northern part of the caldera.
Stratigraphic and chemical data on the products of the last eruptive activity, of the explosive type, of the Latera volcano seem to indicate the progressive emptying of a single magma chamber, as proved by the polygenic collapse of the so called "caldera del Vepe'. After the eruption of the "Vulcanite complessa di Onano' the activity includes explosive events characterized by high, supported eruptive columns, capable of producing widespread pumice fall deposits. During this initial phase, a first calderic collapse takes place which marks the beginning of the formation of the Vepe caldera. The oldest plinian activity is followed by strombolian type eruptions locatable on the northern flanks of the Latera Caldera. A more recent plinian event, proved by widespread pumice fall deposits of phonolitic composition, indicates the end of the phase of activity. The following phase results from the emplacement of the "Vulcanite complessa di Pitigliano', which is the result of a series of eruptive phenomenologies. -from Authors
This paper combines the temporal model of caldera formation presented by Robert Smith and Roy Bailey in 1968 with recent volcanological concepts. Field examples, experimental models and theoretical studies are synthesized to illustrate the process of caldera collapse conceptually as a series of stages of eruption and deformation. During each stage, physical changes occur at the surface, within the underlying magma chamber, and within the subsiding block or blocks that lie between the surface and the top of the magma chamber. The stages are as follows: 1) magma chamber intrusion, 2) initial eruption, downsagging and the onset of subsidence, 3) main subsidence and eruption phase, 4) peripheral extension and eruption quiescence, 5) continued eruption, subsidence and change of eruptive style, and 6) resurgence and extrusion of lava domes and flows. These stages may then be repeated as a subsequent caldera cycle. Every caldera has an individual history and may deform in a different manner at each stage. The paper outlines how these stages can give rise to different caldera types.
Resurgent cauldrons are defined as cauldrons (calderas) in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome. Seven of the best known resurgent cauldrons are: Valles, Toba, Creede, San Juan, Silverton, Lake City, and Timber Mountain. Geologic summaries of these and Long Valley, California, a probable resurgent caldera, are presented. Using the Valles caldera as a model, but augmented by information from other cauldrons, seven stages of volcanic, structural, sedimentary, and plutonic events are recognized in the development of resurgent cauldrons. They are: (I) Regional tumescence and generation of ring fractures; (II) Calderaforming eruptions; (III) Caldera collapse; (IV) Preresurgence volcanism and sedimentation; (V) Resurgent doming; (VI) Major ring-fracture volcanism; (VII) Terminal solfatara and hot-spring activity. These stages define the terminal cycle of resurgent cauldrons, which in the Valles caldera spanned more than 1 million years. The known and inferred occurrence of the seven stages in the eight cauldrons discussed, together with some time control in four cauldrons, indicates that resurgent doming is early in the postcollapse history; hence, it seems part of a pattern and not fortuitous. Doming of the cauldron block by magma pressure is preferred to doming by stock or laccolithic intrusion, although these processes may be subsidiary. Magma rise that produces doming may be explained in several ways, but the principal cause is not known. Nor is it known why some otherwise similar calderas do not have resurgent domes, although size and thickness of the cauldron block and the degree to which it was deformed during caldera collapse may be factors. All known resurgent structures are larger than 8 miles in diameter and are associated with silicic and, presumably, high-viscosity magmas. Genetically, resurgent cauldrons belong to a cauldron group in which subsidence of a central mass takes place along ring fractures and is related to eruption of voluminous ash flows, thereby differing from Kilauean-type calderas. It is proposed that typical Krakatoan-type calderas differ in that collapse is chaotic and ring fractures are not essential to their formation. Krakatoan calderas typically occur in the andesitic volcanoes of island arcs or the eugeosynclinal environment, and their sub-volcanic analogues are not known, whereas resurgent and related Glen Coe-type cauldrons are more common in cratonic or post-orogenic environments as are their sub-volcanic analogues - granitic ring complexes. Granitic ring complexes, such as Lirue, Sande, Ossipee, and Alnsj0, are probably the closest sub-volcanic analogues of resurgent calderas. The source areas of most of the ash-flow sheets of western United States and Mexico are yet to be found. It is suggested that many of them will prove to be resurgent structures. Present evidence suggests that ore deposits are more commonly associated with resurgent cauldrons than with other cauldron types.
