Aleutian Arc Research Articles

Sustained Co‐Eruptive Increase in Seismic Velocity Below Great Sitkin Volcano Due To Magma Extrusion

AbstractVolcanic eruptions carry essential information on the dynamics of volcanic systems. Studies have documented variable eruption styles and eruptive surface deformation. However, co‐eruptive subsurface structural changes remain poorly understood. Here we characterize the seismic velocity changes from July 2019 to July 2023 at Great Sitkin Volcano in the central Aleutian volcanic arc, using single‐station ambient noise interferometry at five three‐component seismic stations. Coincident with the lava effusion since late July 2021, about two months after the explosive eruption on 26 May 2021, we observe a sustained velocity increase, most prominently to the northwest of the caldera. We attribute this velocity increase to the structural changes with magma extrusion, with the spatial variation controlled by the geometry of the magma system or the property of shallow volcaniclastics. Our findings offer insights into understanding co‐eruptive structural modifications at active volcanoes.

Origins and paleoclimatic and paleoceanographic significance of laminated sediments of middle Pleistocene age from the southern Bering Sea

Laminated diatomaceous sediments occur intermittently in the Bering Sea over the past 5 million years. A varve (annually deposited) origin for the laminae has been suggested, but there is currently no consensus. Here, we report results of a study of two laminated intervals dating from ∼528 ka and ∼ 782 ka from IODP Site U1340 on the Bowers Ridge. We combine conventional micropaleontological methods with scanning electron microscope analysis that resolves the seasonal cycle of flux events recorded and demonstrates an annual origin for the laminae. Resting spores of Chaetoceros spp., and more rarely of Thalassiosira antarctica represent early spring blooms with the latter likely due to increased meltwater input. Diatom laminae containing concentrations of Shionodiscus trifultus, Thalassiosira spp., Actinocyclus curvulatus, and Neodenticula seminae, represent flux from the main spring-summer bloom episodes, with the latter indicating influx of seed populations from the North Pacific Alaskan Stream. Concentrations of Coscinodiscus spp. in the uppermost part of the diatom laminae represent “Fall Dump” sedimentation of these stratified-adapted diatoms in response to break down of summer stratification in autumn/early-winter storms. The lithogenic laminae represent mainly winter deposition and rare earth element analysis suggests provenance from the southern Bering Sea shelf and the Aleutian Arc. Productivity was high in the studied intervals with total mass fluxes around 5 times higher than modern values. Variation in lamina thickness and diatom composition contain periodicities of 2–8 years, as well as a bi-decadal variability likely related to influence of the Pacific Decadal Oscillation.

SEISMICITY of NORTHERN EURASIA in 2018–2019

An overview of seismicity in 16 regions of Russia and neighboring countries in 2018–2019 is presented. Seismic monitoring of this territory was carried out by networks of seismic stations of Russia, Azerbaijan, Armenia, Belarus, Kazakhstan, Kyrgyzstan, Moldova, Turkmenistan, Tajikistan, Uzbekistan, Ukraine. In most regions, the seismic process occurred in the background. In Northern Eurasia as a whole, the total seismic energy in 2018 (Еan=17∙1016 J) remained unchanged compared to that for the previous year, and in 2019 fell to Еan=0.65∙1016 J, with annual values of released energy for 2018 and 2019 were significantly lower than its average annual level for the period 2006–2017. (Еan=5.3∙1016 J). The “Angular Uplift earthquake” on December 20, 2018 at 17h01m with MS=7.4 (MwGCMT=7.2), localized in the surface layer of the seismofocal zone of the Commander segment of the Aleutian arc with the epicenter in the Pacific Ocean was the strongest for the period 2018– 2019. Maximum shaking intensity on the tyerritory of Northern Eurasia (I=6–7) was caused by earthquake on July 6, 2018 at 01h40m with MwGCMT=6.2, h=75 km in the seismofocal zone of the Kuril Islands–Southern Kamchatka and Katav-Ivanovsk earthquake on September 4, 2018 at 22h58m in the Chelyabinsk oblast (Ural and Western Siberia region). The Katav-Ivanovsk earthquake is also unique in that it was accompanied by a significant number of aftershocks, which is the first known case for tectonic events within the Urals. In addition, it demonstrated that the natural seismic potential of the Urals exceeds the man-made component by orders of energy

