Lava Extrusion Research Articles

Source Mechanism of Seismic Explosion Signals at Santiaguito Volcano, Guatemala: New Insights From Seismic Analysis and Numerical Modeling

Volcanic activity at the Santiaguito dome complex (Guatemala) is characterized by lava extrusion interspersed with small, regular, gas-and-ash explosions that are believed to result from shallow magma fragmentation; yet, their triggering mechanisms remain debated. Given that the understanding of source processes at volcanoes is essential to risk assessments of future eruptions, this study seeks to shed light on those processes. We use data from a permanent seismic and infrasound network at Santiaguito volcano, Guatemala, established in 2018 and additional temporary stations, including a seismic array deployed during a 13-day field investigation in January 2019 to analyze and resolve the source characteristics of fragmentation leading to gas-and-ash explosions. Seismic data gathered within a distance of 4.5 km from the vent show a weak seismic signal 2–6 s prior to the explosions and associated main seismic signal. To resolve the source location and origin of the seismic signals, we first used ambient noise analysis to assess seismic velocities in the subsurface and then used two-dimensional spectral element modeling (SPECFEM2D) to simulate seismic waveforms. The analyzed data revealed a two-layer structure beneath the array, with a shallow, low-velocity layer (vs= 650 m/s) above deeper, high-velocity rocks (vs= 2,650 m/s). Using this velocity structure, possible source mechanisms and depths were constrained using array and particle motion analyses. The comparison of simulated and observed seismic data indicated that the precursory signal is associated with particle motion in the RZ-plane, pointing toward the opening of tensile cracks at a depth of ∼600 m below the summit; in contrast, the main signal is accompanied by a vertical single force, originating at a shallow depth of about ∼200 m. This suggests that the volcanic explosions at Santiaguito are following a bottom-up process in which tensile fractures develop at depth and enable rapid gas rise which leads to the subsequent explosion. The result indicates that explosions at Santiaguito do not occur from a single source location, but from a series of processes possibly associated with magma rupture, gas channeling and accumulation, and fragmentation. Our study provides a good foundation for further investigations at Santiaguito and shows the value of comparing seismic observations with synthetic data calculated for complex media to investigate in detail the processes leading up to gas-ash-rich explosions found at various other volcanoes worldwide.

Insights into lava dome and spine extrusion using analogue sandbox experiments

Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.

Spectral and geological analyses of domes in western Arcadia Planitia, Mars: Evidence for intrusive alkali-rich volcanism and ice-associated surface features

Small-scale domes with circumferential aprons and concentric aureoles in western Arcadia Planitia (34–41°N, 167–179°E) near Tyndall crater were examined using a suite of datasets including CRISM, THEMIS IR, HiRISE, and CTX. Previous studies based primarily on photogeologic evidence suggested that these domes were analogous to terrestrial felsic cryptodomes to extrusive lava domes. The domes have also been examined using CRISM visible/near infrared to short-wave infrared (VNIR-SWIR) reflectance spectra which indicated the presence of ferrous silicate minerals in association with the domes. This study presents further CRISM spectral evidence for 1) high-Ca pyroxene and glass mixtures with, or possibly without, the presence of olivine on the flanks of some domes, 2) 1.3 μm band absorption features consistent with an Fe-bearing plagioclase or possibly a Fe-rich alkali feldspar in more limited occurrences at the base of some domes, 3) spectral convexity between 3.4 and 3.9 μm associated with rocky, light-toned portions on top of some domes which is attributed to the presence of alkali-rich plagioclase or alkali feldspars. New morphologic observations include a pitted cone and arcs of light-toned, “brain terrain” material on the pole-facing upper margins of some aprons which, in combination with ice-associated “brain terrain” on light-toned outer aureoles suggests an association with ice. The morphology of an assortment of domes and association with alkali feldspars suggests they represent a continuum from intrusive cryptodomes to flat extrusive domes, potentially of felsic composition although formation from viscous alkali-rich mafic magmas is not precluded.

