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Effects of present-day deglaciation in Iceland on mantle melt production rates
Abstract
The ongoing deglaciation in Iceland not only causes uplift at the
surface but also decompression of the mantle below, leading to increased
magma production. Here we study glacially induced decompressional
melting using 3D models of glacial isostatic adjustment in Iceland since
1890. We find that the mean glacially induced pressure rate of change in
the mantle increases the melt production rate by 100-140%, or an
additional 0.21-0.23 km3 of magma per year across Iceland. The greatest
volumetric increase is found directly beneath the largest ice cap
Vatnajökull, co-located with the most productive Icelandic
volcanoes, where approximately 20% of the melt associated with glacial
unloading is generated. If, in addition, melts are being channeled from
the flanks of the melting region towards the central rift, up to 50% of
the additional magma might reach the base of the elastic lithosphere
beneath or close to the Vatnajökull ice cap, equivalent to more
than half of the magma volume extruded during the 2010
Eyjafjallajökull summit eruption per year. Our results are
significantly larger than previous estimates which considered only the
effect of deglaciation of Vatnajökull and mantle melting directly
beneath. Although the ongoing deglaciation in Iceland significantly
increases the melt production rate in the mantle, the increase in melt
supply rate (MSR) at the base of the lithosphere is delayed. If the melt
ascent velocity is lower than 1,000 m/yr, the additional MSR caused by
the last 120 years of deglaciation will continue to increase.
... The link between large-scale ice mass decline and an increase in volcanic eruptions at the end of the last glacial period, ca. 12 ka, is well established (Jull and McKenzie, 1996;Maclennan et al., 2002). A number of questions remain regarding the sensitivity and response time of volcanoes to smaller changes in ice mass, such as those that occur over shorter time scales (e.g., during the Holocene; Tuffen, 2010;Schmidt et al., 2013). The loading and unloading of glaciers change surface pressure and stress relationships in the crust and upper mantle (Schmidt et al., 2013). ...
... A number of questions remain regarding the sensitivity and response time of volcanoes to smaller changes in ice mass, such as those that occur over shorter time scales (e.g., during the Holocene; Tuffen, 2010;Schmidt et al., 2013). The loading and unloading of glaciers change surface pressure and stress relationships in the crust and upper mantle (Schmidt et al., 2013). Numerical models suggest that glacial unloading increases mantle melt production at depth and alters the storage capacity in the crust (Hooper et al., 2011). ...
... Given the range of response times exhibited by Icelandic glaciers to changing climate (10-1000 yr; Wastl et al., 2001) and uncertainties involved in the time taken for new melt produced in the mantle to reach the surface , a lag time of ~600 yr between climate forcing and a reduction in the frequency of volcanic activity would support the argument for the modulation of climatic forcing by glacial expansion. A significant increase in mantle melt production (100%-135%) due to deglaciation between A.D. 1890 and 2010 was modeled in Schmidt et al. (2013). The rate of ice accumulation between the HTM and 5 ka may have been of a magnitude similar to (or slower than) the current rate of ice loss since the LIA. ...
Human-induced climate change is causing rapid melting of ice in many volcanically active regions. Over glacial-interglacial time scales changes in surface loading exerted by large variations in glacier size affect the rates of volcanic activity. Numerical models suggest that smaller changes in ice volume over shorter time scales may also influence rates of mantle melt generation. However, this effect has not been verified in the geological record. Furthermore, the time lag between climatic forcing and a resultant change in the frequency of volcanic eruptions is unknown. We present empirical evidence that the frequency of volcanic eruptions in Iceland was affected by glacial extent, modulated by climate, on multicentennial time scales during the Holocene. We examine the frequency of volcanic ash deposition over northern Europe and compare this with Icelandic eruptions. We identify a period of markedly reduced volcanic activity centered on 5.5-4.5 ka that was preceded by a major change in atmospheric circulation patterns, expressed in the North Atlantic as a deepening of the Icelandic Low, favoring glacial advance on Iceland. We calculate an apparent time lag of ~600 yr between the climate event and change in eruption frequency. Given the time lag identified here, increase in volcanic eruptions due to ongoing deglaciation since the end of the Little Ice Age may not become apparent for hundreds of years.
... We apply these loads to the 2 3 2 km grid described in Schmidt et al. [2013], which includes smaller glaciers that we group with the nearest large ice cap (Table 3). We compute the displacement response to loading by approximating a homogeneous elastic halfspace using the RELAX code [Barbot and Fialko, 2010;Barbot, 2011] with a model domain that extends 2560 km in both north and east directions and 1280 km in depth, large enough that we are confident that model boundary effects do not contaminate our results. ...
... Future studies of the relationship between ice cap load variations and magma generation should include the potential feedback effects introduced by tephra deposition and changes in ice surface albedo. Many studies have demonstrated the link between deglaciation, surface uplift, and magma generation [e.g., Jull and McKenzie, 1996;Pagli and Sigmundsson, 2008;Schmidt et al., 2013], however, no such study has yet included the potential effects of accelerated ice melt due to climate warming [Compton et al., 2015] or the feedback processes introduced by volcanic tephra deposition. ...
... LikeArnad ottir et al.[2009] andSchmidt et al. [2013], we group the Eyjafjallaj€ okull and M yrdalsj€ okull ice caps and thus solve for only four loading values at any given epoch. We invert each epoch independently; inversion results are not constrained to be similar to those at the previous epoch or to follow any predetermined curve. ...
As the global climate changes, understanding short-term variations in water storage is increasingly important. Continuously operating Global Positioning System (cGPS) stations in Iceland record annual periodic motion—the elastic response to winter accumulation and spring melt seasons—with peak-to-peak vertical amplitudes over 20 mm for those sites in the Central Highlands. Here for the first time for Iceland, we demonstrate the utility of these cGPS-measured displacements for estimating seasonal and shorter-term ice cap mass changes. We calculate unit responses to each of the five largest ice caps in central Iceland at each of the 62 cGPS locations using an elastic half-space model and estimate ice mass variations from the cGPS time series using a simple least squares inversion scheme. We utilize all three components of motion, taking advantage of the seasonal motion recorded in the horizontal. We remove secular velocities and accelerations and explore the impact that seasonal motions due to atmospheric, hydrologic, and nontidal ocean loading have on our inversion results. Our results match available summer and winter mass balance measurements well, and we reproduce the seasonal stake-based observations of loading and melting within the 1 σ confidence bounds of the inversion. We identify nonperiodic ice mass changes associated with interannual variability in precipitation and other processes such as increased melting due to reduced ice surface albedo or decreased melting due to ice cap insulation in response to tephra deposition following volcanic eruptions, processes that are not resolved with once or twice-yearly stake measurements.
... GPS geodetic measurements are commonly used to measure glacial isostatic adjustment (GIA), including the viscoelastic response of the Earth due to past deglaciation [e.g., Milne et al., 2001;Johansson et al., 2002;Sella et al., 2007] and the elastic response to present-day ice loss [e.g., Grapenthin et al., 2006;Jiang et al., 2010]. GPS measurements of GIA have been used to infer the rheological properties of the Earth [Milne et al., 2001;Pagli et al., 2007;Árnadóttir et al., 2009;Schmidt et al., 2012Schmidt et al., , 2013 and to quantify the impact of GIA on processes such as decompression melting and volcanism [Pagli and Sigmundsson, 2008;Schmidt et al., 2012Schmidt et al., , 2013. With Earth's changing climate, GPS has become an increasingly useful tool in measuring the effects of modern ice loss on GIA. ...
... GPS geodetic measurements are commonly used to measure glacial isostatic adjustment (GIA), including the viscoelastic response of the Earth due to past deglaciation [e.g., Milne et al., 2001;Johansson et al., 2002;Sella et al., 2007] and the elastic response to present-day ice loss [e.g., Grapenthin et al., 2006;Jiang et al., 2010]. GPS measurements of GIA have been used to infer the rheological properties of the Earth [Milne et al., 2001;Pagli et al., 2007;Árnadóttir et al., 2009;Schmidt et al., 2012Schmidt et al., , 2013 and to quantify the impact of GIA on processes such as decompression melting and volcanism [Pagli and Sigmundsson, 2008;Schmidt et al., 2012Schmidt et al., , 2013. With Earth's changing climate, GPS has become an increasingly useful tool in measuring the effects of modern ice loss on GIA. ...
... Previous GIA modeling for Iceland has resulted in asthenospheric viscosity estimates of 4-10 × 10 18 Pa s [Pagli et al., 2007;Auriac et al., 2013] and 1 × 10 19 Pa s [Árnadóttir et al., 2009;Schmidt et al., 2012Schmidt et al., , 2013. With low-viscosity estimates, the viscous response to unloading over a time period of decades becomes an important component to understanding uplift. ...
Earth's present-day response to enhanced glacial melting resulting from climate change can be measured using Global Positioning System (GPS) technology. We present data from 62 continuously operating GPS instruments in Iceland. Statistically significant upward velocity and accelerations are recorded at 27 GPS stations, predominantly located in the Central Highlands region of Iceland, where present-day thinning of the Iceland ice caps results in velocities of more than 30 mm/yr and uplift accelerations of 1-2 mm/yr2. We use our acceleration estimates to back-calculate to a time of zero velocity, which coincides with the initiation of ice loss in Iceland from ice mass balance calculations and Arctic warming trends. We show, through a simple inversion, a direct relationship between ice mass balance measurements and vertical position and show that accelerated unloading is required to reproduce uplift observations for a simple elastic layer over viscoelastic half-space model.
... This has been demonstrated by thermo-mechanical modelling (e.g. Jull and McKenzie, 1996;Schmidt et al., 2013;Rees Jones and Rudge, 2020) [well understood], and comparable deglaciation-driven trends following the Last Glacial Maximum have been identified in regional and global eruption records (e.g. Nowell et al., 2006;Huybers and Langmuir, 2009;Lin et al., 2022) [well understood]. ...
... Nowell et al., 2006;Huybers and Langmuir, 2009;Lin et al., 2022) [well understood]. The predicted decompression rates of glacial isostatic adjustment can be estimated as a function of depth in the melting region, allowing estimation of melt production rates due to deglaciation, which is currently expected to be of the same order of magnitude as tectonic melt production in Iceland (Sigmundsson et al. 2013;Schmidt et al. 2013). This phenomenon may be less pronounced for the thicker lithosphere and flux-melting regimes of arc systems (Watt et al, 2013), but there is evidence that arc volcanoes show temporary postglacial increases in eruption rate and the eruption of more Fig. 1 Schematics illustrating climate-volcano impacts associated with pre-eruptive processes ("Climate-volcano impacts affecting pre-eruptive processes" section) and how they are expected to unfold in the context of a warming climate evolved magmas (Rawson et al., 2016). ...
The impacts of volcanic eruptions on climate are increasingly well understood, but the mirror question of how climate changes affect volcanic systems and processes, which we term “climate-volcano impacts”, remains understudied. Accelerating research on this topic is critical in view of rapid climate change driven by anthropogenic activities. Over the last two decades, we have improved our understanding of how mass distribution on the Earth’s surface, in particular changes in ice and water distribution linked to glacial cycles, affects mantle melting, crustal magmatic processing and eruption rates. New hypotheses on the impacts of climate change on eruption processes have also emerged, including how eruption style and volcanic plume rise are affected by changing surface and atmospheric conditions, and how volcanic sulfate aerosol lifecycle, radiative forcing and climate impacts are modulated by background climate conditions. Future improvements in past climate reconstructions and current climate observations, volcanic eruption records and volcano monitoring, and numerical models all have a role in advancing our understanding of climate-volcano impacts. Important mechanisms remain to be explored, such as how changes in atmospheric circulation and precipitation will affect the volcanic ash life cycle. Fostering a holistic and interdisciplinary approach to climate-volcano impacts is critical to gain a full picture of how ongoing climate changes may affect the environmental and societal impacts of volcanic activity.
... However, Jull and McKenzie (1996) estimate that the removal of 2 km of ice would increase the melt fraction by approximately 0.2%, though the increase in melt generation as a response to unloading is non-linear. Schmidt et al. (2013) go further, suggesting that the Icelandic uplift due to glacial isostatic adjustments (currently estimated at 25-29 mm/yr (Auriac et al., 2013)) has resulted in an annual melt production increase of 100-135% since 1890. Much of this new material is believed to be concentrated beneath central Iceland, with approximately 20% located in the mantle beneath Vatnajökull, a region containing some of the island's most productive volcanic centres, such as Grímsvötn . ...
... Earth-Science Reviews 177 (2018) 238-247 offset by intrusive processes and glacio-isostatic stresses, the study ultimately asserts that the likelihood of a large volcanic eruption within the region is increased by the retreat of the overlying glacier. The results of Jellinek et al. (2004), Albino et al. (2010) and Schmidt et al. (2013) echo this inference. ...
Developing a comprehensive understanding of the interactions between the atmosphere and the geosphere is an ever-more pertinent issue as global average temperatures continue to rise. The possibility of more frequent volcanic eruptions and more therefore more frequent volcanic ash clouds raises potential concerns for the general public and the aviation industry. This review describes the major processes involved in short- and long-term volcano–climate interactions with a focus on Iceland and northern Europe, illustrating a complex interconnected system, wherein volcanoes directly affect the climate and climate change may indirectly affect volcanic systems. In this paper we examine both the effect of volcanic inputs into the atmosphere on climate conditions, in addition to the reverse relationship – that is, how global temperature fluctuations may influence the occurrence of volcanic eruptions. Explosive volcanic eruptions can cause surface cooling on regional and global scales through stratospheric injection of aerosols and fine ash particles, as documented in many historic eruptions, such as the Pinatubo eruption in 1991. The atmospheric effects of large-magnitude explosive eruptions are more pronounced when the eruptions occur in the tropics due to increased aerosol dispersal and effects on the meridional temperature gradient. Additionally, on a multi-centennial scale, global temperature increase may affect the frequency of large-magnitude eruptions through deglaciation. Many conceptional models use the example of Iceland to suggest that post-glacial isostatic rebound will significantly increase decompression melting, and may already be increasing the amount of melt stored beneath Vatnajökull and several smaller Icelandic glaciers. Evidence for such a relationship existing in the past may be found in cryptotephra records from peat and lake sediments across northern Europe. At present, such records are incomplete, containing spatial gaps. As a significant increase in volcanic activity in Iceland would result in more frequent ash clouds over Europe, disrupting aviation and transport, developing an understanding of the relationship between the global climate and volcanism will greatly improve our ability to forecast and prepare for future events.