The Crustal Magmatic Structure Beneath the Denali Volcanic Gap Imaged by a Dense Linear Seismic Array

AbstractThe crustal structure in south‐central Alaska has been influenced by terrane accretion, flat slab subduction, and a modern strike‐slip fault system. Within the active subduction system, the presence of the Denali Volcanic Gap (DVG), a ∼400 km region separating the active volcanism of the Aleutian Arc to the west and the Wrangell volcanoes to the east, remains enigmatic. To better understand the regional tectonics and the nature of the volcanic gap, we deployed a month‐long north‐south linear geophone array of 306 stations with an interstation distance of 1 km across the Alaska Range. By calculating multi‐component noise cross‐correlation and jointly inverting Rayleigh wave phase velocity and ellipticity across the array, we construct a 2‐D shear wave velocity model along the transect down to ∼16 km depth. In the shallow crust, we observe low‐velocity structures associated with sedimentary basins and image the Denali fault as a narrow localized low‐velocity anomaly extending to at least 12 km depth. About 12 km, below the fold and thrust fault system in the northern flank of the Alaska Range, we observe a prominent low‐velocity zone with more than 15% velocity reduction. Our velocity model is consistent with known geological features and reveals a previously unknown low‐velocity zone that we interpret as a magmatic feature. Based on this feature's spatial relationship to the Buzzard Creek and Jumbo Dome volcanoes and the location above the subducting Pacific Plate, we interpret the low‐velocity zone as a previously unknown subduction‐related crustal magma reservoir located beneath the DVG.

A 6.2 Ma‐Long Record of Major Explosive Eruptions From the NW Pacific Volcanic Arcs Based on the Offshore Tephra Sequences on the Northern Tip of the Emperor Seamount Chain

AbstractWe present a continuous ∼6.2 Ma long record of explosive activity from the Northwest Pacific volcanic arcs based on a composite tephra sequence derived from Ocean Drilling Program Sites 882A and 884B, and core MD01‐2416 on the Detroit Seamount. Geochemical fingerprinting of tephra glass using major and trace element analyses and correlations of tephra layers between the three cores allowed the identification of 119 unique tephras, suggesting eruptions of magnitude (M) of 5.8–7.8. Age estimates for all the identified eruptions were obtained with the help of published and further refined age models for the studied cores, direct 40Ar/39Ar dating of four ash layers, and Bayesian age modeling. The glass compositions vary from low‐ to high‐K2O basaltic andesite to rhyolite and exhibit typical subduction‐related affinity. The majority of the tephras originated from Kamchatka, only a few tephras—from the neighboring Kuril and Aleutian arcs. The glass compositions revealed no temporal trends but made it possible to identify their source volcanic zones in Kamchatka and, in some cases, to determine their source eruptive centers. Our data indicates episodes of explosive activity recorded in the Detroit tephra sequence at ∼6,200, 5,600–5,000, 4,300–3,700 ka, and almost continuous activity since ∼3,000 ka. Within the latter episode, the most active intervals can be identified at 1,700–1,600, 1,150–1,050, and 600–50 ka. Geochemically fingerprinted and dated Detroit tephra sequence form a framework for dating and correlating diverse paleoenvironmental archives across the Northwest Pacific and for studies of geochemical evolution of the adjacent volcanic arcs.

Double Reservoirs Imaged Below Great Sitkin Volcano, Alaska, Explain the Migration of Volcanic Seismicity

AbstractVolcanic seismicity provides essential insights into the behavior of an active volcano across multiple time scales. However, to understand how magma moves as the eruption cycle develops, better knowledge of the geometry and physical properties of the magma plumbing system is required. In this study, using full‐wave ambient noise tomography, we image the three‐dimensional (3‐D) crustal shear‐wave velocity structure below Great Sitkin Volcano in the central Aleutian Arc. The velocity model reveals two low‐velocity anomalies correlating with the migration of volcanic seismicity. With a bulk melt fraction of about 2.5%–9%, these low‐velocity anomalies are interpreted as mushy magma reservoirs. We propose a six‐stage eruption cycle to explain the migration of seismicity and the alternating eruption of the two reservoirs with different recharging histories. These findings have broad implications for the dynamics of magma plumbing systems and the structural control of eruption behaviors.

Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics

Mo isotope compositions have been increasingly used as a source tracer in magmatic systems. Here, we investigate the relative role of subducting oceanic crust, fluid-rich sources (e.g., serpentinite, altered oceanic crust – AOC), subducted sediments and upper mantle in the chemical composition of Aleutian arc magmas with Mo and B systematics. We present elemental and Mo isotope compositions (δ98/95Mo) and B concentrations on Aleutian lavas (n = 59), from Okmok Volcano in the east to the westernmost seamount Piip, showing absence of Mo isotope fractionation during magmatic differentiation. Additionally, we report Mo isotope systematics for serpentinized peridotites from the South-West Indian Ridge (SWIR) (n = 6), AOC (n = 2) and Pacific sediments (DSDP 183, ODP 886) (n = 5) outboard the Aleutian arc. Molybdenum isotope composition and B enrichment (e.g., B/Ce) patterns display a step-function increase along the arc, with low, MORB-like, δ98/95Mo and low B/Ce values in the western section of the arc (B/Ce = 0.15–1.07; δ98/95Mo =−0.38 to +0.01‰), that abruptly increase in the central-eastern volcanoes Korovin, Seguam and Yunaska (B/Ce = 1.20–2.60; δ98/95Mo = +0.03 to +0.30‰) near the intersection of the Amlia Fracture Zone (AFZ) with the trench, but decrease again farther east at Okmok (B/Ce = 0.76 and δ98/95Mo = −0.12‰ on average). These data patterns are interpreted to reflect an along-arc changing source in the Aleutian magmas. AOC and Pacific sediments have predominantly low δ98/95Mo (−0.47 to −0.32 and +0.17 to −1.9‰, respectively), while serpentinites have extremely high δ98/95Mo (up to +1.09‰) and high B/Ce (∼22000). Based on the low δ98/95Mo in sediments and AOC, and lack of correlation between along-arc δ98/95Mo and radiogenic sediment tracers, subducted sediments and AOC do not exert first-order controls on the observed Mo isotope compositions. Rather, low, MORB-like, δ98/95Mo but high Mo enrichments (e.g., Mo/Ce) in the western samples are consistent with slab melting under rutile-bearing eclogitic facies with near absent Mo isotope fractionation from the slab to the arc sources. In turn, the abrupt increase of δ98/95Mo and B/Ce in lavas near the AFZ are best explained by a serpentinite endmember (likely dehydration fluids) at the AFZ that is not evident elsewhere along the arc. Results from this study provide evidence for serpentinites as an additional heavy Mo isotope signature component in subduction zones and demonstrate that high δ98/95Mo coupled with B enrichments are a useful proxy for tracing serpentinite fluids in subduction zones.

Double‐Difference Adjoint Tomography of the Crust and Uppermost Mantle Beneath Alaska

AbstractWe perform adjoint waveform tomography to reveal the P‐wave velocity structure of the crust and uppermost mantle in Alaska by using common‐source double‐difference traveltime data. Our underlying forward modeling tool is a 3D seismic‐wave solver called SPECFEM3D_GLOBE. We select ∼13,000 high‐quality P‐wave arrivals from 147 earthquakes recorded by more than 600 stations. The waveforms are filtered between 15 and 8 s and used to generate about 100,000 differential traveltimes via the cross‐correlation technique. We invert these common‐source differential traveltimes for a P‐wave velocity model down to 150 km depth in Alaska. In the upper mantle, a high‐velocity zone is imaged along the Aleutian volcanic arc, which represents the Pacific plate subducting beneath the North America plate. On the eastern edge of the subducting Pacific plate, the imaged high‐velocity anomaly extends distinctly beyond the Wadati–Benioff zone, indicating an aseismic slab edge. We observe an absence of a low‐velocity mantle wedge beneath the Denali Volcanic Gap (DVG) in our tomographic model. This finding suggests that minimal melt accumulation exists beneath the DVG, which may explain the cessation of magmatism there. Our model also reveals a high‐velocity anomaly near the Wrangell Volcanic Field (WVF), suggesting the possible existence of the Wrangell slab or the high‐velocity slab edge of the Yakutat slab. A potential slab gap shown as a low‐velocity body is detected at 95–125 km depth near the WVF, which could act as a channel to transport mantle materials to feed the cluster of volcanoes in the WVF.