Eruptive history and volcano-tectonic evolution of Paka volcanic complex in the northern Kenya rift: Insights into the geothermal heat source

In this paper, we provide insights into the eruptive history and volcano-tectonic evolution of the Paka volcanic complex in a revised stratigraphic framework. We integrate all available 40Ar/39Ar radiometric dates along with field observations, remote sensing data, and the analysis of surface and cutting samples from a 2552 m deep borehole (PK-01). Chemical analyses distinguish basalt, hawaiite, mugearite, benmorite, and trachyte rock type in Paka. It also indicates that the eruptive products are genetically related by fractional crystallization. Major element mass balance and Rhyolite-MELTS models are consistent with the trachyte being generated by 74–83% fractional crystallization of Paka basalt. Before the growth of the volcano, volcanic activity was characterised by plateau fissure eruptions at 582–405 ka. Eruptions directly related to the Paka edifice are divided into four different volcanic sequences. Sequences 1–4 span the periods 390–278 ka, 247–205 ka, 160–36 ka, and 36–8 ka. The Pre-Paka plateau eruption products outcrop at the base of the volcano flanks and are overlain by edifice-forming trachyte. Initial caldera-related subsidence is expressed by arcuate structures to the west and southeast parts of the volcano. These initial collapses are associated with tuff and pyroclastic eruptions estimated to have occurred at ~38 ka. Following these explosive eruptions, a ~1.5 by ~1.6 km main caldera developed. The lithological information from the drilled geothermal exploration borehole PK-01 indicates a minimum thickness of ~2200 m for the extrusive lava sequences, with the uppermost 1050 m of the strata being directly related to eruptions from the Paka volcano (Phases 1–4). We estimate a minimum total bulk volume of ~50 km3 for the volcanic material erupted from Paka during the last ~390 ka. This translates to an average eruption rate of ~1.2 × 10−4 km3/yr. The associated advected magmatic heat supported an ‘excess’ heat flux ranging between 110 and 138 mW/m2 or about twice present day average continental flux (57 mW/m2) and about 1.3 times the background values in geothermal areas (90 mW/m2). The apparent high heat flux implies a magma-driven geothermal system, where convection of the hydrothermal fluid is occurring above the heat source. Structural mapping revealed normal faults, strike-slip faults, lineaments, and an array of eruptive vents and domes as the main volcano-tectonic structures. We infer at least four faulting and fault reactivation events that have occurred during Paka geological history. The structural orientations in the area are dominated by NNE and NW with subordinate N-S and NNW striking structures. Evidence suggests that the intersection of NNE and NW trending structures may have had a major influence on the volcanism in the area.

Diffusion Timescales of Magmatic Processes in the Moinui Lava Eruption at Mauna Loa, Hawai`i, as Inferred from Bimodal Olivine Populations

Abstract The 2·1 ka Moinui lava flow field, erupted from the southwest rift zone of Mauna Loa, Hawai`i, exhibits striking textural and geochemical variations, that can be used to interpret magma processes pre-, syn- and post-eruption. From this lava flow, the duration of magma storage and storage conditions, the timescales over which magma is transported to the surface, and flow emplacement mechanisms at Mauna Loa are determined. Electron microprobe analysis (EMPA) and diffusion chronometry of olivine crystals identify two distinct crystal populations: a primitive, polyhedral olivine population with core compositions of Fo90–88 and a more evolved, platy olivine population with core compositions of Fo83–82. Fe–Mg diffusion modelling of these olivine populations gives distinct timescales for each population; platy olivines yield timescales of days up to a few weeks, while polyhedral olivines yield timescales of months to years. Despite the nature of a well-insulated pāhoehoe flow, meaning that post-emplacement diffusion continues for some time, a wealth of time information can be retrieved concerning pre-eruptive magmatic processes as well as the processes associated with the lava extrusion. The short timescales obtained from the platy olivine crystals and the observed equilibrium between its cores and ambient melt suggest late-stage nucleation and crystal growth in the shallow conduit and during lava emplacement. Conversely, the longer timescales and olivine-melt disequilibrium of the polyhedral olivine crystals suggests accumulation from a deeper source and subsequent transportation to shallow magma storage beneath the summit of Mauna Loa months, or even years before eruption. The chemical and textural details of the Moinui lava reflect the mode of flow emplacement and may have implications for the interpretation of the distribution of spinifex and cumulate olivine within komatiites; high-temperature, low-viscosity lavas, common in the Archean.