... Several studies document peaks of igneous activity worldwide during late stages of the last deglaciation and the current interglacial ( Figure 1) [Hardarson and Fitton, 1991;Sigvaldason et al., 1992;Zielinski et al., 1994Zielinski et al., , 1996Huybers and Langmuir, 2009], in turn suggesting that climate oscillations [Lisiecki and Raymo, 2007] affect the melting of the Earth's interior. A deglacial-triggering hypothesis [Hardarson and Fitton, 1991], according to which continental lithospheric unloading owing to the melting of ice caps during the transition to interglacials leads to enhanced magma production (which increases by~1% for 1 kbar of pressure decrease if the mantle is at near-solidus temperatures [McKenzie, 1984]), is currently the most accredited explanation for this correspondence [Jull and McKenzie, 1996;Singer et al., 1997;Slater et al., 1998;Huybers and Langmuir, 2009;Hooper et al., 2011;Schmidt et al., 2013]. The impact of climate warming on magma production, however, has been evaluated regardless of surface erosion although the density of upper crustal rocks and sediments exceeds that of ice by approximately 3 times. ...
... Yet they provide a fundamental observational basis on which to calibrate numerical results. In this study, we assume that the Earth possesses a linear rheology, thereby its response scales linearly to the magnitude of the surface loading/unloading [Peltier, 1974;Wu and Hasegawa, 1996;Wu and van der Wal, 2003;Schmidt et al., 2013] and the relative contribution to the decompression melting rates from glaciers growth/melting and glacial erosion can be directly accessed. Assuming a time-invariant geothermal gradient and neglecting lithospheric and sublithospheric deviatoric stresses, variations of the melt productivity at any given depth due to ice building/melting and glacial erosion can be inferred from surface loading/unloading beneath an eroding ice sheet responding to a given climate signal, mimicking a shift between glacial and interglacial conditions. ...
Glacial-interglacial cycles affect the processes through which water and rocks are redistributed across the Earth's surface, thereby linking the solid Earth and climate dynamics. Regional and global scale studies suggest that continental lithospheric unloading due to ice melting during the transition to interglacials leads to increased continental magmatic, volcanic, and degassing activity. Such a climatic forcing on the melting of the Earth's interior, however, has always been evaluated regardless of continental unloading by glacial erosion, albeit the density of rock exceeds that of ice by approximately 3 times. Here we present and discuss numerical results involving synthetic but realistic topographies, ice caps, and glacial erosion rates suggesting that erosion may be as important as deglaciation in affecting continental unloading. Our study represents an additional step toward a more general understanding of the links between a changing climate, glacial processes, and the melting of the solid Earth. Â
... Several studies document peaks of igneous activity worldwide during late stages of the last deglaciation and the current interglacial ( Fig. 1) [Hardarson and Fitton, 1991;Sigvaldason et al., 1992;Zielinski et al., 1994;Zielinski et al., 1996;Huybers and Langmuir, 2009], in turn suggesting that climate oscillations [Lisiecky and Raymo, 2007] affects the melting of the Earth's interior. A deglacial-triggering hypothesis [Hardarson and Fitton, 1991], according to which continental lithospheric unloading owing to the melting of ice-caps during the transition to interglacials leads to enhanced magma production (which increases by ~1% for 1kbar of pressure decrease if the mantle is at near-solidus temperatures [McKenzie, 1984]), is currently the most accredited explanation for this correspondence [Jull and McKenzie, 1996;Singer et al., 1997;Slater et al., 1998;Huybers and Langmuir, 2009;Hooper et al., 2011;Schmidt et al., 2013]. The impact of climate warming on magma production, however, has been evaluated regardless of surface erosion although the density of upper crustal rocks and sediments exceeds that of ice by approximately three times. ...
... Yet, they provide a fundamental observational basis on which to calibrate numerical results. In this study, we assume that the Earth posses a linear rheology, thereby its response scales linearly to the magnitude of the surface loading/unloading [Peltier, 1974;Wu and Hasegawa, 1996;Wu and van der Wal, 2003;Schmidt et al., 2013] and the relative contribution to the decompression melting rates from glaciers growth/melting and glacial erosion can be directly accessed. Assuming a time invariant geothermal gradient and neglecting lithospheric and sub-lithospheric deviatoric stresses, variations of the melt productivity at any given depth due to ice building/melting and glacial erosion can be inferred from surface loading/unloading beneath an eroding ice sheet responding to a given climate signal, mimicking a shift between glacial and interglacial conditions. ...
Glacial-interglacial cycles affect the processes through which water and rocks are redistributed across the Earth's surface, thereby linking the solid Earth and climate dynamics. Regional and global scale studies suggest that continental lithospheric unloading due to ice melting during the transition to interglacials leads to increased continental magmatic, volcanic and degassing activity. Such a climatic forcing on the melting of the Earth's interior, however, has always been evaluated regardless of continental unloading by glacial erosion, albeit the density of rock exceeds that of ice by approximately three times. Here, we present and discuss numerical results involving synthetic but realistic topographies, ice caps and glacial erosion rates suggesting that erosion may be as important as deglaciation in affecting continental unloading. Our study represents an additional step towards a more general understanding of the links between a changing climate, glacial processes and the melting of the solid Earth.
... The data were projected into an ITRF08 Eurasian fixed reference frame (Altamimi et al., 2012) and corrected for GIA model predictions for a layered earth model based on ice history of the four major glaciers in Iceland since 1890 (Árnadóttir et al., 2009;Schmidt et al., 2013, Peter Schmidt personal communication 2014. Árnadóttir et al. (2009) used vertical deformation rates estimated from the ÍSNET nationwide GPS campaigns in 1993 and 2004 to evaluate the optimal earth parameters for their GIA model. ...
... This contraction signal was also Time series for N-and E-components from selected GPS stations around Eyjafjallajökull from 1992 to May 2009. The data are in Eurasian fixed ITRF08 reference frame, corrected for GIA according toSchmidt et al. (2013; Peter Schmidt, personal communication) (scaling horizontal correction by 1.6) and coseismic offset due to the June 2000 SISZ earthquakes fromPedersen et al., (2003). Dashed lines show approximate trends based on available data. ...
The ice-capped Eyjafjallajökull volcano, south Iceland, had been dormant for 170 years when the first signs of reawakening of the volcano were captured by seismic and geodetic measurements in 1994. These were the first clear observed signs of unrest followed by 16years of intermittent magmatic unrest culminating in 2010 when two eruptions broke out on the flank and at the summit. We analyze seismic data from 1991 through 2008 and GPS data from 1992 to May 2009 to infer magma movements beneath the volcano. The relocated earthquakes reveal an overall pipe-like pattern northeast of the summit crater, sporadically mapping the pathway of magma from the base of the crust towards an intrusion in the upper crust. During the study period, three major seismic swarms were recorded. Two of them, in 1994 and 1999-2000, occurred in the upper and intermediate crust and accompanied crustal deformation centered at the southeastern flank. No uplift was detected during the 19- to 25-km-deep 1996 swarm, near the crust-mantle boundary, but the horizontal, ~E-W oriented T-axes indicate a period of tension/opening, suggesting magma intruding up into the base of the crust. The GPS measured deformation during 1999-2000 can be modeled as intrusion of a horizontal, circular sill with volume of 0.030±0.007km3 at 5.0±1.3km depth. The less constrained 4.5- to 5-km-deep sill model for the 1994 episode indicates a three times smaller intruded volume (0.011km3) than during 1999-2000. In the years between/following the intrusions, contraction was observed at the southeastern flank. The contraction from 2000.5 to 2009.3 can be fitted by a circular sill model with a volume contraction of -0.0015±0.0003km3/year at 5.5±2.0km depth. The less well constrained model for 1994.7 to 1998.6 gives a volume contraction of -(0.0009-0.0010) km3 at a fixed depth of 5km. The accumulated volume changes (~-0.013km3 for the second period, ~0.0037km3 for the first period) are much larger than expected due to solidification and cooling of magma alone and might partly be explained by the underestimated volume of intruded magma, mass loading effects within the crust due to the intruded magma and possibly, and to less extent, degassing (CO2).
... Glacial-deglacial cycles can influence the rate of decompression melting of magma (Huybers and Langmuir, 2009;Jull and McKenzie 1996;Slater et al., 1998;Schmidt et al., 2013;Maclannan et al., 2002). During glaciation ice weight results in lithospheric flexure, preventing adiabatic decompression melting of magma, Slater et al., (1998) explains that mantle upwelling is halted until the solidus pressure state is returned. ...
... During glaciation ice weight results in lithospheric flexure, preventing adiabatic decompression melting of magma, Slater et al., (1998) explains that mantle upwelling is halted until the solidus pressure state is returned. When deglaciation begins during an interglacial phase, pressure reductions through ice mass loss allows for lithospheric rebound, temporarily increasing the rate of magma production through melt generation by ~100-140% (Schmidt et al., 2013). ...
Future climatic change as a result of anthropogenic climate warming may potentially promote the occurrence of geological phenomena. This paper critically reviews and evaluates existing literature which seeks to determine causal links between climate change and increased geological activity. This paper builds upon the work of McGuire (2010) in order to develop an unbiased evaluation of the extent to which contemporary climate change may alter the frequency of geological events. Patterns of climatic change associated with volcanic, seismic and submarine landslide occurrences during the late Quaternary are discussed, followed by a critical review of the mechanism by which climate change may alter geological processes. This paper further identifies aleatory, epistemic and ontological uncertainties which exist and impede our understanding of current climate science and geological system responses. The extent to which contemporary climate change may trigger a geological response remains uncertain and controversial, as assuming uniformitarianism, it is likely that the frequency of volcanic, seismic and submarine landslide events may increase in response to climate changes. In order for the true extent to be accurately determined, uncertainties must be reduced, for which areas of further study have been identified and recommended.
... As far as volcanic activity is concerned, the loading and unloading of glaciers may change the surface pressure and stress relationships in the crust and upper mantle (Schmidt et al., 2013). These changes in surface loading exerted by large variations in glacier size are known to affect the rates of volcanic activity during glacial-interglacial transitions. ...
... Volcanic stratospheric sulfur injections (VSSI) from global volcanic activity, summed over centuries, have varied by an order of magnitude between the highly active 13th century -marking the inception of the Little Ice Age -and the 1st century CE . Even larger variations have likely occurred during the warm Early Holocene, when the rapid melting of large ice sheets during deglaciation regionally triggered a strong acceleration in volcanic activity (Maclennan et al., 2002;Sigmundsson et al., 2010;Watt et al., 2013) through feedback chains that may also operate during the 21st and 22nd centuries with projected changes in the cryosphere under global warming (Schmidt et al., 2013;Tuffen, 2010). Understanding how future volcanic activity may affect climate is strongly dependent on understanding the statistical nature of volcanic activity: its variability and the degree of temporal clustering of eruptions (Bethke et al., 2017;Man et al., 2021;Tuel et al., 2017). ...
The injection of sulfur into the stratosphere by volcanic eruptions is the dominant driver of natural climate variability on interannual to multidecadal timescales. Based on a set of continuous sulfate and sulfur records from a suite of ice cores from Greenland and Antarctica, the HolVol v.1.0 database includes estimates of the magnitudes and approximate source latitudes of major volcanic stratospheric sulfur injection (VSSI) events for the Holocene (from 9500 BCE or 11 500 years BP to 1900 CE), constituting an extension of the previous record by 7000 years. The database incorporates new-generation ice-core aerosol records with a sub-annual temporal resolution and a demonstrated sub-decadal dating accuracy and precision. By tightly aligning and stacking the ice-core records on the WD2014 chronology from Antarctica, we resolve long-standing inconsistencies in the dating of ancient volcanic eruptions that arise from biased (i.e., dated too old) ice-core chronologies over the Holocene for Greenland. We reconstruct a total of 850 volcanic eruptions with injections in excess of 1 teragram of sulfur (Tg S); of these eruptions, 329 (39 %) are located in the low latitudes with bipolar sulfate deposition, 426 (50 %) are located in the Northern Hemisphere extratropics (NHET) and 88 (10 %) are located in the Southern Hemisphere extratropics (SHET). The spatial distribution of the reconstructed eruption locations is in agreement with prior reconstructions for the past 2500 years. In total, these eruptions injected 7410 Tg S into the stratosphere: 70 % from tropical eruptions and 25 % from NH extratropical eruptions. A long-term latitudinally and monthly resolved stratospheric aerosol optical depth (SAOD) time series is reconstructed from the HolVol VSSI estimates, representing the first Holocene-scale reconstruction constrained by Greenland and Antarctica ice cores. These new long-term reconstructions of past VSSI and SAOD variability confirm evidence from regional volcanic eruption chronologies (e.g., from Iceland) in showing that the Early Holocene (9500–7000 BCE) experienced a higher number of volcanic eruptions (+16 %) and cumulative VSSI (+86 %) compared with the past 2500 years. This increase coincides with the rapid retreat of ice sheets during deglaciation, providing context for potential future increases in volcanic activity in regions under projected glacier melting in the 21st century. The reconstructed VSSI and SAOD data are available at https://doi.org/10.1594/PANGAEA.928646 (Sigl et al., 2021).