ПРОДОЛЬНЫЕ СДВИГИ В ОСТРОВНОЙ ДУГЕ ПРИ НОРМАЛЬНОМ ПОДДВИГЕ ОКЕАНИЧЕСКОЙ ПЛИТЫ: ПРИМЕР КАМЧАТКИ

Crustal deformation in Kamchatka since the Middle Pleistocene is driven by two processes: transverse extension as a result of its eastern part displacement towards the ocean following the submerged part of the oceanic plate, and transverse shortening just north of the subduction zone as the Komandor block of the Aleutian arc gets closer to Kamchatka. The areas of differently directed movements and deformations of Kamchatka are separated by the left-lateral horizontal flexure appearing in plan as curved uplifts of the Eastern Range, the deep water trench, and consequently, the axis of the East volcanic belt. Kinematically, the flexure replaces the expected transverse left-lateral strike-slip fault between the two differently deformed parts of Kamchatka. The revealed right-lateral displacements along the longitudinal faults in the northern part of the uplift of the Eastern Range in Kamchatka result from bedding-plane slip in the flexure limb. The high alluvial terrace of the Kamchatka River, displaced landforms, and faulted layers of the soil-pyroclastic cover were dated by the tephrochronological method. Age determinations imply that the recurrence of displacements along the faults of the flexure is about once every 3-5 thousand years, and the average rate of lateral displacements during the second half of the Holocene is about 1 mm per year. Comparison of the fault movement rates and the maximum value of the accumulated lateral displacement of the Kamchatka River valley suggest that horizontal movements began no later than the second half of the Middle Pleistocene. This also means that the Kamchatka River valley already existed at that time, and its antecedent part was formed when down-cutting into the bedrock of the uplifting Eastern Range. The total amount of displacement along the faults of the flexure since the second half of the Middle Pleistocene is estimated at about 1 km. On the whole, the results suggest the possibility of lateral movements along longitudinal faults in an island arc with the oceanic plate direction being normal to it.

SEISMICITY of NORTHERN EURASIA in 2016–2017

The review of the seismicity of Northern Eurasia in 2016–2017 contains a description of seismic networks, features of the seismic regime and individual significant earthquakes in 16 regions of Russia and neighboring countries. Seismic monitoring of this territory was carried out by networks of seismic stations in Russia, Azerbaijan, Armenia, Belarus, Kazakhstan, Kyrgyzstan, Latvia, Moldova, Turkmenistan, Tajikistan, Uzbekistan and Ukraine. In most regions, the seismic process proceeded in the "background" mode. In Northern Eurasia as a whole, a decrease in the seismic energy released in 2016 was noted relative to its long-term average value, which was also observed in 2015. The increase in the released energy in 2017 did not lead to reaching the level of 2015. The Near Aleutian earthquake on July 17, 2017 with Mw=7.8 was the maximum for the period under consideration. It was one of the strongest instrumentally recorded events in the western part of the Aleutian arc. The maximum shaking intensity in the territory of Northern Eurasia (I=7) was caused by two earthquakes – the Aketao earthquake on November 25, 2016 with Mw=6.6, localized in China, near its borders with Tajikistan and Kyrgyzstan, and the Karamyk earthquake on May 3, 2017 with Mw=6.0, Tajikistan. Seismic activity of platform territories is confirmed by the Mariupol earthquake on August 7, 2016 with Mw=4.6 and intensity I=5 in Mariupol.