Late-stage groundmass differentiation as a record of magma stagnation, fragmentation, and rewelding

In the 2011 eruption of Shinmoedake of the Kirishima volcano group, sub-Plinian eruptions were followed by lava extrusion with intermittent Vulcanian explosions. The interstices of microlites and relatively large nanolites (> 0.4 nm width) in the groundmass of pyroclasts (“groundmass interstices”) were studied to reveal shallow magmatic processes that lead to different eruption styles. The pumice of the sub-Plinian eruption has the least differentiated groundmass interstices composition. The groundmass interstices of the dense juvenile fragments of the Vulcanian explosions are divided into two groups. The first group is the most differentiated as expected from their lava-like texture, whereas the second group is less differentiated and covers the range of sub-Plinian pumice. The Vulcanian pumice overlaps the dense juvenile fragments despite their high vesicularity. This seemingly contradictory relationship of composition in the interstitial groundmass indicates the clastogenic lava origin of the less-differentiated group of dense juvenile fragments. In contrast, magmas stagnated in the shallow conduit maintained elevated temperature and water content, allowing groundmass interstices to differentiate by microlite growth. These magmas then erupted as pumice in the Vulcanian explosions and were effused as lava that was fragmented by subsequent explosions to be later sampled as a dense juvenile fragment. The groundmass crystallinity increased by 9.1 vol.% in 5–45 days between the sub-Plinian and Vulcanian eruptions, increasing initial melt and magma viscosity from 106.1–7.4 and 107.0–8.3 Pa s to 106.9–8.4 and 108.2–9.7 Pa s, respectively. This viscosity increase by nanolite crystallization could have facilitated the stress fragmentation of conduit magma, leading to Vulcanian explosions. Post-fragmentation expansion of the sub-Plinian pumice could have been suppressed by this viscosity increase, resulting in their low vesicularity. Late-stage groundmass differentiation can thus control shallow magmatic processes.

UAS-based tracking of the Santiaguito Lava Dome, Guatemala

Imaging growing lava domes has remained a great challenge in volcanology due to their inaccessibility and the severe hazard of collapse or explosion. Changes in surface movement, temperature, or lava viscosity are considered crucial data for hazard assessments at active lava domes and thus valuable study targets. Here, we present results from a series of repeated survey flights with both optical and thermal cameras at the Caliente lava dome, part of the Santiaguito complex at Santa Maria volcano, Guatemala, using an Unoccupied Aircraft System (UAS) to create topography data and orthophotos of the lava dome. This enabled us to track pixel-offsets and delineate the 2D displacement field, strain components, extrusion rate, and apparent lava viscosity. We find that the lava dome displays motions on two separate timescales, (i) slow radial expansion and growth of the dome and (ii) a narrow and fast-moving lava extrusion. Both processes also produced distinctive fracture sets detectable with surface motion, and high strain zones associated with thermal anomalies. Our results highlight that motion patterns at lava domes control the structural and thermal architecture, and different timescales should be considered to better characterize surface motions during dome growth to improve the assessment of volcanic hazards.

Subduction with arrested volcanism: Compressional regime in volcanic arc gap formation along east Mindanao, Philippines

The eastern portion of the Philippine island arc system, from the Bicol Peninsula through Leyte island to the Pacific Cordillera of east Mindanao, is the overriding plate underneath which the West Philippine Basin (part of the Philippine Sea Plate) and Palau Basin oceanic plates subduct along the Miocene-Pliocene to present-day Philippine Trench. Southward younging of the trench, coupled with the presence of the left-lateral Philippine Fault Zone parallel to it, is recognized. Young volcanic centers (<7Ma to present) are well-developed in the Bicol Peninsula and in Leyte island but not in the Pacific Cordillera. There are only two volcanic centers that are exposed in the Pacific Cordillera, the Paco volcanic center in the north and the Leonard Kniassef in the south. There is a 250 km volcanic arc gap distance between the two volcanic centers. Reported inland and offshore geologic structures, from onramp structures to thrust faults, and strain rate computations suggest a predominantly compressional regime in the Pacific Cordillera from the Miocene to the present. This is consistent with the hinge advance character of the Philippine Trench and the recognized E-W directed σ1 in this part of the island arc system. It is forwarded that the prevailing compressional regime inhibited the extrusion of lavas leading to the non-formation of surficial volcanic centers between the Paco and Leonard Kniassef volcanic centers. Furthermore, the compression can also partly account for the presence of porphyry copper deposits in this part of Mindanao.