... Volcanic stratospheric sulfur injections (VSSI) from global volcanic activity, summed over centuries, have varied by an order of magnitude between the highly active 13 th centurymarking the inception of the Little Ice Ageand the 1 st century AD (Toohey and Sigl, 2017). Even larger variations have likely occurred during the warm early Holocene, when the rapid melting of large ice sheets during deglaciation regionally triggered a strong acceleration in volcanic activity (Maclennan et al., 2002;Sigmundsson et al., 2010;Watt et al., 2013) through feedback chains that may also operate 95 during the 21 st and 22 nd centuries with projected changes of the cryosphere under global warming (Schmidt et al., 2013;Tuffen, 2010). Understanding of how future volcanic activity may affect climate is strongly dependent on understanding the statistical nature of volcanic activity: its variability and the degree of temporal clustering of eruptions (Bethke et al., 2017;Man et al., 2021;Tuel et al., 2017). ...
The injection of sulfur into the stratosphere by volcanic eruptions is the dominant driver of natural climate variability on interannual-to-multidecadal timescales. Based on a set of continuous sulfate and sulfur records from a suite of ice cores from Greenland and Antarctica, the HolVol v.1.0 database includes estimates of the magnitudes and approximate source latitudes of major volcanic stratospheric sulfur injection (VSSI) events for the Holocene (from 9500 BCE or 11500 year BP to 1900 CE), constituting an extension of the previous record by 7000 years. The database incorporates new-generation ice-core aerosol records with sub-annual temporal resolution and demonstrated sub-decadal dating accuracy and precision. By tightly aligning and stacking the ice-core records on the WD2014 chronology from Antarctica we resolve long-standing previous inconsistencies in the dating of ancient volcanic eruptions that arise from biased (i.e. dated too old) ice-core chronologies over the Holocene for Greenland. We reconstruct a total of 850 volcanic eruptions with injections in excess of 1 TgS, of which 329 (39 %) are located in the low latitudes with bipolar sulfate deposition, 426 (50 %) are located in the Northern Hemisphere (NH) extratropics and 88 (10 %) are located in the Southern Hemisphere (SH) extratropics. The spatial distribution of reconstructed eruption locations is in agreement with prior reconstructions for the past 2,500 years, and follows the global distribution of landmasses. In total, these eruptions injected 7410 TgS in the stratosphere, for which tropical eruptions accounted for 70 % and NH extratropics for 25 %. A long-term latitudinally and monthly resolved stratospheric aerosol optical depth (SAOD) time series is reconstructed from the HolVol VSSI estimates, representing the first Holocene-scale reconstruction constrained by Greenland and Antarctica ice cores. These new long-term reconstructions of past VSSI and SAOD variability confirm evidence from regional volcanic eruption chronologies (e.g., from Iceland) in showing that the early Holocene (9500–7000 BCE) experienced a higher number of volcanic eruptions (+16 %) and cumulative VSSI (+86 %) compared to the past 2,500 years. This increase coincides with the rapid retreat of ice sheets during deglaciation, providing context for potential future increases of volcanic activity in regions under projected glacier melting in the 21st century. The reconstructed VSSI and SAOD data are available at https://doi.pangaea.de/10.1594/PANGAEA.928646 (Sigl et al., 2021).
... Crustal stress changes (i.e. isostatic adjustment) have been proposed as a key driver of volcanic activity across glacial-interglacial transitions since magma reservoirs, lithospheric pathways and mantle melt production rates are sensitive (Schmidt et al. 2013;Crowley et al. 2015) to unloading (i.e. pressure release) leading to increased eruptive flux and/or shifts in composition, though lag times between deglaciation and enhanced volcanism are spatially and temporally variable, and may be modulated by local geologic controls (Schindlbeck et al. 2018). ...
We describe the stratigraphy, age, geochemistry and correlation of tephra from west to east across the northern Patagonian Andes (c. 40–41°S) with a view to further refining the eruptive history of this region back to the onset of the Last Glacial Termination (~18 cal. ka). Eastwards across the Andes, rhyodacite to rhyolitic tephra markers of dominantly Puyehue-Cordón Caulle source are persistently recognised and provide a stratigraphic context for more numerously erupted intervening tephra of basalt to basaltic–andesite composition. Tephra from distal eruptive centres are also recognised.
West of the Andean Cordillera, organic-rich cores from a small closed lake basin (Lago Pichilafquén) reveal an exceptional high-resolution record of lowland vegetation–climate change and eruptive activity spanning the last 15 400 years. Three new rhyodacite tephra (BT6-T1, -T2 and -T4) identified near the base of the Pichilafquén record, spanning 13.2 to 13.9 cal. ka bp, can be geochemically matched with correlatives in basal andic soil sequences closely overlying regolith and/or basement rock. The repetitiveness of this tephrostratigraphy across this Andean transect suggests near-synchronous tephra accretion and onset of up-building soil formation under more stable (revegetating) ground-surface conditions following rapid piedmont deglaciation on both sides of the Cordillera by at least ~14 cal. ka bp.
... While an increasing number of studies are examining links between global climate and volcanism (Huybers and Langmuir, 2009;Watt et al., 2013;Edwards et al., 2020), most of the hypotheses are still being tested. In Iceland, Schmidt et al. (2013) have proposed that rapid late Holocene deglaciation may be contributing to magma generation at some volcanoes, while preliminary studies from Chile have so far not found evidence for connections between glaciations and melt production but have suggested that changes in crustal stresses could impact tapping of magmas stored in the crust (Rawson et al., 2016). The potential impacts for volcanic hazards are mixed as rising air temperatures lead to a negative glacier mass balance. ...
We created a global database of glacierized volcanoes, using a projection optimized for each volcano, to identify locations where land ice (glaciers and ice sheets) and volcanoes co-exist on Earth. Our spatial database melds the Smithsonian Global Volcanism Database (SGVD) and the Randolph Glacier Inventory 6.0 (RGI). We identified all Holocene volcanoes within the SGVD that have glacier ice within radii of 1 km, 2.5 km, and 5 km, and thus have the potential to impact or be impacted by surrounding ice. Our analysis shows that 245 Holocene volcanoes have glacier ice within the specified radii, which are covered partly or fully by 2584 unique glaciers or the Antarctic Ice Sheet. The volcanoes are located in all major volcano-tectonic settings, although the majority (72%) are in subduction zones built on continental crust (greater than 25 km thick). They also cover the majority of the typical compositional ranges for igneous rocks (basalt to rhyolite). Twenty-nine volcanoes, or 12%, have at least 90% ice cover within 5 km, which together comprise 36% of global glacier area on volcanoes. About 20,000 people live within 5 km of a glacierized volcano, while 160 million people live within 100 km of a glacierized volcano and could be impacted by lahars and/or disruption of their water sources during future eruptions. By merging our database with existing ice thickness model estimates we find 850 ± 290 km 3 of ice within 5 km of volcanic vents globally. We compare the eruption history, ice volume, and nearby population estimates to identify the most dangerous volcanoes on Earth. The combination of volcano locations and ice thickness estimates allows us to identify 20 (out of 245) glacierized volcanoes that are most likely to experience 'thick' ice eruptions, while the vast majority are more likely to experience 'thin' ice eruptions.
... Understanding the causality between volcanic eruptions and climate-related growth and decay of glaciers is important for improving eruption forecasting, managing volcanic hazards and for using volcanoes as an unbiased record of paleoenvironment. Ideas concerning climate-related forcing of volcanism are embedded in the literature (Glazner et al., 1999;Grove, 1974;Jellinek et al., 2004;Maclennan et al., 2002;Mathews, 1958;Mora and Tassara, 2019;Schmidt et al., 2013;Sternai et al., 2016), however, such ideas and models have not been investigated for many tectonic environments. ...
Glacial loading and unloading can influence the rates and timing of magmatism in tectonic settings where the lithosphere is relatively thin and hot. In continental arcs, where the lithosphere is substantially thicker, nominal loading by surface glaciers is unlikely to affect the production of subduction-related magmas. However, geologic evidence suggests that a causal linkage between volcanism and glaciations in arcs does exist. The last deglaciation of the Garibaldi volcanic belt in southwestern British Columbia, Canada saw a ∼5-fold volumetric increase in volcanism over the mean Pleistocene (background) rate that coincided with ∼400 m of rapid postglacial crustal uplift. Here, we use a cylindrical, thin plate bending approximation to calculate the horizontally oriented stresses developed in the crust during glacial loading/unloading (up to ±12 MPa). The distribution of compressive and extensional stresses is shown to modulate magma transport and storage by facilitating or suppressing magma ascent by dike propagation. We use Monte Carlo simulations to track the depth of upward migrating magma bodies in the crust over a single glacial cycle (i.e., during crustal depression and rebound). During loading, volcanism is suppressed as magma becomes trapped by compressive deviatoric stress in the upper part of the crust. During unloading, relaxation of this stress acts to release accumulated magma from the upper crust and causes an increase in eruption rate immediately following deglaciation. This model of "glacial pumping" of a magma-charged lithosphere suggests a causal linkage between volcanism and glaciation in volcanic arcs that explains the persistent observation that the volume and timing of volcanism is correlated with the major climate cycles.
... Historic records (see Gudmundsson 1987;Haraldsson 2012;Höskuldsson et al. 2013) suggest a volcanic eruption in Iceland approximately once every five years. Recent studies point to the possibility of increased eruption frequency due to less pressure from melting glaciers (Pearce 2012;Schmidt et al. 2013;Compton et al. 2015), posing a greater threat to the local population. The particles in volcanic ash vary in size and weight. ...
... In combination with measurements of the changing REE composition of erupted basalts, the most recent estimate is of a magma velocity of about 100 m/yr. The effect of the deglaciation of Iceland on mantle melting may be important for CO 2 outgassing over the past 120 kyr [9] and is thought to have an ongoing influence [13]. ...
Partial melting of asthenospheric mantle generates magma that supplies volcanic systems. The timescale of melt extraction from the mantle has been hotly debated. Microstructural measurements of permeability typically suggest relatively slow melt extraction (1 m/yr) whereas geochemical (Uranium-decay series) and geophysical observations suggest much faster melt extraction (100 m/yr). The deglaciation of Iceland triggered additional mantle melting and magma flux at the surface. The rapid response has been used to argue for relatively rapid melt extraction. However, at least to some extent, this unusual episode must be unrepresentative. We develop a one-dimensional, time-dependent and nonlinear (far from steady-state), model forced by deglaciation. For the most recent, and best mapped, Icelandic deglaciation, 30 m/yr is the best estimate of melt velocity. This is a factor of about 3 smaller than previously claimed, but still relatively fast. We translate these estimates to other mid-ocean ridges and find that fast melt extraction prevails.
... Between 1995 and 2013, Iceland experienced −9.5 ± 1.5 Gt/yr of average ice mass change, with negative acceleration (meaning an increasing rate of mass loss) (Björnsson et al., 2013). In Iceland, ice mass changes and volcanic activity are linked, as unloading caused by ice melt can affect the frequency and character of volcanic eruptions (e.g., Jull and McKenzie, 1996;Gee et al., 1998;Pagli and Sigmundsson, 2008;Schmidt et al., 2013). Detailed and accurate measurements of glacier mass balance are therefore important to understand not only sea level rise, but also volcanic activity in Iceland and potential international consequences. ...
The Gravity Recovery and Climate Experiment (GRACE) satellites have measured anomalies in the Earth's time-variable gravity field since 2002, allowing for the measurement of the melting of glaciers due to climate change. Many techniques used with GRACE data have difficulty constraining mass change in small regions, such as Iceland, often requiring broad averaging functions in order to capture trends. These techniques also capture data from nearby regions, causing signal leakage. Alternatively, Slepian functions may solve this problem by optimally concentrating data both in the spatial domain (e.g., Iceland) and spectral domain (i.e., the bandwidth of the data). We use synthetic experiments to show that Slepian functions can capture trends over Iceland without meaningful leakage and influence from ice changes in Greenland. We estimate a mass change over Iceland from GRACE data of approximately -9.3 ± 1.0 Gt/yr between March 2002 and November 2016, with an acceleration of 1.1 ± 0.5 Gt/yr2.
... Magma generation here occurs due to pressure-release melting, as the mantle upwells beneath rift zones. Although the net change in overburden pressures from variations in ice cover have been relatively small, the high rates of change associated with glacial activity can produce significant short-term fluctuations in magmatic output (Jull & McKenzie, 1996;Pagli & Sigmundsson, 2008;Schmidt et al., 2013). Carbon readily partitions into magmas during partial melting (Rosenthal et al., 2015) and is released as a CO 2 -rich fluid/vapor as the magma ascends through the crust, making volcanism the primary pathway for transporting carbon from the Earth's mantle to the atmosphere (Dasgupta & Hirschmann, 2010). ...
Climate cycles may significantly affect the eruptive behavior of terrestrial volcanoes due to pressure changes caused by glacial loading, which raises the possibility that climate change may modulate CO2 degassing via volcanism. In Iceland, magmatism is likely to have been influenced by glacial activity. To explore if deglaciation therefore impacted CO2 flux we coupled a model of glacial loading over the last ∼120 ka to melt generation and transport. We find that a nuanced relationship exists between magmatism and glacial activity. Enhanced CO2 degassing happened prior to the main phase of late‐Pleistocene deglaciation, and it is sensitive to the duration of the growth of the ice sheet entering into the LGM, as well as the rate of ice loss. Ice sheet growth depresses melting in the upper mantle, creating a delayed pulse of CO2 out‐gassing as the magmatic system recovers from the effects of loading.
... where the volume integration is evaluated over the domain V and X is the degree of melting by volume fraction, which can be related to the degree of melting by weight fraction F by (McKenzie (1984); Schmidt et al. (2013)) ...