Caldera collapse at Glencoe, Scotland, was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned. Intracaldera depocenters had the form of grabens or half grabens, locally with downsag and bounding monoclinal flexures, and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history. This piecemeal caldera collapse is in contrast to previous interpretations, in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault. Collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast. A dominant northwest-trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through Glencoe. The main graben was transected orthogonally by two cross grabens. Each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity, which built tuff cones, followed by magmatic fountaining that produced lava-like silicic ignimbrites. The three lava-like ignimbrites total more than 300 m thick. Two later eruptions produced eutaxitic silicic ignimbrites, together more than 300 m thick, and contemporaneous progressive downsag was associated with the formation of extensional fractures (crevasses) hundreds of meters deep. Sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence. Unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and/or lacustrine deposition between each ignimbrite eruption. The changes in drainage and sedimentation, and the development of coarse debrisavalanche breccias, reflect tectonic faulting during the periods between major eruptions. The duration of the period including the five caldera-forming eruptions was probably ~0.5 m.y., and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments (>0.5 km/m.y. normal displacement) is similar to that of the most actively subsiding sedimentary basins. Glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems, which, with knowledge that caldera volcanoes commonly overlie faults, suggests that piecemeal calderas may be more common than previously recognized. The research also shows that the emplacement of granites (s.l.) was episodic rather than synchronous throughout the magmatic province, and it suggests that numerous plutons in the province, e.g., in Donegal, Ireland, may have formed at sites of central volcanoes that are no longer preserved.
Caldera morphology on the six historically active shield volcanoes that comprise Isabela and Fernandina islands, the two westernmost islands in the Galápagos archipelago, is linked to the dynamics of magma supply to, and withdrawal from, the magma chamber beneath each volcano. Caldera size (e.g., volumes 2-9 times that of the caldera of Kilauea, Hawai'i), the absence of well-developed rift zones and the inability to sustain prolonged low-volumetric-flow-rate flank eruptions suggest that magma storage occurs predominantly within centrally located chambers (at the expense of storage within the flanks). The calderas play an important role in the formation of distinctive arcuate fissures in the central part of the volcano: repeated inward collapse of the caldera walls along with floor subsidence provide mechanisms for sustaining radially oriented least-compressive stresses that favor the formation of arcuate fissures within 1-2 km outboard of the caldera rim. Variations in caldera shape, depth-to-diameter ratio, intra-caldera bench location and the extent of talus slope development provide insight into the most recent events of caldera modification, which may be modulated by the episodic supply of magma to each volcano. A lack of correlation between the volume of the single historical collapse event and its associated volume of erupted lava precludes a model of caldera formation linked directly to magma withdrawal. Rather, caldera collapse is probably the result of accumulated loss from the central storage system without sufficient recharge and (as has been suggested for Kilauea) may be aided by the downward drag of dense cumulates and intrusives.
Collapse of a large caldera can cause spatial and temporal perturbations of stress, and formation of "caldera faults." The stress variations influence the direction of slip vectors on the fault planes; hence, stress estimation is important for the study of caldera-forming processes. In our paleostress estimation, the stress variations in the collapse of the ca. 14 Ma Kumano caldera in Japan have been revealed. A stress inversion method based on the Wallace-Bott hypothesis was used to compute the orientation of the principal stress axes (sigma;1 σ2σ3) and the stress ratio φ=(sigma;2-σ3)/(φ1- σ3), where 0≤φ≤1. The caldera faults formed simultaneously with the caldera-forming ash-flow tuff eruption. Therefore, paleostress solutions obtained from slip data measured on such faults show the spatial and temporal changes of the stress at the time of the caldera collapse. The computed stress ratio φ characterizes a pair of stress fields. In the early stage, the stress field with φ∼1.0 shows a semi-radial trajectory of stress σ2 and an eastern concentric trajectory of stress σ3. This stress regime, resulting from pre-collapse tumescence, counteracts the gravitational force and thus produces smaller net vertical stress. The regional tumescence above an inflated magma chamber is the most plausible source of the stress field, and it is consistent with the timing of the caldera formation. In the late stage, the stress field with φ∼0.5 shows the semi-radial trajectory of stress σ2 and the west-convex and concentric trajectory of stress σ3. Change of the stress ratio φ from 1.0 to 0.5 implies that increase in the relative magnitude of the stress σ1 caused the deeper subsidence of the caldera floor. Stress variations may be of significant value for reconstructing the structural history of the caldera.