The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Subduction disruption, slab tears: ca. 1 Ma true collision of an ~30‐km‐thick oceanic plateau segment recorded by Yakutat slab nascent tear magmatism

AbstractDistinguishing the initiation of actual collision from flat‐slab subduction of oceanic buoyant highs along convergent margins is elusive because both can lead to inboard deformation and disrupt magmatic arcs. Volcanoes with nascent tear magmatic signatures provide a means to document both the occurrence and timing of actual oceanic buoyant high collision. There is a ~40‐year debate on when the true collision of the Yakutat plateau began in Alaska. Three newly identified ca. 1 Ma volcanoes with a north‐to‐south trench perpendicular orientation, nascent tear geochemical signatures, overlaying an imaged Yakutat slab tear, provide constraints on the timing of Yakutat collision and slab tearing. The ca. 1 Ma slab tear is coincident with Yakutat slab segmentation, northern continental Aleutian Arc rejuvenation, cessation of Wrangell Arc magmatism, increased collisional zone exhumation and eastern Yakutat trench abandonment. The documentation of nascent slab tear volcanoes may help resolve similar debates in other convergent margin settings.

Trigger Mechanisms of Gas Hydrate Decomposition, Methane Emissions, and Glacier Breakups in Polar Regions as a Result of Tectonic Wave Deformation

Trigger mechanisms are proposed for gas hydrate decomposition, methane emissions, and glacier collapse in polar regions. These mechanisms are due to tectonic deformation waves in the lithosphere–asthenosphere system, caused by large earthquakes in subduction zones, located near the polar regions: the Aleutian arc, closest to the Arctic, and the Antarctica–Chilean and Tonga–Kermadec–Macquarie subduction zones. Disturbances of the lithosphere are transmitted over long distances (of the order of 2000–3000 km and more) at a speed of about 100 km/year. Additional stresses associated with them come to the Arctic and Antarctica several decades after the occurrence of seismic events. On the Arctic shelf, additional stresses destroy the microstructure of metastable gas hydrates located in frozen rocks at shallow depths, releasing the methane trapped in them and leading to filtration and emissions. In West Antarctica, these wave stresses lead to decreases in the adhesions of the covered glaciers with underlying bedrock, sharp accelerations of their sliding into the sea, and fault occurrences, reducing pressure on the underlying rocks containing gas hydrates, which leads to their decomposition and methane emissions.

Multiple Magma Sources Beneath the Okmok Caldera as Inferred From Local Earthquake Tomography

AbstractOkmok volcano located on the northeastern part of the Umnak Island is one of the most active volcanoes in the Aleutian Arc. It was initially built as a large shield volcano, but 12,000 and 2,050 years ago, it experienced two caldera‐forming eruptions that destroyed the central part of the volcano. The post‐caldera eruptions have occurred mostly along the inner perimeter of the caldera from cinder and tuff cones. Here, we use the local earthquake data of the Alaska Volcano Observatory (AVO) in the time period from 2003 to 2017 to build a model with the 3D distributions of the P and S wave velocities and Vp/Vs ratio. At depths of more than 10 km, we observe a vertically aligned anomaly of high Vp/Vs ratio interpreted as a long‐lived conduit likely responsible for the volcano evolution since its origin. Above this conduit, we reveal a large anomaly of high Vp/Vs ratio representing the main magma reservoir that is connected with several shallow anomalies located below the centers of recent eruptions in the caldera. One of such anomalies represents a large shallow reservoir below the Cone A, which was the source of most of Okmok's historical eruptions. The most recent eruption occurred in 2008 and was fed by a magma diapir that was initially formed in the central magma reservoir and then slowly ascended through a ductile silica rich layer at depths from 6 to 2 km. This interpretation is consistent with the petrology studies and modeling of ground deformations.