Dissolved Gas and Metal Composition of Hydrothermal Plumes From a 2008 Submarine Eruption on the Northeast Lau Spreading Center

Extrusion of lava onto the seafloor transports heat and mass from the lithosphere to the hydrosphere and the biosphere. During this process, large amounts of dissolved gases can be released into the ocean as hydrothermal plumes and serve as nutrients for microbial activity. Here we report the dissolved gas and metal composition of hydrothermal plumes associated with a submarine eruption at the Northeast Lau Spreading Center (NELSC) in the SW Pacific Ocean in November 2008. During this eruption, two different types of plumes were observed in the water column: a shallow event plume rich in H2 and poor in 3He and CH4, and a deep near-seafloor plume with high CH4, metals and 3He and intermediate H2 concentrations. Both were generated at the same time and at the same location. While the high abundance of H2 in the event plume points to ongoing reactions between hot rock and seawater, the distinct chemical characteristics of the near-seafloor plume likely result from the release of a mature fluid stored in the crust. The plume chemistry of the event plume favors a seawater-lava interaction event plume generation model. However, the heat budget asks for an additional process releasing enough heat to lift the plume within the time frame of this short event. The extremely high H2 concentrations suggest that the eruption was locally more explosive than assumed. A more explosive eruption style might enhance the heat flux from lava cooling.

Late-Mesozoic tectonic evolution and magmatic history of the Zhe-Gan-Wan region, SE China: evidence from the Shiling rhyolite and other Yanshanian granitoids

Yanshanian granitoids (178–120 Ma, Jurassic to Cretaceous), which intruded into Precambrian crystalline base- ment and Paleozoic strata, are widely distributed at the junction of the Zhejiang, Jiangxi and Anhui provinces (the ZGW region) in southern China, along with coeval volcanic rocks. This paper summarizes zircon U–Pb age data, geochemi- cal characteristics and Sr-Nd isotopic characteristics of the Late Jurassic to Middle Cretaceous granitoids and volcanic rocks from the ZGW region. We demonstrate that members of the Shiling rhyolite formed during two different periods of magmatic activity at 154.7 ± 2.5 Ma and 139–134 Ma and that igneous rocks from the different periods have distinct geochemical characteristics. Jurassic igneous rocks of the ZGW region have relatively low SiO2and high Al 2O3contents, and show enrichment of large ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs). They are strongly enriched in LREE and depleted in HREEs with weakly negative Eu anomalies and strongly negative Nb, Ta anomalies. Rb and Y concentrations follow the trends of I-type and S-type granites. By contrast, Cretaceous igneous rocks of this region are characterized by high SiO2and low Al2O3contents with negative Eu anomalies. They have typical wing- shaped rare earth element (REE) distribution patterns and show enrichment of Rb, Th, U, Nb, Ta and depletion of Ba, Sr, P and Ti. They have affinity to A-type peraluminous granites or highly-fractionated felsic rocks. Overall, the igneous rocks evolved from high-Sr low-Yb to low-Sr high-Yb, which might reflect the evolution of the tectonic setting from subduction to lithospheric extension/thinning, i. e., a transition from a continental margin subduction setting during the Late Jurassic to a within-plate extensional setting during the Early Cretaceous, at c. 142 ± 3 Ma. The repeated alternation between lava extrusion and extension and extensional fault kinematics in the late Mesozoic is related to the changes of direction and rate of plate movement of the Izanagi and Pacific plates.

Lithofacies architecture, composition, and age of the Carboniferous Teplice Rhyolite (German–Czech border): Insights into the evolution of the Altenberg-Teplice Caldera