Here we review the studies of pluton emplacement and caldera collapse, proposing a model linking the two processes. The shallow rise of magma occurs by dyking, and its emplacement occurs along major anisotropies. Many plutons are emplaced at subhorizontal discontinuities, forming sills. These will eventually grow, forming laccoliths, the most common mechanism to store shallow magma. Calderas are a surface expression of these reservoirs, and can be approximated by a piston cylinder, which sinks due to: (1) underpressure in the reservoir, producing outward-dipping reverse faults; (2) overpressure within a sill-like chamber undergoing doming; and (3) overpressure within a laccolith generating apical tensile stresses; in the latter two cases, inward-dipping normal faults form. We suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions. Their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions. Tabular intrusions, generating ring-faults mainly at their tips, form calderas with a similar width to the intrusion; laccoliths may generate ring-faults along the intrusion roof, forming calderas narrower than the intrusion. The outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the reservoir.
Calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation. Yet, it is only in the last 25 years that a thorough three-dimensional study of calderas has been carried out, utilising studies of eroded calderas, geophysical analysis of their structures and analogue modelling of caldera formation. As more data has become available on calderas, their individuality has become apparent. A distinction between ‘caldera’, ‘caldera complex’, ‘cauldron’, and ‘ring structure’ is necessary, and new definitions are given in this paper. Descriptions of calderas, based on dominant composition of eruptives (basaltic, peralkaline, andesitic–dacitic, rhyolitic) can be used, and characteristics of each broad group are given. Styles of eruption may be effusive or explosive, with the former dominant in basaltic calderas, and the latter dominant in andesitic–dacitic, rhyolitic and peralkaline calderas.
Resurgent doming consists of the uplift, usually accompanied by volcanic activity, of part of a collapse caldera. Analogue models were used to investigate the architecture of resurgent domes. Dry sand simulates the brittle crust; uprising silicone, located at the base of the sand-pack, simulates magma. The deformation pattern depends mainly upon: (1) the ratio (aspect ratio) between the thickness of the sand overburden and the width of the silicone intrusion; (2) the duration of experiment. For aspect ratios ≈1, two concentric domes develop; the first-formed outer dome is bordered by inward-dipping reverse ring faults, while the inner dome by outward-dipping normal ring faults. The layers inside the dome are uniformly dipping. For aspect ratios ≈0.4, the dome shows a crestal depression, surrounded by radial fractures, followed by an apical extrusion of silicone. The internal structure of the dome is made up of domed layers. Independently from the aspect ratio, the duration of the experiment enhances silicone extrusion. A consistent structure is observed in most resurgent domes in nature. The comparison between experiments and nature suggests that two distinct resurgence modes occur, mainly depending on the aspect ratio (thickness/width) of the crust overlying the magma chamber. Aspect ratios ≈1 develop a resurgent block with uniformly-dipping layers and peripheral volcanic activity (Ischia and Pantelleria type). Aspect ratios ≈0.4 develop a resurgent dome with a crestal depression, domed layers within and peripheral and internal volcanic activity (Valles and Long Valley type).
The Ethiopian Rift is characterized by several Quaternary calderas. Remote sensing and field analyses were used to investigate the regional structural control on three calderas (Fantale, Gariboldi, Gedemsa) in the axial part of the rift. These calderas are located along the Wonji Fault Belt (WFB), a zone of Quaternary NNE–SSW normal faults and extensional fractures. The three calderas show E–W elongation and major E–W vent alignments, oblique with regard to the mean NW–SE extension direction. No significant evidence of E–W tectonic structures has been found near the calderas, the only relevant systems being those of the WFB. Conversely, left-lateral E–W-trending faults are present at the rift borders and on the Nubia and Somalia plateaus, implying a predominant pre-rift activity. The E–W fractures were partly reactivated during rifting, possibly controlling the development of the magma chambers. Thus, the E–W elongation of the calderas would be the surface expression of such a control, rather than the result of regional extension. An evolutionary model on the role of different structures on magmatism at different crustal levels within the rift is proposed.