Observations of the time lag between the last deglaciation and a surge in volcanic activity in Iceland constrain the average melt ascent velocity to be $\geq50$ $\mathrm{m/yr}$. Although existing theoretical work has explained why the surge in eruption rates increased $5$-$30$ fold from the steady-state rates during the last deglaciation, they cannot account for large variations of Rare Earth Element (REE) concentrations in the Icelandic lavas. Lavas erupted during the last deglaciation are depleted in REEs by up to $70\%$; whereas, existing models, which assume instantaneous melt transport, can only produce at most $20\%$ depletion. Here, we develop a numerical model with finite melt ascent velocity and show that the variations of REEs are strongly dependent on the melt ascent velocity. When the average melt ascent velocity is $100$ $\mathrm{m/yr}$, the variation of $\mathrm{La}$ calculated by our model is comparable to that of the observations. In contrast, when the melt ascent velocity is $1,000$ $\mathrm{m/yr}$ or above, the model variation of $\mathrm{La}$ becomes significantly lower than observed, which explains why previous models with instantaneous melt transport did not reproduce the large variations. We provide the first model that takes account of the diachronous response of volcanism to deglaciation. We show by comparing our model calculations of the relative volumes of different eruption types (subglacial, finiglacial and postglacial) and the timing of the bursts in volcanic eruptions with the observations across different volcanic zones that the Icelandic average melt ascent velocity during the last deglaciation is likely to be $\sim100$ $\mathrm{m/yr}$.
... The first model suggests a mass gain over Iceland due to GIA, while the second suggests a mass loss. Schmidt et al. (2013) uses a 3-D model of GIA in Iceland since largest ice cap and the section of mantle beneath it. Sørensen et al. (2017) improves on prior models of GIA in Iceland by incorporating both GRACE and relative sea level (RSL) data, and their model predicts a pattern consistent with that predicted for PGR using the ICE-5G history. ...
The Gravity Recovery and Climate Experiment (GRACE) satellites have measured anomalies in the Earth’s time-variable gravity field since 2002, allowing for the measurement of the melting of glaciers due to climate change. Many techniques used with GRACE data have difficulty constraining mass change in small regions such as Iceland, often requiring broad averaging functions in order to capture trends. These techniques also capture data from nearby regions, causing signal leakage. Alternatively, Slepian functions may solve this problem by optimally concentrating data both in the spatial domain (e.g., Iceland) and spectral domain (i.e., the bandwidth of the data). In this project, we use synthetic experiments to show that Slepian functions can capture trends over Iceland without meaningful leakage and influence from ice changes in Greenland. We estimate a mass change over Iceland from GRACE data of approximately -9.68 ± 0.99 Gt/yr between January 2002 andNovember 2016, with an acceleration of 1.07 ± 0.50 Gt/yr^2 .
... GIA potentially plays a role in modulating climate cycles. Postglacial rebound acts to reduce the pressure in the mantle, and this has been implicated in promoting terrestrial volcanism (Sigmundsson et al., 2010;Schmidt et al., 2013;Praetorius et al., 2016). Huybers and Langmuir (2009) argue that the CO 2 release associated with increased volcanism during the last deglaciation may have been sufficient to promote further ice melt, raising the possibility that glacial rebound, CO 2 release, and ice dynamics are part of a positive feedback loop. ...
Glacial isostatic adjustment (GIA)
describes the response of the solid Earth, the gravitational field, and the
oceans to the growth and decay of the global ice sheets. A commonly studied
component of GIA is postglacial rebound, which specifically relates to
uplift of the land surface following ice melt. GIA is a relatively rapid
process, triggering 100 m scale changes in sea level and solid Earth
deformation over just a few tens of thousands of years. Indeed, the
first-order effects of GIA could already be quantified several hundred years
ago without reliance on precise measurement techniques and scientists have
been developing a unifying theory for the observations for over 200 years.
Progress towards this goal required a number of significant breakthroughs to
be made, including the recognition that ice sheets were once more extensive,
the solid Earth changes shape over time, and gravity plays a central role in
determining the pattern of sea-level change. This article describes the
historical development of the field of GIA and provides an overview of the
processes involved. Significant recent progress has been made as concepts
associated with GIA have begun to be incorporated into parallel fields of
research; these advances are discussed, along with the role that GIA is
likely to play in addressing outstanding research questions within the field
of Earth system modelling.
... 12,16 ). Modeling by Schmidt et al. 40 suggests that the magnitude of pressure drop during deglaciation would be sufficient to cause significant increase in mantle melt production. Consequently, while ice unloading would enhance volcanism at high latitudes, elevated sea level would reduce volcanism at low latitudes. ...
It is a longstanding observation that the frequency of volcanism periodically changes at times of global climate change. The existence of causal links between volcanism and Earth's climate remains highly controversial, partly because most related studies only cover one glacial cycle. Longer records are available from marine sediment profiles in which the distribution of tephras records frequency changes of explosive arc volcanism with high resolution and time precision. Here we show that tephras of IODP Hole U1437B (northwest Pacific) record a cyclicity of explosive volcanism within the last 1.1 Myr. A spectral analysis of the dataset yields a statistically significant spectral peak at the ~100 kyr period, which dominates the global climate cycles since the Middle Pleistocene. A time-domain analysis of the entire eruption and δ18O record of benthic foraminifera as climate/sea level proxy shows that volcanism peaks after the glacial maximum and ∼13 ± 2 kyr before the δ18O minimum right at the glacial/interglacial transition. The correlation is especially good for the last 0.7 Myr. For the period 0.7-1.1 Ma, during the Middle Pleistocene Transition (MPT), the correlation is weaker, since the 100 kyr periodicity in the δ18O record diminishes, while the tephra record maintains its strong 100 kyr periodicity.
... Looking ahead, the thinning and potential removal of ice cover from the WARS volcanic province could have profound impacts for future volcanic activity across the region. Research in Iceland has shown that with thinning ice cover, magma production has increased at depth as a response to decompression of the underlying mantle (Jull & McKenzie 1996;Schmidt et al. 2013). Moreover, there is evidence that, worldwide, volcanism is most frequent in deglaciating regions as the overburden pressure of the ice is first reduced and then removed (Huybers & Langmuir 2009;Praetorius et al. 2016). ...
The West Antarctic Ice Sheet overlies the West Antarctic Rift System about which, due to the comprehensive ice cover, we have only limited and sporadic knowledge of volcanic activity and its extent. Improving our understanding of subglacial volcanic activity across the province is important both for helping to constrain how volcanism and rifting may have influenced ice-sheet growth and decay over previous glacial cycles, and in light of concerns over whether enhanced geothermal heat fluxes and subglacial melting may contribute to instability of the West Antarctic Ice Sheet. Here, we use ice-sheet bed-elevation data to locate individual conical edifices protruding upwards into the ice across West Antarctica, and we propose that these edifices represent subglacial volcanoes.We used aeromagnetic, aerogravity, satellite imagery and databases of confirmed volcanoes to support this interpretation. The overall result presented here constitutes a first inventory of West Antarctica's subglacial volcanism. We identified 138 volcanoes, 91 of which have not previously been identified, and which are widely distributed throughout the deep basins of West Antarctica, but are especially concentrated and orientated along the > 3000 km central axis of the West Antarctic Rift System.
... The recurrence rate of both Icelandic volcanism and ash clouds over northern Europe has varied over the last 7000 years (Fig. 4, Supplementary File 4). Variation in the frequency of Icelandic volcanism over time can be explained by periodic changes in rifting activity in Iceland and the influence of surface loading (glacier extent) on rates of volcanism (Larsen et al., 1998;Schmidt et al., 2013). The recurrence rate of ash clouds over northern Europe and all Icelandic eruptions shows a general increase in the last 1500 years. ...
Fine ash produced during explosive volcanic eruptions can be dispersed over a vast area, where it poses a threat to aviation, human health and infrastructure. Here, we focus on northern Europe, which lies in the principal transport direction for volcanic ash from Iceland, one of the most active volcanic regions in the world. We interrogate existing and newly produced geological and written records of past ash fallout over northern Europe in the last 1000 years and estimate the mean return (repose) interval of a volcanic ash cloud over the region to be 44 ± 7 years. We compare tephra records from mainland northern Europe, Great Britain, Ireland and the Faroe Islands, with records of proximal Icelandic volcanism and suggest that an Icelandic eruption with a Volcanic Explosivity Index rating (VEI) ≥ 4 and a silicic magma composition presents the greatest risk of producing volcanic ash that can reach northern Europe. None of the ash clouds in the European record which have a known source eruption are linked to a source eruption with VEI < 4. Our results suggest that ash clouds are more common over northern Europe than previously proposed and indicate the continued threat of ash deposition across northern Europe from eruptions of both Icelandic and North American volcanoes.
... In fact, changes in lithospheric stress and melt potential and opening of melt pathways appear to govern the process. The postglacial period, in particular, provides evidence for feedbacks of this kind, the most obvious candidate being deformation of the lithosphere by on-and off-loading of large ice sheets (e.g., Maclennan et al., 2002;Schmidt et al., 2013). If eruptions change or modify climate, which in turn has the potential to initiate eruptions, the stage is set for chicken-and-egg scenarios where potential feedback loops and thresholds may yield complex responses (Fig. 1b). ...
... Icelandic volcanoes erupted far more often while last-glacial ice sheets thinned and retreated than they did before or since [66,67]. Decreasing weight of thinning glaciers exerts less pressure on vents and the mantle, and so magma chambers exolve gas and tend to erupt. ...
This edited volume, showcasing cutting-edge research, addresses two primary questions - what are the main drivers of change in high-mountains and what are the risks implied by these changes? From a physical perspective, it examines the complex interplay between climate and the high-mountain cryosphere, with further chapters covering tectonics, volcano-ice interactions, hydrology, slope stability, erosion, ecosystems, and glacier- and snow-related hazards. Societal dimensions, both global and local, of high-mountain cryospheric change are also explored. The book offers unique perspectives on high-mountain cultures, livelihoods, governance and natural resources management, focusing on how global change influences societies and how people respond to climate-induced cryospheric changes. An invaluable reference for researchers and professionals in cryospheric science, geomorphology, climatology, environmental studies and human geography, this volume will also be of interest to practitioners working in global change and risk, including NGOs and policy advisors.
... Icelandic volcanoes erupted far more often while last-glacial ice sheets thinned and retreated than they did before or since [66,67]. Decreasing weight of thinning glaciers exerts less pressure on vents and the mantle, and so magma chambers exolve gas and tend to erupt. ...
... Thus, model of GIA can be important in predicting volcanic activities and earthquakes (cf. [39,58]). Another crucial application of the GIA model is in the safety assessment of radioactive waste which must reside underground for over 150,000 years before it looses most of its toxicity. ...
This study examines the block lower-triangular preconditioner with element-wise Schur complement as the lower diagonal block applied on matrices arising from an application in geophysics. The element-wise Schur complement is a special approximation of the exact Schur complement that can be constructed in the finite element framework. The preconditioner, the exact Schur complement and the element-wise Schur complement are analyzed mathematically and experimentally.
The preconditioner is developed specifically for the glacial isostatic adjustment (GIA) model in its simplified flat Earth variant, but it is applicable to linear system of equations with matrices of saddle point form.
In this work we investigate the quality of the element-wise Schur complement for symmetric indefinite matrices with positive definite pivot block and show spectral bounds that are independent of the problem size. For non-symmetric matrices we use generalized locally Toeplitz (GLT) sequences to construct a function that asymptotically describes the spectrum of the involved matrices.
The theoretical results are verified by numerical experiments for the GIA model. The results show that the so-obtained preconditioned iterative method converges to the solution in constant number of iterations regardless of the problem size or parameters.
... While actual unloading was more prolonged and likely occurred in stages, this modeling demonstrates the viability of glacial unloading to produce the observed increases in early postglacial volcanic production in the neovolcanic zones of Iceland. The recent studies of Pagli and Sigmundsson (2008) and Schmidt et al. (2013) highlight the potential effects of glacial melting even in recent time, with estimates of up to 0.23 km 3 /year of new magma being produced beneath Iceland in association with isostatic uplift rates up to 25 mm/year during the last century. ...
New observations and geochemical analyses of volcanic features in the 170-km-long Western Volcanic Zone (WVZ) of Iceland constrain spatial and temporal variations in volcanic production and composition associated with the last major deglaciation. Subglacial eruptions represent a significant portion of the late Quaternary volcanic budget in Iceland. Individual features can have volumes up to ∼48 km^3 and appear to be monogenetic. Subaqueous to sub-aerial transition zones provide minimum estimates of ice sheet thickness at the time of eruption, although water-magma interactions and fluctuating lake levels during eruption can lead to complex lithological sequences. New major and trace element data for 36 glacial and postglacial eruptive units, combined with observations of lava surface quality, passage zone heights, and 3He exposure ages of some glacial units, indicate a maximum in volcanic production in the WVZ during the last major ice retreat. Anomalously high volcanic production rates continue into the early postglacial period and coincide with significant incompatible element depletions and slightly higher CaO and SiO2 and lower FeO content at a given MgO. Subglacial units with strong incompatible element depletions also have lava surfaces that lack evidence of subsequent glaciation. These units likely formed after the onset of deglaciation, when rapidly melting ice sheets increased decompression rates in the underlying mantle, leading to anomalously high melting rates in the depleted upper mantle. This process also can explain the eruption of extremely depleted picritic lavas during the early postglacial period. These new observations indicate that the increased volcanic activity associated with glacial unloading peaked earlier than previously thought, before Iceland was completely ice free.
Strong compressive and shear stresses generated by glacial loading and unloading have a direct impact on near‐surface geological processes. Glacial stresses are constantly evolving, creating stress perturbations in the lithosphere that extend significant distances away from the ice. In the Arctic, periodic methane seepage and faulting have been recurrently associated with glacial cycles. However, the evolution of the Arctic glacial stress field and its impact on the upper lithosphere have not been investigated. Here, we compute the evolution in space and time of the glacial stresses induced in the Arctic lithosphere by the North American, Eurasian and Greenland ice sheets during the latest glaciation. We use glacial isostatic adjustment (GIA) methodology to investigate the response of spherical, viscoelastic Earth models with varying lithospheric thickness to the ice loads. We find that the GIA‐induced maximum horizontal stress (σH) is compressive in regions characterized by thick ice cover, with magnitudes of 20–25 MPa in Fennoscandia and 35–40 MPa in Greenland at the last glacial maximum. Simultaneously, a tensile regime with σH magnitude down to −16 MPa dominates across the forebulges with a mean of −4 MPa in the Fram Strait. At present time, σH in the Fram Strait remains tensile with an East‐West orientation. The evolution of GIA‐induced stresses from the last glaciation to present could destabilize faults along tensile forebulges, for example, the west‐coast of Svalbard. A more tensile stress regime as during the Last Glacial Maximum would have more impact on pre‐existing faults that favor gas seepage from gas reservoirs.