The Teplice Rhyolite (TR) intra-caldera deposits of the Altenberg-Teplice Caldera (ATC, Central Europe) were investigated by lithofacies analysis, stratigraphy, and whole-rock chemistry as well as laser ablation U–Pb zircon dating. The aim was to demonstrate that the joint application of these methods can discern complex stratigraphy of ancient volcanic successions, susceptible to erosion and alteration, taking the TR as case study. The TR was regarded as a formation and divided into nine members that represent three main eruptions, each one separated by periods of volcanic quiescence. The first two eruptions mainly portrayed opening and clearing of vents, which were dominated by lithic-rich ignimbrites with lateral lithofacies changes (fiamme-rich to crystal-rich) covered by lava flows/domes. Eruption style was low pyroclastic fountaining alternated with effusive lava outpour. A sustained buoyant eruption column was exclusively interpreted at the outset of the second eruption based on the presence of a basal fallout deposit. The third eruption recorded three different phases: further vent opening, a waxing phase, and final lava extrusion. The vent opening phase was similar to those occurred in previous eruptions. The waxing phase produced voluminous, massive, poorly-sorted, very crystal-rich (>50vol.%) ignimbrites, exhibiting progressive increase in crystal contents with height and chemical composition variations that indicate deposition by progressive aggradation. Low pyroclastic fountaining formed these high-particle concentration currents that waxed and reached quasi-steady conditions. Eruption ended with extrusion of a lava flow. The TR ignimbrites were emplaced hot and were partially to densely welded, as evidenced by eutaxitic texture, deformed glass shards, spherulites, perlitic cracks, and microcrystalline to granophyric textures. Horizontal and vertical lithofacies variations in pyroclastic deposits, together with progressive infill and association between TR feeding systems and faulting, demonstrate that eruptions developed through several fissure vents or faults. A volcanic explosivity index of 7 was estimated according to a minimum ejecta volume of ∼320km3 (DRE). Volcanic architecture of the TR can be linked to sequential caldera subsidence that occurred synchronously with progressive deposition. Regional faulting (NE–SW-trending rhyolitic dykes) related to ATC feeding conduits suggests that progressive subsidence and final trap-door caldera collapse, possibly with a piece-meal component (faulted caldera floor), originated from post-Variscan tensional stress regime (∼NNW–SSE direction). The TR Formation has a general rhyolitic composition with A-type granite affinities and a normal chemical zoning. U–Pb zircon dating yielded ages from 323 to 312Ma, which, compared to age data from other ATC rocks, indicate that the TR was generated within a time span of ∼325–317Ma.

Multi-year Satellite Observations of Sulfur Dioxide Gas Emissions and Lava Extrusion at Bagana Volcano, Papua New Guinea

Bagana, arguably the most active volcano in Papua New Guinea, has been in a state of near-continuous eruption for over 150 years, with activity dominated by sluggish extrusion of thick blocky lava flows. If current extrusion rates are representative, the entire edifice may have been constructed in only 300–500 years. Bagana exhibits a remarkably high gas flux to the atmosphere, with persistent sulfur dioxide (SO2) emissions of several thousand tons per day. This combination of apparent youth and high outgassing fluxes is considered unusual among persistently active volcanoes worldwide. We have used satellite observations of SO2 emissions and thermal infrared radiant flux to explore the coupling of lava extrusion and gas emission at Bagana. The highest gas emissions (up to 10 kt/day) occur during co-extrusive intervals, suggesting a degree of coupling between lava and gas, but gas emissions remain relatively high (~2,500 t/d) during inter-eruptive pauses. These passive emissions, which clearly persist for decades if not centuries, require a large volume of degassing but non-erupting magma beneath the volcano with a substantial exsolved volatile phase to feed the remarkable SO2 outgassing: an additional ~1.7–2 km3 basaltic andesite would be required to supply the excess SO2 emissions we observe in our study interval (2005 to present). That this volatile phase can ascend freely to the surface under most conditions is likely to be key to Bagana's largely effusive style of activity, in contrast with other persistently active silicic volcanoes where explosive and effusive eruptive styles alternate.