Suswa volcano, located at 1°10′S, 36°20′E, is Quaternary in age (<0.4 Ma), dominantly trachytic-phonolitic in composition, and has two calderas. Regional extension was a fundamental control on caldera collapse, providing pathways for the siting, drainage and recharge of magma chambers. Caldera I collapse was associated with magmatic overpressure from volatile exsolution, magma-water interaction, influx of denser magma and magma drainage at depth. Trachybasalt ash, trachyte globular-ash ignimbrites, trachyte pumice lapilli air-fall tuffs and carbonate-trachyte ignimbrites characterize the initial subsidence. Air-fall tuffs, erupted during caldera collapse at Longonot, are interbedded, suggesting a regional collapse event. Incremental, but dominantly Valles-type, collapse continued with the eruption of trachyte agglutinate flows from concentric ring-fractures outside the caldera ring-fault (Ring-Feeder Zone) and trachyte pumice lapilli air-fall tuffs from west caldera I.
Following caldera I collapse, phonolite lava flows were erupted from the caldera floor. Centrally-erupted phonolite lava flows led to the construction of Ol Doinyo Onyoke lava cone. A pit-crater on the cone was a precursor to the collapse of caldera II, both of which were generated entirely by magma withdrawal. Regional decompression caused ring-fault bounded, block-resurgence of the caldera floor
Calderas, 4–30 km in diameter, are the major igneous centers of Trans-Pecos Texas. Twelve calderas occur in two north-northwest-trending belts. Eight, including two in adjacent Chihuahua, occur in a western alkali-calcic belt. Four are in an eastern alkalic belt. Additional calderas are probably associated with two major ash-flow sheets in the eastern belt. Sources exist for all major tuffs of Trans-Pecos, other than these two, so additional calderas are unlikely. All calderas were formed between 38 and 28 Ma ago. The two calderas in Chihuahua were formed 30 and 28 Ma ago and show significant differences in composition and eruptive style from the earlier calderas. Calderas are underlain by individual magma chambers that were active for no more than about 1 Ma. The caldera cycle in Texas is highly variable. Minor volcanism commonly occurred before ash-flow tuff eruption and caldera collapse. Tumescence occurred in at most two calderas. Ash-flow eruption and collapse were simultaneous in all calderas, as shown by thick, intracaldera tuffs that are interbedded with breccias shed from caldera walls. Collapse occurred along discrete fracture systems in most of the western calderas but along hinge zones in the eastern calderas and at least part of the two young calderas in Chihuahua. Sedimentation within calderas was minor. Preresurgent volcanism ranged from minor to extensive and was best developed in the western calderas, all of which show some form of resurgence. No eastern calderas were resurgent. Resurgence is expressed in many ways in western calderas, but only a few fit the classic model. Ring-fracture volcanism was irregularly developed. Hydrothermal alteration and ore deposition were abundant and spatially and genetically related to calderas in the western alkali-calcic belt. Alteration and ore deposition were minor to nonexistent in the eastern alkalic belt.