Mechanical models are summarized for the co-eruptive behavior of basaltic eruptions, predicting the evolution of eruption rate during the eruption. The models apply to magma flow in a channel after its establishment, feeding an eruption site, and draining magma from an underlying magma body. The channel has a circular or rectangular cross-section, and the underlying magma body has the shape of a sphere or an oblate ellipsoid. Piston-type caldera collapse is also considered. Magma flow rate scales with the evolution of pressure driving the flow. The driving pressure, and therefore, the magma flow rate, decline exponentially. The decay constant is dependent on the properties of the channel, the magma body, and the host rock. The model predictions are compared to observations of recent basaltic eruptions in Iceland: The Eyjafjallajökull 2010 flank eruption on Fimmvörðuháls, the Grímsvötn 2011 eruption, and 2014–2015 activity and the Holuhraun eruption in the Bárðarbunga volcanic system.
Previously glaciated landscapes are often covered with glacial sediments with poorly sorted particles. The thickness of glacial sediments can reach hundreds of meters. These sediments, which have experienced high intensity erosion through glacial transportation, are susceptible to chemical weathering processes because of the increased reactive mineral surfaces induced via erosion. Infiltrated meteoric water will first attack the glacial sediments, becoming (partially) saturated with weathering products, before interacting with the local bedrock underneath. As a consequence, the glacial deposits will hinder the weathering processes of the underlying local bedrock. In this study, we test this hypothesis by imaging the weathered layer of a previously glaciated terrain in the Medicine Bow Mountains, Wyoming using full-3D seismic ambient-noise tomography. To the best of our knowledge, our study provides the first full-3D seismic velocity model of the Critical Zone structure under a previously glaciated landscape. Our 3D seismic velocity model reveals that the saprolite weathered from local bedrock is thinner where the overlying glacial deposit layer is thicker. This anti-correlation suggests that the glacial deposits may locally protect the underlying bedrock from chemical weathering by depleting reactive chemicals in the meteoric water and (partially) saturate the meteoric water with weathering products. Our study may provide a potential counter mechanism against glaciation-induced acceleration of chemical weathering of the continents, which regulates atmospheric concentration of carbon dioxide and the global climate.
Partial melting of asthenospheric mantle generates magma that supplies volcanic systems. The timescale of melt extraction from the mantle has been hotly debated. Microstructural measurements of permeability typically suggest relatively slow melt extraction (1 m/yr) whereas geochemical (Uranium-decay series) and geophysical observations suggest much faster melt extraction (100 m/yr). The deglaciation of Iceland triggered additional mantle melting and magma flux at the surface. The rapid response has been used to argue for relatively rapid melt extraction. However, this episode must, at least to some extent, be unrepresentative, because the rates of magma eruption at the surface increased about thirty-fold relative to the steady state. Our goal is to quantify this unrepresentativeness. We develop a one-dimensional, time-dependent and nonlinear (far from steady-state), model forced by the most recent, and best mapped, Icelandic deglaciation. We find that 30 m/yr is the best estimate of the steady-state maximum melt velocity. This is a factor of about 3 smaller than previously claimed, but still relatively fast. We translate these estimates to other mid-ocean ridges accounting for differences in passive and active upwelling and degree of melting. We find that fast melt extraction greater than about 10 m/yr prevails globally.
The evidence for periods of increased volcanic activity following deglaciation, such as following ice sheet retreat after the Last Glacial Maximum, has been examined in several formerly glaciated areas, including Iceland, Alaska, and the Andean Southern Volcanic Zone. Here we present new evidence supporting the theory that during episodes of cooling in the Holocene, Icelandic volcanic activity decreased. By examining proximal and distal tephra records from Iceland spanning the last 12,500 years, we link two observed tephra minima to documented periods of climatic cooling and glacial advance, at 8.3 to 8 and 5.2 to 4.9 cal kyr BP. We simulate these periods in atmosphere-ocean and ice sheet models to assess the potential validity of the postglacial ‘unloading effect’ on Icelandic volcanic systems. We conclude that an increase in glacial cover may have decreased shallow magma ascent rates, thus limiting eruption potential and producing apparent quiescent periods in proximal and distal tephra records. However, several major uncertainties remain regarding the theory, including geographical and temporal preservation biases and the importance of any unloading effects against other factors, and these will require more prolonged investigation in future research.
Surface processes and magmatism condition the structural evolution of continental rifts and passive margins through mechanical and thermal effects on the lithosphere rheology. However, their interrelationships in extensional settings are largely unknown. Here, I use coupled thermo-mechanical geodynamic and landscape evolution numerical modeling to assess the links between erosion of rift shoulders, sedimentation within the rift basin and extensional rock melting. Results suggest that, when the crust is thinner than ~40 km, the extension rate is slower than ~2 cm/yr and the mantle potential temperature is below ~1230 °C, efficient surface processes may double crustal melting by Moho lowering and inhibit mantle decompression melting by ~50% through sediment loading within the rift basin. It is thus likely that surface processes significantly influenced the magmatic activity of a number of extensional settings worldwide-e.g. the Mediterranean, the Gulf of California, the Iberia-Newfoundland margin, and the South China Sea. Because magmatism and surface processes affect jointly the geological carbon cycle, the surface processes forcing on extensional rock melting investigated here involves an additional means of linkage between plate tectonics and climate changes.
Ash from volcanic eruptions can severely interrupt air traffic, as the eruption of Eyjafjallajökull volcano in 2010 impressively demonstrated. This study used historic eruption and meteorological data to develop two volcanic ash scenarios using Icelandic volcanoes. The scenarios demonstrate the potential scale of events in terms of duration and intensity and enable an investigation of responses either during a long period of continuous risk assessment and maintenance or when facing a large-scale severe interruption of air traffic, while under current regulations.
Aviation experts were invited to discuss the scenarios to help create a picture of the current resilience of the aviation sector and to identify opportunities for improvement in risk management. The research demonstrates that under both scenarios the impact on air traffic would be significant. Weaknesses in current response exercises to volcanic events were identified, suggesting a need to address more extreme scenarios and test responses to events of longer duration. The method employed in this study served as an example to assess the effects of possible impacts of volcanic eruptions on aviation in the North Atlantic and could be applied to other parts of the world.
The eruption of the Eyjafjallajökull volcano in 2010 was an unprecedented event for European aviation and emphasized the need for advancements in the corresponding risk management of the stakeholders involved. This study researches progress since 2010, as significant regulatory changes have been introduced to improve European and North Atlantic aviation risk management with regards to volcanic ash. A participatory stakeholder workshop with scenario narratives was set up in which stakeholders discussed obstacles in the general management of aviation during volcanic ash eruptions as well as under extreme eruption scenarios. This paper presents recommendations developed from the workshop.
The research found that a better understanding is needed of the impacts that long lasting ash episodes may have on aviation. Events of long duration require improved availability of staff, e.g., with staff exchange between related agencies. Furthermore, it is recommended that staff be trained to meet accelerated demands and restructured tasks during a crisis that may last for months. It is also suggested that more challenging response exercises be used to drive stakeholders out of their comfort zone.
The study provides recommendations on information exchange between the stakeholders. During an event, the large amounts of information received from scattered sources may be quite challenging. A single point of information for stakeholders could be set up to structure the information and reduce confusion. Communication products, such as maps, must be better aligned with end-user needs. Ensuring the comprehensibility of difficult features, such as the representation of uncertainty in ash distribution modelling and produced data, requires discussion with end-users prior to an event.
The study stresses the need for further funding of research on the impact of ash on jet engines since lack of knowledge in this area limits the benefits of advances in ash forecasting. The application of the Safety Risk Assessment approach needs to be coordinated across nations. Strengthening society’s resilience as a whole to such events, calls for a comprehensive long-term contingency plan, including alternative transportation if aircrafts are grounded.
Tectonic landforms reveal that the West Antarctic Ice Sheet (WAIS) lies atop a major volcanic rift system. However, identifying subglacial volcanism is challenging. Here we show geochemical evidence of a volcanic heat source upstream of the fast-melting Pine Island Ice Shelf, documented by seawater helium isotope ratios at the front of the Ice Shelf cavity. The localization of mantle helium to glacial meltwater reveals that volcanic heat induces melt beneath the grounded glacier and feeds the subglacial hydrological network crossing the grounding line. The observed transport of mantle helium out of the Ice Shelf cavity indicates that volcanic heat is supplied to the grounded glacier at a rate of ~ 2500 ± 1700 MW, which is ca. half as large as the active Grimsvötn volcano on Iceland. Our finding of a substantial volcanic heat source beneath a major WAIS glacier highlights the need to understand subglacial volcanism, its hydrologic interaction with the marine margins, and its potential role in the future stability of the WAIS.
Between 5 and 6 million years ago, during the so-called Messinian salinity crisis, the Mediterranean basin became a giant salt repository. The possibility of abrupt and kilometre-scale sea-level changes during this extreme event is debated. Messinian evaporites could signify either deep- or shallow-marine deposits, and ubiquitous erosional surfaces could indicate either subaerial or submarine features. Significant and fast reductions in sea level unload the lithosphere, which can increase the production and eruption of magma. Here we calculate variations in surface load associated with the Messinian salinity crisis and compile the available time constraints for pan-Mediterranean magmatism. We show that scenarios involving a kilometre-scale drawdown of sea level imply a phase of net overall lithospheric unloading at a time that appears synchronous with a magmatic pulse from the pan-Mediterranean igneous provinces. We verify the viability of a mechanistic link between unloading and magmatism using numerical modelling of decompression partial mantle melting and dyke formation in response to surface load variations. We conclude that the Mediterranean magmatic record provides an independent validation of the controversial kilometre-scale evaporative drawdown and sheds new light on the sensitivity of magmatic systems to the surface forcing.
In geodynamic numerical models of volcanic systems, the volcanic basement hosting the magmatic reservoir is often assumed to exhibit constant elastic parameters with a sharp transition from the host rocks to the magmatic reservoir. We assess this assumption by deriving an empirical relation between elastic parameters and temperature for Icelandic basalts by conducting a set of triaxial compression experiments between 200 °C and 1000 °C. Results show a significant decrease of Young's modulus from ∼38 GPa to less than 4.7 GPa at around 1000 °C. Based on these laboratory data, we develop a 2D axisymmetric finite-element model including temperature-dependent elastic properties of the volcanic basement.
Surface displacements and edifice deformations at active volcanoes can occur when magma reservoirs begin to inflate as new magma enters them. Volcanoes are also subjected to a variety of external lithospheric stresses that are thought to be responsible for triggering volcanic unrest or modifying ongoing activity. However, despite many observations, it is uncertain whether these phenomena can actually interfere with magma chamber dynamics since it is not clear why some volcanoes are more subjected to these interactions than others. In order to determine whether external stresses interfere with volcanic activity, a viscoelastic 3D Finite Element Mogi-based model of Kīlauea volcano's magma chamber was implemented. First, the model was used to replicate an inflation cycle without external stresses. Its results were then compared with the ones obtained if the same model was subjected to tidal stress modulation and a strong (M w =7.7) tectonic earthquake. The model showed that tidally-induced pressurization is not sufficiently large to modify the pressure in a 5km deep volcanic magma chamber, but it suggested how the magma chamber pressure build-up rate can be influenced by tidal pressurization and thus why some volcanoes seem to experience tidal interferences more than others. Furthermore, the model's results suggested why magma chambers are about the same size as calderas both on the Earth and on other Solar System silicate planets. System. Finally, it was used to propose an explanation of why a short-lived eruption at Kīlauea volcano, Hawai'i, began 30min after the 1975 magnitude 7.7 (M w) Kalapana earthquake.
Linear systems with two-by-two block matrices are usually preconditioned by block lower- or upper-triangular systems that require an approximation of the related Schur complement. In this work, in the finite element framework, we consider one special such approximation, namely, the element-wise Schur complement. It is sparse and its construction is perfectly parallelizable, making it an appropriate ingredient when building preconditioners for iterative solvers executed on both distributed and shared memory computer architectures. For saddle point matrices with symmetric positive (semi-)definite blocks we show that the Schur complement is spectrally equivalent to the so-constructed approximation and derive spectral equivalence bounds. We also illustrate the quality of the approximation for nonsymmetric problems, where we observe the same good numerical efficiency.
Furthermore, we demonstrate the computational and numerical performance of the corresponding preconditioned iterative solution method on a large scale model benchmark problem originating from the elastic glacial isostatic adjustment model discretized using the finite element method.
During melting in the upper mantle the preferred partitioning of water into the melt will effectively dehydrate the solid residue. Linear extrapolation of laboratory experiments suggests that dehydration can produce a sharp viscosity contrast (increase) of a factor 500 across the dry solidus. In this study we show that the suggested magnitude of dehydration stiffening in a plume–ridge setting is incompatible with the present glacial isostatic adjustment (GIA) in Iceland. Using GPS observations of current GIA in Iceland, we find that the data are best fit by a viscosity contrast over the dry solidus in the range 0.5–3. A viscosity contrast higher than 10 requires a mantle viscosity below the dry solidus lower than 4-8×1018Pas, depending on the thickness of the dehydrated layer. A viscosity contrast of 100 or more demands a mantle viscosity of 1018Pas or less. However, we show here that a non-linear extrapolation of the laboratory data predicts a viscosity contrast as low as a factor 3–29, assuming conditions of constant strain rate to constant viscous dissipation rate. This is compatible with our GIA results and suggests that the plume–ridge interaction beneath Iceland is governed by a non-linear rheology and controlled by a combination of kinematic and dynamic boundary conditions rather than buoyant forces alone.