FOCAL MECHANISMS OF EARTHQUAKES IN THE SUBDUCTION ZONE OF THE WESTERN PACIFIC PLATE

Deformation features of the subducting Pacific lithospheric plate are considered according to the data on earthquake focal mechanisms. The territory includes the convergent boundaries between the Pacific Plate and the North American (in the Aleutian arc region), the Okhotsk, the Eurasian and the Philippine plates.It has been shown that the angle of subducting Pacific Plate in the Aleutian subduction zone affects the focal mechanisms of earthquakes that occurred in the upper, 35 km part of the oceanic plate in the zone of its bending. There occur normal-fault earthquakes at a steep-angle subduction and rare thrust earthquakes at a shallow-angle subduction. The azimuthal orientation of P-axes of the focal mechanism solutions in the upper (1–70 km) contact zone corresponds to the Pacific Plate displacement vector when the plate fragments are subducting west-northwestwards. There occurs a change in azimuthal orientation of the compression axes in the subducting plate at a depth of more than 70 km: the axes occupy different azimuthal sectors showing difference in the orientation of their slope, with the orientations of the T-axes become multidirectional.The calculation of seismotectonic deformations was carried out based on the data on focal mechanisms of 7768 earthquakes. It was revealed that the Exx and Ezz deformation fields are the most homogeneous at depths of 1–70 km. The pattern of seismotectonic deformations changes abruptly for deep parts of the subducting plate (105–200, 200–400, and 400–700 km), there are observed heterogeneous deformation fields Exx, Eyy and Еzz with alternating episodes of extension and shortening.There has been proposed the author’s scheme of the influence of the upper mantle convection structure on the geometry of the subducting plate (slab) as a potential catalyst for the processes responsible for the separation of seismic activity zones and the change of earthquake types with depth and in different parts of the extended subduction zone.

Structure of the magma plumbing system beneath Semisopochnoi Island (Aleutian Arc) inferred from seismic tomography

Semisopochnoi Island is a remote and little-studied volcanic island in the western part of the Aleutian Arc. The existence of several active volcanic centers and a 5000–7000-year-old large caldera makes this island an important site for volcanic hazard assessment in the Northern Pacific. Based on local seismicity data recorded by six permanent seismic stations, we created the seismic tomography model, including the 3D distributions of Vp, Vs, and Vp/Vs ratios to a depth of 10 km. This model provides the first geophysical insight into the interior structure of Semisopochnoi Island and sheds light on the processes in the magma plumbing system beneath all volcanic centers on the island. At depths of 5–10 km, we observed a columnar-shaped high Vp/Vs-ratio anomaly below the caldera in the central part of the island, which likely represents the steady magma conduit. This conduit is headed by a prominent high Vp/Vs-ratio anomaly located 3–5 km directly below the caldera, which represents the magma reservoir feeding Cerberus and other Holocene-aged volcanic centers on Semisopochnoi Island.

A Data-Based Minimal Model of Episodic Inflation Events at Volcanoes

Space geodetic time series, be they ground-based or space-based, have increased in length and accuracy. These series can now be mined for information on the qualitative dynamics of volcanic systems directly from surface deformation data. Here, we study three volcanoes: Akutan and Okmok that are part of the Aleutian arc, and Piton de la Fournaise on la Reunion Island. All three are continuously monitored by the Global Positioning System (GPS) and exhibit common stair step–shaped inflation cycles sometimes referred to as to as “episodic inflation events”. Here we seek to characterize the corresponding dynamical regime of pressure build-up within their plumbing system. To do so, we make use of Multichannel Singular Spectrum Analysis (M-SSA), a data-adaptive, non-parametric time series analysis methodology that allows for 1) the reliable detection and extraction of such patterns even when the corresponding signal lies close to, or even below, the data scatter; and 2) the extraction of information relevant to the underlying qualitative dynamics without a priori assumptions on the underlying physical mechanisms. For our three volcanoes, we find that the inflation cycles resemble the relaxation oscillations of a simple oscillator that involves a nonlinear dissipative mechanism. This finding provides important guidelines for physics-based models of episodic inflation cycles. In fact, the three volcanoes share a plumbing system composed of several interconnected storage bodies. Guided by the qualitative M-SSA–inferred dynamics, we formulate a simple physical model of two magma bodies connected by a conduit in which the viscosity of the fluid varies with temperature or magma crystallization. We show that such a model possesses internal relaxation oscillations similar to those of a simple oscillator. These oscillations correspond to repetitive events with sharp variations in the rate of magma transport and they can account for episodic events of pressure build-up in magma bodies, with no need for a time-dependent magma flux into or out of the system. We also show that the model’s number of degrees of freedom is consistent with the amount of information extracted from M-SSA data analysis. The approach presented here relies on the robust statistical analysis of deformation time series to constrain the phenomenology of pressure build-up within a volcanic plumbing system; it provides a novel framework for understanding the dynamics of volcanic systems.