Sabaloka is one of the best exposed and most accessible of a large number of Younger Granite complexes in Sudan. These complexes have close affinities with the Younger Granites of western Africa and like them range widely in age. Sabaloka itself probably dates from the Proterozoic or early Palaeozoic. The paper includes a detailed map and description of the complex and presents the results of 20 new whole-rock chemical analyses. Of the two main centres at Sabaloka, the large Cauldron Complex comprises a subsided block of basement overlain by up to 2 km of volcanic rocks and circumscribed by a polygonal zone of ring-fracturing. The fracture system was intruded by a ring-dyke of porphyritic microgranite after eruption of the volcanic rocks, and at about the same time a boss of mica granite with associated tin-tungsten mineralization was injected into the subsided block. There is also gravimetric evidence of subsurface granite intrusions in both the north and south of the cauldron, but no indications of any large mass of basic rock. Nearly all of the volcanic and intrusive rocks of the Cauldron Complex are thoroughly acidic, but a thin group of basaltic lavas lies at the base of the volcanic succession and a few minor intrusions are of basic and intermediate composition. The acidic rocks include metaluminous and subaluminous types, but peralkaline rocks are either absent or very minor in amount and altered beyond recognition. Lavas dominate the lower part of the volcanic succession whereas rhyolitic ignimbrites compose most of the upper part. Of the two main episodes of subsidence which formed the cauldron the first followed upon eruption of the lavas and produced a structural basin centred on the eastern margin of the present complex. Subsequent establishment of the ring-fracture system appears to have been consequent upon an extension of the magma chambers to the north, and was accompanied by voluminous ash-flow eruptions and the formation of a caldera. The second major subsidence post-dated all the volcanic rocks still preserved, and was probably followed by resurgent doming in the north, though the evidence on this point is not conclusive. The Cauldron Complex is classified as a 'Valles type' of caldera volcano. The much smaller Tuleih Complex lies north of the Cauldron and includes a boss of quartz-syenite and subacid granite together with a plexus of smaller intrusions which include peralkaline intermediate and acidic rocks of comenditic character. The age of these intrusions relative to the Cauldron Complex is not known. The chemistry of these various rock types reflects in many respects their close similarity to the Younger Granite association of western Africa, although the rocks of the Cauldron Complex are somewhat poorer in soda than most analysed acidic rocks from the Nigerian Younger Granites.
There are seven major, late Quaternary ( less than equivalent to 1 m. y. ), trachytic caldera volcanoes in the Kenya Rift Valley. These are situated in the inner troughs of the rift. Their positions are not apparently related to transverse lineaments or basement structures. The structural development was complex, involving combinations of the formation of large calderas and small calderas/pit craters, the growth of lava and tuff cones and the eruption of fissure basalts. Two groups of volcanoes are broadly distinguished. The southern group is overwhelmingly of trachytic composition. In the remaining, more northerly volcanoes, basalts and mugearites form bimodal associations with trachytes.
Experiment has always played a key role in scientific methodology. Experiments are used to explore new phenomena, provide systematic observations, determine model parameters, and to test theoretical models and numerical results. As the field of volcanology has become increasingly quantitative over the last several decades, experiments have thus become increasingly valuable. Laboratory experiments using analogue materials allow the study of complex interacting processes such as heat and mass transport, phase changes, and complex rheologies, all features of real volcanic systems. A significant difficulty, however, of laboratory experiments used to understand volcanic processes is the problem of scale - volcanic systems operate of a much wider range of length, time and property scales than can usually be studied in the lab. If experiments cannot be scaled they can at least serve as a tool to investigate processes and interactions. In this presentation, I review the value of laboratory experiments, the challenges, some successes, and some shortcomings. I will use three examples to illustrate how laboratory experiments can be combined with field observations and numerical simulations to provide new insight into volcanic processes: 1) solidification on the surface of lava lakes, 2) fragmentation of magma during rapid decompression, 3) the effect of bubbles and crystals on the rheology of magmas. In all three cases, the experiments have some limitations because analogue materials are used. However, they also permit a systematic study of the physicochemical processes that operate in real volcanic systems.
According to present concepts, a caldera is a more or less circular volcanic depression larger than a crater which is caused by subsidence. It is commonly considered that the subsided mass consists of a block or blocks encircled by a ring fracture. Caldera collapse is generally correlated with a major explosive eruption. The present investigation is concerned with six features which do not conform well with the favored caldera model. Attention is given to downsagged calderas, the distribution of postcaldera vents in calderas, vent rings, the size of calderas and cauldrons, incremental caldera growth, and caldera-forming events. It is found that no single structural or genetic model applies to all calderas. Thus, the fact of subsidence may be the only common feature. It is pointed out that most known ring dikes occur in Precambrian crust. This may mean that the subsiding piston mechanism operates best where the crust is sufficiently rigid and strong.