The Tertiary igneous rocks of Greenland, Iceland, the Faeroes and Britain have been the subject of study and debate for more than a hundred years. Iceland is of particular significance because the coincidence of a mantle plume with the Mid-Atlantic Ridge combines the two fundamental forces that promote magmatism, namely the elevated mantle potential temperature induced by the Iceland plume and adiabatic decompression in response to spreading at the ridge. Furthermore, the exposed Iceland crust contains evidence of major ridge-jumps over the last 16 million years and this relocation of the magmatic focus has been a prominent process in the evolution of the island. The control oil ridge-jumping is clearly related to the interaction of the mantle plume with the overlying lithospheric plate. This process has had a significant impact on the investigation of magmatic, tectonic and sedimentary processes. The bulk of the Tertiary region is made of subaerial tholeiitic flood basalts separated by minor clastic interbeds, usually of volcanic origin. The relatively monotonous Tertiary lithology is interrupted where central volcanoes occur with their buried palaeotopography, evolved rocks, hydrothermal alteration and stratigraphic complexities. It has become clear that the range of chemical composition of Tertiary basalt is much more, limited than that seen among Pleistocene and Holocene basalt, and depleted basalt appears, surprisingly, to be absent from the Tertiary succession. These observations call be explained by processes of crustal accretion operating today in the active rift zones of Iceland. It is a widely held assumption that V-shaped ridges observed in the gravity field around the Reykjanes Ridge imply variation in plume temperature and plume activity. Temporal variations in some isotope ratios in the Tertiary lava flows seem to coincide with the formation of the V-shaped features, and this could be consistent with a pulsating plume model.
Global warming causes retreat of ice caps and ice sheets. Can melting glaciers trigger increased volcanic activity? Since 1890 the largest ice cap of Iceland, Vatnajokull, with an area of similar to 8000 km(2), has been continuously retreating losing about 10% of its mass during last century. Present-day uplift around the ice cap is as high as 25 mm/yr. We evaluate interactions between ongoing glacio-isostasy and current changes to mantle melting and crustal stresses at volcanoes underneath Vatnajokull. The modeling indicates that a substantial volume of new magma, similar to 0.014 km(3)/yr, is produced under Vatnajokull in response to current ice thinning. Ice retreat also induces significant stress changes in the elastic crust that may contribute to high seismicity, unusual focal mechanisms, and unusual magma movements in NW-Vatnajokull.
Currently the North Atlantic ridge is overriding the Iceland plume. Due to several ridge jumps the plume has been virtually ridge-centred since 20–25 Ma giving rise to extensive melting and crust formation. This review gives an overview over the results of the geophysical and, to minor extent, the geochemical research on the general structure of the Icelandic crust and the mantle beneath Iceland. In the first part, results mostly from topography/bathymetry, gravity, seismics/seismology, magnetotellurics, and geodynamical numerical modelling are summarised. They support the main conclusion that the Icelandic crust is up to ca. 40 km thick, whereby the lower crust and the uppermost mantle have an anomalously small density contrast and a gradual transition rather than a well-defined Moho. The interpretation of a good electrical conductor at 10–15 km depth as a molten layer is irreconcilable with a thick crust, so that alternative explanations have to be sought for this still enigmatic feature. In the second part, results from different branches of seismology, geochemistry, and numerical modelling on the Iceland plume are reviewed and discussed. For the upper mantle, combining seismological models, geodynamical models and crustal thickness data suggests that the plume has a radius of 100–120 km and an excess temperature of 150–200 K, while the structure of the plume head is less well known. The volume flux is likely to be 5–6 km3 /a, and numerical modelling indicates that water and its loss upon melting have a substantial impact on melt production and on the dynamics and distribution of segregating melt. Geochemical studies indicate that the plume source is quite heterogeneous and very probably contains material from the lower mantle. An origin of the plume somewhere in the lower mantle is also supported by several seismological findings, but evidence is not unambiguous yet and has still to be improved.
Some 11% of Iceland is covered by glaciers. They contain 3,600 km 3 of water, equivalent to a 35-m-thick ice layer spread evenly over the whole country; if melted, it would raise global sea level by 1 cm. This is Iceland's greatest water storage, corresponding to the precipitation of 20 years. Dynamic in nature, these glaciers are responsive to climate fluctuations and affect their environment profoundly. Also, they lie over active volcanoes; these induce jökulhlaups that can threaten areas of habitation. The country's glaciers feed its largest rivers and currently provide at least one-third of its total runoff. Since a general glacier recession set in at the end of the 19th century, the largest icecap, Vatnajökull, has decreased by about 10% in volume (300 km 3), contributing 1 mm to the concurrent rise in sea level. During the last ten years, ice losses have accelerated, thereby detracting 2.7% (84 km 3) from the total icecap volume. Typically, radiation provides two-thirds of the melt energy, turbulent fluxes one-third. However, transitory volcanic eruptions and continuous geothermal activity at the bed of Vatnajökull added some 5.5 km 3 to surface melting during the 1990s, with one particular volcanic eruption melting 4.0 km 3 . In all of Iceland's major icecaps, surges account for a significant portion of total mass transport through the principal outlet glaciers, playing an important role in outlet dynamics and hydrology. Taking the 20th century as a whole, surges contributed at least 10% to the total ice transport to ablation areas of Vatnajökull. Plausible future climate scenarios, coupled with models of mass balance and ice dynamics, suggest that the main icecaps will lose 25% to 35% of their present volume within half a century, leaving only small glaciers on the highest peaks after 150–200 years. Glacier meltwater runoff will peak after 50 years, then decline to present-day values by 100 years from now. When the glaciers have disappeared, the entire river discharge will come directly from precipitation.
1-D models were calculated for the velocity of shear waves, polarized vertically (SV) and horizontally (SH) from dispersed Rayleigh and Love surface waves. These had been recorded in Iceland by the ICEMELT broad-band seismic network, with about half of the waves coming from near-distance earthquakes (
The amount of melt generated per unit pressure drop during adiabatic upwelling, the isentropic melt productivity, cannot be determined directly from experiments and is commonly assumed to be constant or to decrease as melting progresses. From analysis of one- and two-component systems and from calculations based on a thermodynamic model of peridotite partial melting, we show that productivity for reversible adiabatic (i.e. isentropic) depressurization melting is never constant; rather, productivity tends to increase as melting proceeds. Even in a one-component system with a univariant solid-liquid boundary, the 1/T dependence of (partial S/partial T)P and the downward curvature of the solidus (due to greater compressibility of liquids relative to minerals) lead to increased productivity with increasing melt fraction during batch fusion (and even for fractional fusion in some cases). Similarly, for multicomponent systems, downward curvature of contours of equal melt fraction between the solidus and the liquidus contributes to an increase in productivity as melting proceeds. In multicomponent systems, there is also a lever-rule relationship between productivity and the compositions of coexisting liquid and residue such that productivity is inversely related to the compositional distance between coexisting bulk solid and liquid. For most geologically relevant cases, this quantity decreases during progressive melting, again contributing to an increase in productivity with increasing melting. These results all suggest that the increases in productivity with increasing melt fraction (punctuated by drops in productivity upon exhaustion of each phase from the residue) predicted by thermodynamic modelling of melting of typical mantle peridotites using MELTS are neither artifacts nor unique properties of the model, but rather general consequences of adiabatic melting of upwelling mantle.
This study explores the thermodynamics of adiabatic decompression
melting of peridotitic mantle containing pyroxenite veins that have
lower solidi than the peridotite. When a vein of lower
solidus-temperature material melts adjacent to more refractory material,
additional heat will flow into the melting region to increase its
melting productivity. If pyroxenite veins have a solidus-depletion
gradient ((∂Tm/∂F)P) like that of
olivine or peridotite, then the melting of the veins is enhanced by up
to a factor of 4 by this heat. However, the solidus-depletion gradient
of pyroxenites is apparently lower than that of peridotites; thus
pyroxenite melting would be even more enhanced. If pyroxenite veins have
a gentler solidus-pressure (T-P) dependence (i.e., lower
(∂Tm/∂P)F) than that of peridotite
solidi, then although these veins will experience enhanced melting while
they are the only melting assemblage, they will stop melting soon after
their peridotite matrix begins to melt. During large-scale peridotite
melting the material ascends along a T-P path close to that of the
peridotite solidus, so that the mixture's temperature remains lower than
the solidus of the residual pyroxenite, and pyroxenite melting ceases
throughout the shallower sections of the melting column. If pyroxenitic
material makes up a large fraction of the mantle mixture (˜20%),
then the heat consumed by deep pyroxenite melting cools the ascending
mantle mixture enough so that peridotite melting begins ˜5-10 km
shallower than it would in the absence of precurser pyroxenite melting.
After recycling into the mantle, the melt extraction residue will again
melt if it is reheated to ambient mantle temperatures and rises again to
shallow depths.
Recent revisions to the geomagnetic time scale indicate that global plate motion model NUVEL-1 should be modified for comparison with other rates of motion including those estimated from space geodetic measurements. The optimal recalibration, which is a compromise among slightly different calibrations appropriate for slow, medium, and fast rates of seafloor spreading, is to multiply NUVEL-1 angular velocities by a constant, α, of 0.9562. We refer to this simply recalibrated plate motion model as NUVEL-1A, and give correspondingly revised tables of angular velocities and uncertainties. Published work indicates that space geodetic rates are slower on average than those calculated from NUVEL-1 by 6±1%. This average discrepancy is reduced to less than 2% when space geodetic rates are instead compared with NUVEL-1A.
BASALTIC magmatism results when upwelling mantle crosses the peridotite solidus. The greater the overstep of the solidus, the greater the degree of mantle melting and the larger the volume of magma produced. In Iceland most of the active volcanism occurs along the central spreading axis, although some occurs in isolated off-axis volcanoes such as Snaefellsjökull. Because the mantle beneath Snaefellsjökull is not upwelling as vigorously as that beneath the spreading axis, it oversteps its solidus by a much smaller amount and consequently the degree of melting is less. The composition of the resulting magma will be much more sensitive to small perturbations in the amount of upwelling than will the mantle beneath the ridge axis. We show here that the reduction of pressure in the mantle caused by the unloading of ice was sufficient to affect magma composition. Unloading was accompanied by a clear shift towards less undersaturated magma compositions, which reflect a transient increase of about 0.5% in the degree of mantle melting, coupled with a decrease in depth of melting.
Recent studies have shown that the extraction of water from the mantle due to partial melting beneath mid-ocean ridges may increase the viscosity of the residuum by 2–3 orders of magnitude. We examine this rheological effect on mantle flow and melting of a ridge-centered mantle plume using three-dimensional numerical models. Results indicate that the viscosity increase associated with dehydration prevents buoyancy forces from contributing significantly to plume upwelling above the dry solidus. Consequently, upwelling in the primary melting zone is driven passively by plate spreading and melt production rates are substantially lower than predicted by models that do not include the rheological effect of dehydration. Predictions of along-axis crustal thickness, bathymetric, and gravity variations are shown to be consistent with observations at Iceland and along the Mid-Atlantic Ridge. Furthermore, these predictions result from a model of a plume with relatively high excess temperature (180°C) and narrow radius (100 km) — properties that are consistent with estimates previously inferred from geochemical and seismological observations. Calculations of incompatible trace-element concentrations suggest that observed along-axis geochemical anomalies primarily reflect incompatible element heterogeneity of the plume source.
The South Iceland Seismic Zone (SISZ) is an E-W transform zone, where
the relative spreading of the North American and Eurasian plates across
southern Iceland is accommodated by motion on many parallel N-S
right-lateral strike slip faults, rather than left-lateral motion on a
single E-W through going fault. Historically, earthquake sequences with
main shocks reaching M7 have occurred in the SISZ, many initiating in
the eastern part of the zone with subsequent events further west. A
magnitude 6.3 earthquake occurred in the western part of the SISZ on
May29, 2008. Aftershock locations and global centroid-moment-tensor
solutions indicate rupture on at least two parallel N-S faults. The
rupture on the second fault, located about 4 km west of the initial
event,appears to have initiated less than one second after the main
shock,suggesting dynamic triggering. The May 2008 earthquakes are a
continuation of the June 2000 sequence, when two Mw=6.5 events struck
the eastern and central part of the SISZ. The June 2000 main shocks
ruptured two parallel N-S faults, spaced about 17 km apart, occurring
about 3 1/2 days apart. Here, we present a geodetic and seismic study
of the May 2008 earthquakes based on continuous and annual GPS
measurements, as well as InSAR and aftershock locations. The GPS
network was surveyed in April, a month before the events and remeasured
immediately after. Maximum coseismic displacements of about 15 cm
(horizontal) were recorded at the closest continuous GPS stations on
each side of the two faults. We also measured continuously at about 20
GPS benchmarks for more than a month after the event. A small transient
(about 1 cm) was recorded during the first 10 days following the
earthquake. This transient motion does not appear to be caused by
poro-elastic rebound due to pressure changes in the ground water
system, as was observed following the June 2000 earthquakes. The
aftershocks lineate at least two N-S structures as well as an E-W
conjugate fault. Preliminary modeling of GPS and InSAR data indicate
that most of the surface deformation can be explain with strike-slip at
shallow depth on two N-S structures, with little or no slip on the E-W
lineament. Our first estimate using a layered-elastic Earth model
indicates slip of about 2 m at shallow depth. The relatively high ratio
of slip to fault length was also evident in the June 2000 events. Such
high stress drop events indicate that the SISZ is a young and immature
fault zone.
Surges are common in all the major ice caps in Iceland, and historical reports of surge occurrence go back several centuries. Data collection and regular observation over the last several decades have permitted a detailed description of several surges, from which it is possible to generalize on the nature of surging in Icelandic glaciers. Combining the historical records of glacier-front variations and recent field research, we summarize the geographic distribution of surging glaciers, their subglacial topography and geology, the frequency and duration of surges, changes in glacier surface geometry during the surge cycle, and measured velocity changes compared to calculated balance velocities. We note the indicators of surge onset and describe changes in ice, water and sediment fluxes during a surge. Surges accomplish a significant fraction of the total mass transport through the main outlet glaciers of ice caps in Iceland and have important implications for their hydrology. Our analysis of the data suggests that surge-type glaciers in Iceland are characterized by gently sloping surfaces and that they move too slowly to remain in balance given their accumulation rate. Surge frequency is neither regular nor clearly related to glacier size or mass balance. Steeply sloping glaciers, whether hard- or soft-bedded, seem to move sufficiently rapidly to keep in balance with the annual accumulation.