The Impact of Ice Caps on the Mechanical Stability of Magmatic Systems: Implications for Forecasting on Human Timescales

Monitoring the activity of subglacial volcanoes along the Aleutian Arc in Alaska is important to the safety of local populations, as well as air traffic flying through the region. However, observations of volcanic unrest are limited by accessibility and resources, particularly at glacier-covered systems, making investigations of their stability challenging. Westdahl Peak, a subglacial volcano on Unimak Island in the Aleutian Arc has experienced significant unrest and uplift since its most recent VEI three eruption in 1991-1992. Given the magnitude of observed uplift, previous investigations suggested the potential for eruption by 2010, but no such event has occurred. One hypothesis to explain this prolonged unrest is that the 1-km thick glacier may increase the stability of the magma system. However, the impact of ice caps and glaciers on the short-term stability of volcanoes is not well understood. In this study, thermomechanical finite element models are used to evaluate how the stability of a glaciated volcano is impacted by variations in ice cap thickness, magma chamber depth, geometry, magma flux rate, and seasonal changes in ice cover thickness. Our numerical experiments indicate that the presence of an ice cap (1–3 km thick) increases the average repose interval for a magma system. Among models with different magma chamber geometries, depths, and flux rates, the greatest increases in repose interval are observed in prolate systems where the increase is up to 57% for a chamber located at 5 km-depth. Spherical and oblate also experience smaller, yet significant, increases in repose interval. Additionally, the percentage increase in repose interval is not impacted by variations in magma flux rate for a given ice cap thickness and magma chamber geometry. However, flux rates do influence the timing of eruptions when the system is experiencing seasonal variations in ice thickness. Our results show that systems with low flux rates are more likely to fail when the ice thickness is at its lowest. The numerical estimates further suggest that the ice cap on Westdahl Peak, which is ∼1 km, may slightly increase the stability of the magma system. In general, given flux rates and magma chamber geometries estimated for the Westdahl system, the repose interval can increase by ∼7 years due to the Westdahl glacier. This increase is small on a geologic scale but is significant on human time scales and the impact of glaciers must be considered in future forecasting efforts.

Structure of the magma plumbing system beneath Semisopochnoi Island (Aleutian Arc) inferred from seismic tomography

Structure of the magma plumbing system beneath Semisopochnoi Island (Aleutian Arc) inferred from seismic tomography

No evidence for tephra in Greenland from the historic eruption of Vesuvius in 79 CE: implications for geochronology and paleoclimatology

Abstract. Volcanic fallout in polar ice sheets provides important opportunities to date and correlate ice-core records as well as to investigate the environmental impacts of eruptions. Only the geochemical characterization of volcanic ash (tephra) embedded in the ice strata can confirm the source of the eruption, however, and is a requisite if historical eruption ages are to be used as valid chronological checks on annual ice layer counting. Here we report the investigation of ash particles in a Greenland ice core that are associated with a volcanic sulfuric acid layer previously attributed to the 79 CE eruption of Vesuvius. Major and trace element composition of the particles indicates that the tephra does not derive from Vesuvius but most likely originates from an unidentified eruption in the Aleutian arc. Using ash dispersal modeling, we find that only an eruption large enough to include stratospheric injection is likely to account for the sizable (24–85 µm) ash particles observed in the Greenland ice at this time. Despite its likely explosivity, this event does not appear to have triggered significant climate perturbations, unlike some other large extratropical eruptions. In light of a recent re-evaluation of the Greenland ice-core chronologies, our findings further challenge the previous assignation of this volcanic event to 79 CE. We highlight the need for the revised Common Era ice-core chronology to be formally accepted by the wider ice-core and climate modeling communities in order to ensure robust age linkages to precisely dated historical and paleoclimate proxy records.