A 48-mGal gravity low coincides with Long Valley caldera and is mainly attributed to low-density caldera fill. Gravity measurements by Unocal Geothermal have been integrated with U.S. Geological Survey data, vastly improving gravity station coverage throughout the caldera. A strong regional gravity trend is mainly attributed to isostasy. A ``best fitting'' (based on regional control of basement densities) Airy-Heiskanen isostatic model was used for the regional correction. A three-dimensional, multiple-unit gravity modeling program with iterative capabilities was developed to model the residual gravity. The density structure of Long Valley caldera and vicinity was modeled with 22 discrete density units, most of which were based on geologic units. Information from drill hole lithologies, surface geology, and structural geology interpretations constrain the model. Some important points revealed by the three-dimensional gravity modeling are that (1) the volume of ejected magma associated with the Bishop Tuff eruption is greater than previously thought, (2) the caldera structure is strongly influenced by precaldera topography and the extensions of major, active faults, (3) the main west ring fracture is coincident with the Inyo Domes-Mono Craters fracture system, (4) a relatively low-density region probably underlies the caldera, and (5) a silicic magma chamber may underlie Devils Postpile.
Ground movements (bradyseism) at Campi Flegrei, Italy, have been explained by a classical model that involves the intrusion of new magma to shallow depth, or by models which emphasize both the magmatic and aquifer effects. The authors describe a model for the ground deformations that involves only hydrothermal fluids, of magmatic or meteoric/marine origin, with no direct involvement of the magma, other than as a heat source. They explain the bradyseism at Campi Flegrei by a hydrothermal model, using the porphyry systems (Henley and McNabb, 1978; Burnham, 1979; Fournier, 1999) as an analogue of the Campi Flegrei system. In this view. Campi Flegrei might very well represent a modern analogue of a mineralized porphyry system, as has been demonstrated for White Island, New Zealand (Rapien et al., 2003). The authors used fluid and melt inclusion data from Campi Flegrei and other volcanoes of the Neapolitan area (Vesuvius, Ponza and Ventotene) to demonstrate the linkage with porphyry systems. Fluid inclusions in all the above volcanic systems show clear evidence of various stages of silicate melt/hydrosaline melt/aqueous fluid/CO2 immiscibility during the magmatic evolution and its transition from magmatic to hydrothermal stage, comparable to the plastic, lithostatic domain in porphyry systems. In contrast, convectively driven fluids are found only in the volcaniclastic sediments of the Campi Flegrei caldera (in the geothermal wells of San Vito and Mofete fields), and are representative of the brittle, hydrostatic domain. The coexistence of liquid-dominated and vapor-dominated inclusions in the same fluid inclusion assemblage is strong evidence of boiling conditions during inclusion trapping, whereas fluid inclusions with daughter crystals trapped in samples from deeper, hotter levels indicate a high concentration of solute (brines), as confirmed by drilling.
Quaternary volcanism and faulting in the Kenya rift valley are focused in a narrow zone along the rift axis. Beginning ~1 Ma, large trachytic volcanoes were built within an E-W to ENE-WSW extensional stress field. Volcanic fissures and associated pyroclastic cones were aligned ~N-S, parallel to the maximum horizontal stress (SHmax). Mt. Suswa, a large volcano located ~1°S latitude, collapsed sometime between 240 and 100 ka, and the resultant caldera is elongate N83°E, parallel to the minimum horizontal stress (Shmin). At the time of Suswa's collapse, or shortly thereafter, the regional stress field rotated 45° in a clockwise direction. After ~100 ka, the trachytic volcanoes in the northern rift also began large-scale caldera collapse. The calderas of Kakorinya, Silali, Emuruangogolak and Paka formed at about 92, 64, 38 and 10 ka, respectively (Dunkley et al., 1993). The calderas of these volcanoes are oriented N75°W, N58°W, N56°W and N55°W, progressively rotating toward parallelism with the new N45°W Shmin. Likewise, some of the youngest fissures and trains of small-scale cones on the flanks of the northern volcanoes are aligned NE-SW, parallel to SHmax. We suggest that the magma chambers beneath each of the trachytic volcanoes grew to an elliptical shape by stressinduced spalling of the chamber walls, analogous to the formation of breakouts in boreholes and tunnels. The caldera long axes therefore represent the time-averaged shallow crustal Shmin direction during the life of the underlying magma chambers. The actual collapse of the calderas, beginning with Suswa, may have been triggered by the sudden rotation of the stress field, which formed new fracture systems and increased the ease with which magmas could be ejected from flank eruptive centers or fissures. The link between stress field rotation and catastrophic caldera collapse may have implications for geologic risk assessment in the East African rift and other volcanically active areas.