Climate warming at the end of the last glaciation caused ice caps on
Icelandic volcanoes to retreat. Removal of surface ice load is thought
to have decreased pressures in the underlying mantle, triggering
decompression melting, enhanced magma generation and increased volcanic
activity. Present-day climate change could have the same effect,
although there may be a time lag of hundreds of years between magma
generation and eruption. However, in addition to increased magma
generation, pressure changes associated with ice retreat should also
alter the capacity for storing magma within the crust. Here we use a
numerical model to evaluate the effect of the current decrease in ice
load on magma storage in the crust at the Kverkfjöll volcanic
system, located partially beneath Iceland's largest ice cap. We compare
the model results with radar and global positioning system measurements
of surface displacement and changes in crustal stress between 2007 and
2008, during the intrusion of a deep dyke at Upptyppingar. We find that
although the main component of stress recorded during dyke intrusion
relates to plate extension, another component of stress is consistent
with the stress field caused by the retreating ice cap. We conclude that
the retreating ice cap led to enhanced capture of magma within the
crust. We suggest that ice-cap retreat can promote magma storage, rather
than eruption, at least in the short term.
The 39-day long eruption at the summit of Eyjafjallajökull volcano in April-May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·10¹¹ kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05 km³). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27 km³) transported by 3-10 km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·10¹⁰ kg (70 Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km² in Europe and the North Atlantic. Of order 10¹⁰ kg (2%) are considered to have been transported longer than 600-700 km with <10⁸ kg (<0.02%) reaching mainland Europe.
1] Modeling of melt formation and transport in all tectonic settings requires the inclusion of water, since water has large effects on mantle solidi as well as physical properties of liquids. To facilitate the inclusion of water in melting models this paper presents a new parameterization for melt fraction as a function of pressure, temperature, water content and modal cpx, based on knowledge gained from recent advances in the fields of thermodynamic modeling as well as experimental investigations of peridotite melting and hydrous equilibria. The parameterization is computationally efficient and can be modified easily as better experimental data become available. We compare it to other published parameterizations and test it in simple calculations of adiabatic decompression melting (mid-ocean ridge) and hydrous melting (subduction zone). Components: 8542 words, 12 figures, 3 tables.
1] We modeled fundamental mode Love and Rayleigh waves to study the seismic properties of the upper mantle beneath the Reykjanes Ridge. These waves were generated by regional earthquakes occurring in the North Atlantic to the south of Iceland and were recorded by stations located on Iceland. Over 12,000 measurements of the phase, group arrival time, and amplitude of narrow-pass-filtered waveforms (over the period range of 9.5– 100 s) were used to solve for mantle shear wave velocity structure and anisotropy. In a vertical plane oriented normal to the ridge axis, the velocity structure contains a broad and deep low-velocity zone in the upper mantle beneath the ridge. A joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures ($75°). Our study shows that plume material spreads broadly outward beneath the Reykjanes Ridge from Iceland and is not confined to a narrow lithospheric channel. At distances >200 km from the ridge, shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals (mainly above 50 km depth), which can be interpreted as past, horizontal, ridge-perpendicular flow. Within ±200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge. The transition to this type of anisotropy coincides with the appearance of increased hot spot–ridge interaction $20 Myr ago, indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time. (2007), Surface wave tomography of the upper mantle beneath the Reykjanes Ridge with implications for ridge – hot spot interaction, J. Geophys. Res., 112, B08313, doi:10.1029/2006JB004785.
Reports experiments carried out between 9 and 16 kbar (0.9-1.6 GPa) using natural, primitive mid-ocean ridge basalt compositions and synthetic analogs of mid-ocean ridge basalts to investigate the effects of pressure, temperature, and variable bulk composition on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and plagioclase or spinel. -from Authors
The results of geochemical and geophysical surveys can be used to to inform and refine the development of dynamic models of mantle convection. Iceland is an excellent place to study the interaction between a spreading centre and mantle plume because it has been well characterised by several geophysical surveys and it is straightforward to collect large numbers of basalt samples from the active ridge zones. Seismic surveys of the Northern Volcanic Zone (NVZ) of Iceland have shown that the crustal thickness increases from ~ 20 km near the northern coast to 32-40 in central Iceland. This change in crustal thickness reflects an increasing rate of melt production near the centre of the plume. Crustal thickness measurements alone cannot be used to determine whether this increased melting rate reflects higher mantle temperatures or increased mantle upwelling rates. Fortunately, the composition of mantle melts are sensitive to the extent and depth of melting, and a comparison of geochemical and geophysical results from the Theistareykir region of northern Iceland and the Herdubreid region of central Iceland allows us to investigate the influence of the plume on melt generation. The observed Rare Earth Element (REE) composition and crustal thickness of the Theistareykir area can be reproduced by a melting model where mantle with a potential temperature of ~1480°C upwells under the spreading ridge and the mantle upwelling is driven by plate separation alone. However, plate-driven upwelling models cannot simultaneously reproduce the composition of the Herdubreid region lava and the observed crustal thickness. Forward and inverse techniques show that plate-driven models that match the crustal thickness underestimate the La concentration by more than a factor of 2, and models that reproduce the compositions underestimate the crustal thickness by a factor of 4-5. Therefore one of the assumptions involved in the plate-driven upwelling models is not appropriate for central Iceland. A new set of models was developed in which mantle upwelling rates are allowed to differ from those of plate-driven upwelling in order to investigate the role of plume -driven mantle upwelling. The lava composition and crustal thickness of the Herdubreid region can be reproduced by models where the upwelling rates near the base of the melting region ( >100 km depth) are ~ 10 times higher than those expected from plate-driven upwelling alone and the mantle potential temperature is 1480-1520°C. About half of the melt generation under central Iceland results from plume -driven upwelling, with the remainder caused by plate-driven upwelling of hot material. This result is in agreement with numerical models of ridge-centred plumes.
A broad uplift occurs in Iceland in response to the retreat of ice caps,
which began circa 1890. Until now, this deformation signal has been
measured primarily using GPS at points some distance away from the ice
caps. Here, for the first time we use satellite radar interferometry
(interferometric synthetic aperture radar) to constrain uplift of the
ground all the way up to the edge of the largest ice cap,
Vatnajökull. This allows for improved constraints on the Earth
rheology, both the thickness of the uppermost Earth layer that responds
only in an elastic manner and the viscosity below it. The
interferometric synthetic aperture radar velocities indicate a maximum
displacement rate of 24±4 and 31±4 mm/yr at the edge of
Vatnajökull, during 1995-2002 and 2004-2009, respectively. The
fastest rates occur at outlet glaciers of low elevation where ice
retreat is high. We compare the observations with glacial isostatic
adjustment models that include the deglaciation history of the Icelandic
ice caps since 1890 and two Earth layers. Using a Bayesian approach, we
derived probability density functions for the average Earth model
parameters for three satellite tracks. Based on our assumptions, the
three best fit models give elastic thicknesses in the range of 15-40 km,
and viscosities ranging from 4-10× 1018 Pa s.
Lithological variations in the mantle source regions under mid-ocean
ridges and ocean islands have been proposed to play a key role in
controlling melt generation and basalt composition. Here we combine
compositional observations from Icelandic basalts and modeling of
melting of a bilithologic peridotite-pyroxenite mantle to demonstrate
that, while short-length scale major element variation is present
in the mantle under Iceland, source heterogeneity does not make an
important contribution to excess melt production. By identifying the
major element characteristics of end-member Icelandic melts, we find
enriched melts to be characterized by low SiO2 and CaO, but
high FeO. We quantitatively compare end-member compositions to
experimental partial melts generated from a range of lithologies,
pressures and melt fractions. This comparison indicates that a single
source composition cannot account for all the major element variation;
depleted Icelandic melts can be produced by depleted peridotite melting,
but the major element composition of enriched melts is best matched by
melting of mantle sources that have been refertilized by the addition of
up to 40% mid-ocean ridge basalt. The enriched source beneath Iceland is
more fusible than the source of depleted melts, and as such will be
overrepresented in accumulated melts compared with its abundance in the
source. Modeling of peridotite-pyroxenite melting, combined with our
observational constraints on the composition of the Icelandic mantle,
indicates that crustal thickness variations in the North Atlantic must
be primarily due to mantle temperature and flow field variations.
During melting in the upper mantle the preferred partitioning of water into the melt will effectively dehydrate the solid residue. Linear extrapolation of laboratory experiments suggests that dehydration can produce a sharp viscosity contrast (increase) of a factor 500 across the dry solidus. In this study we show that the suggested magnitude of dehydration stiffening in a plume–ridge setting is incompatible with the present glacial isostatic adjustment (GIA) in Iceland. Using GPS observations of current GIA in Iceland, we find that the data are best fit by a viscosity contrast over the dry solidus in the range 0.5–3. A viscosity contrast higher than 10 requires a mantle viscosity below the dry solidus lower than 4-8×1018Pas, depending on the thickness of the dehydrated layer. A viscosity contrast of 100 or more demands a mantle viscosity of 1018Pas or less. However, we show here that a non-linear extrapolation of the laboratory data predicts a viscosity contrast as low as a factor 3–29, assuming conditions of constant strain rate to constant viscous dissipation rate. This is compatible with our GIA results and suggests that the plume–ridge interaction beneath Iceland is governed by a non-linear rheology and controlled by a combination of kinematic and dynamic boundary conditions rather than buoyant forces alone.
A third Global Positioning System (GPS) survey of a regional network surrounding the Krafla volcanic system, north Iceland, was conducted in 1992 following a major crustal spreading episode which began in this system in 1975. Differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum, rift-normal expansion rate of 4.5 cm/year near the rift, decreasing to 3 cm/year at large distances. The time-averaged spreading rate in north Iceland, 1.8 cm/year, cannot account for this deformation. The vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance. Three different models are applied to study the postdike injection ground deformation: (1) stress redistribution in an elastic layer over a viscoelastic half-space, (2) stress redistribution in an elastic-viscous layered medium, and (3) continued opening at depth on the dike plane in an elastic half-space. Using model 1, the effects of historical episodes in the region are subtracted from the observed displacement fields, and the remaining motion is modeled as relaxation following the recent Krafla rifting episode. The best fit model involves a half-space viscosity of 1.1×1018 Pa s, a relaxation time of 1.7 years, and an elastic layer thickness for northeast Iceland of 10 km. The vertical field indicates that the Krafla dike complex rifted the entire elastic layer. Using model 2, the motion 1987-1990 and 1990-1992 can be simulated adequately given the survey errors, but the 1987-1992 deformation is poorly fitted, suggesting that a more realistic geophysical model is required. Using model 3, a range of dikes will fit the deformation field.
This paper reports experiments carried out between 9 and 16 kbar (0.9–1.6 GPa) using natural, primitive mid-ocean ridge basalt compositions and synthetic analogs of mid-ocean ridge basalts to investigate the effects of pressure, temperature, and variable bulk composition on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and plagioclase or spinel. For this low-variance, five-phase assemblage, equations involving pressure, melt NaK # ([Na2O+K2O]/[Na2O+K2O+CaO]; weight ratio), melt Mg # (Mg/[Mg+Fe2+]; total iron as Fe2+), and weight percent TiO2 in the melt predict temperature and major element compositions of magmas produced by melting spinel and plagioclase lherzolites at upper mantle pressures. The equations are estimated using a selected set of data from this experimental study and published experimental studies that report compositions of glasses coexisting with olivine, low-Ca pyroxene, augite, and plagioclase and/or spinel. An experimental test of a liquid compositionally similar to melts produced in a subset of peridotite-basalt sandwich experiments is presented. The composition tested was reported as multiply saturated (with orthopyroxene + augite + spinel + olivine) in the sandwich experiment, but it does not crystallize these phases at the conditions of the experiment. We exclude this liquid and the subset it represents (data from Fujii and Scarfe [1985] and Falloon and Green [1987]) from the data set used to constrain the melting equilibria. With estimates of the melt NaK #, melt Mg #, and weight percent TiO2 of the melt the quantitative descriptions of the melting equilibria can be used to predict the temperatures and major element compositions of melts from lherzolite. Methods are described for estimating these compositional parameters with the nonmodal batch melting equation (for Na2O, CaO, K2O, and TiO2) and a mass balance calculation (for MgO and FeO) from the initial composition and phase proportions of the mantle source, the amount of melt produced and the nature of the melting process, and the stoichiometric coefficients of the mantle melting reaction.
Gravity data from Iceland and its surroundings are analysed and modelled with respect to seismic data. A Bouguer gravity map of Iceland is recomputed based on admittance between the topography and the gravity and with corrections for glacial ice sheets. From seismic data and with the help of relations between the residual topography and the depth to seismic boundaries we construct maps of the main seismic boundaries, including the Moho. By inversion calculations we recomputed these maps, assuming different density values for Seismic Layer 4 to fit the observed gravity field. We found that the average density of Layer 4 has to be in the range 3050-3150 kg m-3 in order to fit both seismic and gravity data. Thus we conclude that Layer 4 is a transition zone between the mantle and the oceanic crust in Iceland. Furthermore by assuming that the upper-mantle density variations necessary to compensate for the gravity effect of crustal layers, are caused by thermal variations in the upper mantle, we calculate the depth to the 1200 °C isotherm to be at 30-50 km depth below Iceland but rising up to less than 20 km below parts of the volcanic zone in Northern Iceland. We conclude that the temperature within the Seismic Layer 4 is close to 600 °C at its top, increasing to approximately 950 °C at its bottom (Moho), which makes a widespread layer of partially molten material within Layer 4 unlikely. By use of cross spectral analysis of the gravity field and the external topographic load at short wavelengths, we conclude that the elastic plate thickness in Iceland can hardly exceed 6 km. In addition we point out that the residual isostatic anomalies have circular forms east of the eastern volcanic zone but are near parallel to the ridge axis on the western side. This form of the anomalies may be caused by pressure from the eastward moving mantle plume below the eastern volcanic zone.
When general-purpose finite element analysis software is used to model glacial isostatic adjustment (GIA), the first-order effect of prestress advection has to be accounted for by the user. We show here that the common use of elastic foundations at boundaries between materials of different densities will produce incorrect displacements, unless the boundary is perpendicular to the direction of gravity. This is due to the foundations always acting perpendicular to the surface to which they are attached, while the body force they represent always acts in the direction of gravity. If prestress advection is instead accounted for by the use of elastic spring elements in the direction of gravity, the representation will be correct. The use of springs adds a computation of the spring constants to the analysis. The spring constant for a particular node is defined by the product of the density contrast at the boundary, the gravitational acceleration, and the area supported by the node. To be consistent with the finite element formulation, the area is evaluated by integration of the nodal shape functions. We outline an algorithm for the calculation and include a Python script that integrates the shape functions over a bilinear quadrilateral element. For linear rectangular and triangular elements, the area supported by each node is equal to the element area divided the number of defining nodes, thereby simplifying the computation. This is, however, not true in the general nonrectangular case, and we demonstrate this with a simple 1-element model. The spring constant calculation is simple and performed in the preprocessing stage of the analysis. The time spent on the calculation is more than compensated for by a shorter analysis time, compared to that for a model with foundations. We illustrate the effects of using springs versus foundations with a simple two-dimensional GIA model of glacial loading, where the Earth model has an inclined boundary between the overlying elastic layer and the lower viscoelastic layer. Our example shows that the error introduced by the use of foundations is large enough to affect an analysis based on high-accuracy geodetic data.
Measurements of the lake level of Lake Langisjór at the SW edge of the Vatnajökull ice cap indicate a tilt of 0.26 ± 0.06 μrad/year away from the ice cap in the years 1959–1991. The tilt is too large to be explained as an elastic Earth response to ice retreat this century, or to be caused by change in the gravitational pull of the ice cap, but it can be explained by sub-lithospheric viscous adjustment. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnajökull during the Little Ice Age. The inferred sub-lithospheric viscosity is 1 × 1018 − 5 × 1019 Pa s.
Iceland is one of the few places on Earth where a divergent plate boundary can be observed on land. Direct observations of crustal deformation for the whole country are available for the first time from nationwide Global Positioning System (GPS) campaigns in 1993 and 2004. The plate spreading across the island is imaged by the horizontal velocity field and high uplift rates (>=10 mmyr-1) are observed over a large part of central and southeastern Iceland. Several earthquakes, volcanic intrusions and eruptions occurred during the time spanned by the measurements, causing local disturbances of the deformation field. After correcting for the largest earthquakes during the observation period, we calculate the strain rate field and find that the main feature of the field is the extension across the rift zones, subparallel to the direction of plate motion. Kinematic models of the horizontal plate spreading signal indicate a slightly elevated rate of spreading in the Northern Volcanic Zone (NVZ) (23 +/- 2 mmyr-1), while the rates at the other plate boundary segments agree fairly well with the predicted rate of plate spreading (~20 mmyr-1) across Iceland. The horizontal ISNET velocities across north Iceland therefore indicate that the excessive spreading rate (>30 mmyr-1) observed by GPS in 1987-1992 following the 1975-1984 Krafla rifting episode was significantly slower during 1993-2004. We model the vertical velocities using glacial isostatic adjustment (GIA) due to the recent thinning of the largest glaciers in Iceland. A layered earth model with a 10-km thick elastic layer, underlain by a 30-km thick viscoelastic layer with viscosity 1 × 1020 Pa s, over a half-space with viscosity ~1 × 1019 Pa s can explain the broad area of uplift in central and southeastern Iceland. A wide area of significant residual uplift (up to 8 mmyr-1) is evident in north Iceland after we subtract the rebound signal from the observed rates, whereas the Reykjanes Peninsula and the Western Volcanic Zone (WVZ) appear to be subsiding at a rate of 4-8 mmyr-1. We observe a coherent pattern of small but significant residual horizontal motion (up to 3 mmyr-1) away from Vatnajökull and the smaller glaciers that is most likely caused by glacial rebound. Our study demonstrates that the velocity field over a large part of Iceland is affected by deglaciation and that this effect needs to be considered when interpreting deformation data to monitor subglacial volcanoes in Iceland.
We present partial melting experiments at 2-3 GPa on a basaltic pyroxenite (G2) similar in composition to typical oceanic crust. The 3.0 GPa solidus is located at 1310 +/- 12°C and the liquidus is 1500-1525°C. Clinopyroxene, garnet, quartz, and rutile are subsolidus phases. Garnet, quartz, and rutile are absent above 1475°C, 1365°C, and 1335°C, respectively. At the solidus, the garnet mode is low (18 wt.%) because clinopyroxene is unusually aluminous (13.8-15.5 wt.% Al2O3). In adiabatically upwelling mantle near 2-3 GPa, G2-like pyroxenite begins melting 35-50 km deeper than peridotite. The calculated near-solidus adiabatic productivity for G2 is ~13%/GPa and averages ~59%/GPa through the melting interval, suggesting substantial partial melting deep in basalt source regions: G2 is ~60% molten at the 3 GPa peridotite solidus. Small percentages of pyroxenite in the source significantly affect oceanic crust production and composition, as the proportion of pyroxenite-derived melt contributed to oceanic crust formation is 5 to >10 times the pyroxenite proportion in the source. Given the overall depleted isotopic character of mid-ocean ridge basalt (MORB), oversampling of fertile G2-like pyroxenite limits the abundance of such lithologies to ~
This paper reports experiments carried out between 1.5 and 2.3 GPa using synthetic analogs of the molten peridotite system to investigate the effects of pressure (P), temperature (T), and variable bulk chemistry on the composition of melts multiply saturated with the miner- als present in the upper oceanic mantle: olivine, orthopyroxene, augite, and spinel (garnet). The multidimensional surface defined by melts coexisting with the lherzolite mineral assemblage produced in this study as well as from the literature is fit as a function of the variables: P, melt Mg number, and melt mole percent NaOo.,, KOo.,, Cr203, and TiO2, using multiple regression techniques. Forward models of polybaric, near-fractional melting using the parameterization presented here demonstrate that small extent, pooled magmas produced at greater initial pres- sures of melting differ in composition from small extent, pooled magmas produced at lower ini- tial pressures of melting. Furthermore, the small extent, pooled magmas generated at shallower initial pressures of melting fractionate at low P to magmas that are similar to the high-Na:O, low-FeO mid-ocean ridge basalts of the global array of Klein and Langmuir ( 1987), while the small extent, pooled magmas generated at greater initial pressures of melting fractionate at low P to magmas that do not resemble any mid-ocean ridge basalts present in the global data set. Preliminary data relevant for melting garnet-lherzolite indicate that the systematics of the key major element indicators of depth of melting (FeO and SiO2 in the melt) observed in melts gen- erated in the spinel stability field persist in melts generated in the lower P portion of the garnet stability field.
The dynamics of crustal rifting in Iceland depend strongly on the lower crustal rheology, which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume. While magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt, observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle. Since viscosity is also sensitive to the degree of partial melt present, viscosity estimates for these regions could shed light on the factors responsible for these observations. In this study we utilize horizontal and vertical displacement vectors determined in GPS campaigns in northeast Iceland since 1986. These are modeled in terms of steady state tectonic loading plus postseismic/postdiking relaxation following the 1975-1984 Krafla rifting episode, as first proposed by Foulger and others. With the elastic part of the model fixed by external constraints, these data have a high sensitivity to the viscosity structure beneath Iceland. Lower crust and upper mantle viscosities of about 3×1019 Pas and 3×1018 Pas, respectively, yield the closest agreement with the data. Our lower crustal viscosity estimate is consistent with the low attenuation and low (subsolidus) temperature for the lower curst inferred in recent studies. Inversions for fissure opening during the Krafla rifting episode yield about 7 m of opening centered on the Krafla rift, as is observed. Allowing for contemporaneous deep rifting on vertical faults along the Askja segment partially accounts for the observed increase in separation across the rift during 1987-1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the Askja rift during in 1987-1990. Recent deep normal faulting beneath the Askja rift and further south might explain all of these remaining features.
A striking feature of Icelandic volcanism is the effect that the last ice age had on volcanic activity. After the final retreat of ice ∼11 kyr BP, the average eruption rate is estimated to have been 20–30 times greater than it is today. This increase has been attributed to the release of pooled magma through differential tectonic movements during the unloading of ice. However recent work has shown that deglaciation can account for the increase in mantle melting by decreasing the pressure in the upper mantle. We present geochemical data and volume estimates of erupted magmas from Iceland's northern neovolcanic zone which show that the average composition of magmas erupted during the last glacial period in Iceland are significantly more enriched in incompatible trace elements than postglacial and interglacial lavas. The difference in light rare earth element concentrations cannot be accounted for by liquid–crystal fractionation. Averaging the compositions of glacial and postglacial magmas also eliminates the likelihood that the compositional change is due to variations in source composition. An increase in mantle melting from deglaciation can account for both the magma eruption rate and observed changes in trace element concentrations. Finite transport times for magma to travel from the source region to the surface can be estimated from the delay in timing of the increased eruption rates and the end of the last glacial period. This gives transport times of about 1–3 kyr and is consistent with estimates from (226Ra/230Th) activity ratios measured in ocean island and mid-ocean ridge basalts.
We present a map of the depth to the base of the upper crust and the total crustal thickness across Iceland constrained by seismic refraction results, receiver function analysis and gravity modelling. Upper crustal thicknesses (as defined by Ó.G. Flóvenz, J. Geophys. 47 (1980) 211–220) lie in the range of approximately 2–11 km, with the thinnest upper crust below active and extinct central volcanoes and the thickest upper crust close to the flanks of the rift zones. The thickest crust (40–41 km) lies above the centre of the Iceland mantle plume, where active upwelling and high mantle temperatures enhance melt production. Thick crust (∼35 km) is also found in eastern Iceland, between the current plume centre and the Faroe–Iceland Ridge. Elsewhere, the crust thins away from the plume centre. The thinnest crust (≤20 km) is found in the active rift in the northern part of the Northern Volcanic Zone, where melt production has been affected by a ridge jump, and in the far southwest of Iceland. The uppermost mantle below Iceland is characterised by reduced densities below the rift zones, suggesting higher mantle temperatures and the possible presence of partial melt in these regions.
The equations governing the movement of the melt and the matrix of a partially molten material are obtained from the conservation of mass, momentum, and energy using expressions from the theory of mixtures. The equations define a length scale δc called the compaction length, which depends only on the material properties of the melt and matrix. A number of simple solutions to the equations show that, if the porosity is initially constant, matrix compaction only occurs within a distance ∼δc of an impermeable boundary. Elsewhere the gravitational forces are supported by the viscous stresses resulting from the movement of melt, and no compaction occurs. The velocity necessary to prevent compaction is known as the minimum fluidization velocity. In all cases the compaction rate is controlled by the properties of the matrix. These results can only be applied to geological problems if the values of the permeability, bulk and shear viscosity of the matrix can be estimated. All three depend on the microscopic geometry of the melt, which is in turn controlled by the dihedral angle. The likely equilibrium network provides some guidance in estimating the order of magnitude of these constants, but is no substitute for good measurements, which are yet to be carried out. Partial melting by release of pressure at constant entropy is then examined as a means of produced melt within the earth.The principal results of geological interest are that a mean mantle temperature of 1350°C is capable of producing the oceanic crustal thickness by partial melting. Local hot jets with temperatures of 1550°C can produce aseismic ridges with crustal thicknesses of about 20 km on ridge axes, and can generate enough melt to produce the Hawaiian Ridge. Higher mantle temperatures in the Archaean can produce komatiites if these are the result of modest amounts of melting at depths of greater than 100 km, and not shallow melting of most of the rock. The compaction rate of the partially molten rock is likely to be rapid, and melt-saturated porosities in excess of perhaps 3 per cent are unlikely to persist anywhere over geological times. The movement of melt through a matrix does not transport major and trace elements with the mean velocity of the melt, but with a slower velocity whose magnitude depends on the distribution coefficient. This effect is particularly important when the melt fraction is small, and may both explain some geochemical observations and provide a means of investigating the compaction process within the earth.
Gravity data from Iceland and its surroundings are analysed and modelled
with respect to seismic data. From seismic data and with help of
relations between the residual topography and the depth to seismic
boundaries we construct maps of the main seismic boundaries including
the Moho. By inversion calculations we recomputed these maps, assuming
different density values for seismic layer 4 to fit the observed gravity
field. We found that the average density of layer 4 has to be in the
range 3050 - 3150 kg/m3 in order to fit both seismic and gravity data.
Thus we conclude that layer 4 is a transition zone between mantle and
the oceanic crust in Iceland. Furthermore by assuming that the upper
mantle density variations necessary to compensate the gravity effect of
crustal layers are caused by thermal variations in the upper mantle, we
calculate the depth to the 1200 C isotherm to be at 30-50 km depth below
Iceland but rising up to less than 20 km below parts of the volcanic
zone in Northern Iceland. We conclude that the temperature within the
seismic layer four is close to 600 C at its top, increasing to
approximately 950 C at its bottom (Moho), which makes a widespread
layer of partially molten material within layer 4 unlikely. By use of
cross spectral analysis of the gravity field and the external
topographic load at short wavelengths, we conclude that the elastic
plate thickness in Iceland can hardly exceed 6 km. In addition we point
out that the residual isostatic anomalies have circular forms east of
the eastern volcanic zone but are near parallel to the ridge axis on the
western side. This form of the anomalies may be caused by pressure from
the eastward moving mantle plume below the eastern volcanic zone.