Mineral resorption triggers explosive mixed silicate–carbonatite eruptions

Historic eruptions of Earth's only active carbonatite volcano, Oldoinyo Lengai (Tanzania), have repeatedly switched from low energy carbonatite lava extrusion to highly energetic explosive silicate volcanism, most recently in 1966–67 and 2007–08. The explosive eruptions produce strongly Si-undersaturated peralkaline silicate ashes with unusually high (Na + K)/Al of 3.4–6.3 when compared to the average peralkalinity of ∼0.8 in the East African Rift System. A series of experiments in the carbonatite–clinopyroxene system at 750–1150°C, 0.1 GPa, reveal that augitic clinopyroxene breaks down peritectically at >900°C yielding strongly peralkaline conjugated silicate- and carbonatite melts. The clinopyroxene-derived silicate melt dissolves (Na,K)2O from the (Na,K)2CO3-component of the carbonatite leading to high peralkalinities and to liberation of excess CO2, since the solubility of carbon dioxide in silicate liquids is ≪1 wt.% at subvolcanic pressures. Carbonatite injection into subvolcanic clinopyroxene-rich crystal mushes hence explains the occurrence of strongly peralkaline silicate melts and provides a mechanism for CO2-driven explosive eruptions. The silicate melt compositions mostly depend on the (Na + K)/Ca ratio of the intruding carbonatite, the silicate ashes erupted in 1966–67 and 2007–08 require an interaction of a clinopyroxene-rich crystal mush with a slightly less evolved alkali-carbonatite than presently erupted at Oldoinyo Lengai. The mechanism identified here, where mineral breakdown induced melt hybridization triggers volatile saturation and highly explosive volcanism is generally applicable to igneous systems that involve carbonatites or other low-viscosity CO2-bearing alkaline silicate melts.

3D electrical imaging of the inner structure of a complex lava dome, Puy de Dôme volcano (French Massif Central, France)

Lava domes result from extrusion of massive lava, frequent explosions and collapses. This contribution focuses on a complex trachytic lava dome, the Puy de Dôme volcano, located in the Chaîne des Puys volcanic field (French Massif Central, France). We performed Electrical Resistivity Tomography (ERT) acquisitions on the entire edifice in order to investigate its overall inner structure as well as to detail its summit area. The resulting large ERT dataset integrated a recently developed 3D inversion code based on an unstructured discretization of the geometrical model. The 3D inversion models obtained refine the existing geological model of the Puy de Dôme's inner structure obtained by previous geophysical studies. These results also highlight the strong fracturing and fumarolic alteration that affect the summit part of the volcano.

A Recent Volcanic Eruption Discovered on the Central Mariana Back-Arc Spreading Center

Submarine volcanic eruptions are difficult to detect because they are hidden from view at the bottom of the ocean and far from land-based sensors. However, most of Earth’s volcanic activity is in the oceans along tectonic plate boundaries, and modern tools of oceanography now allow us to find and study recent eruptions in the deep sea. The first known historical eruption on the Mariana back-arc spreading center was discovered in December 2015 during exploration of the southern back-arc for new hydrothermal vent sites. A water-column survey along the axis of the back-arc showed hydrothermal plumes over the site characterized by low particle concentrations and relatively high reduced chemical anomalies. A dive with the autonomous underwater vehicle Sentry collected high-resolution (1 m) multibeam sonar bathymetry over the site, followed by a near-bottom photographic survey of a smaller area. The photo survey revealed the presence of a pristine, dark, glassy lava flow on the seafloor with no sediment cover. Venting of milky hydrothermal fluid indicated that the lava flow was still warm and therefore very young. A comparison of multibeam sonar bathymetry collected by R/V Falkor in December 2015, to the most recent previous survey of the area by R/V Melville in February 2013, revealed large depth changes in the same area, effectively bracketing the timing of the eruption within a window of less than 3 years. The bathymetric comparison shows the eruption produced a string of lava flows with maximum thicknesses of 40–138 m along a distance of 7.3 km (from latitude 15∘22.3′ to 15∘26.3′N) between depths of 4050–4450 m bsl (meters below sea level), making this the deepest known historical submarine volcanic eruption on Earth. The cross-axis width of the lava flows is 200–800 m. The Sentry bathymetry shows that the new lava flows are constructed of steep-sided hummocky pillow mounds and are surrounded by older flows with similar morphology. In April and December 2016, two dives were made on the new lava flows by remotely operated vehicles Deep Discoverer and SuBastian. The pillow lavas have many small glassy buds on the steep flanks of the mounds, locally thick accumulations of hydrothermal sediment near the tops of mounds, and small cones of radiating pillows at their summits. The 2015–2016 observations show a rapidly declining hydrothermal system on the lava flows, suggesting that the eruption had occurred only months before its discovery in December 2015. The morphology of the pillow lavas is similar to other historical eruption sites, so the greater depth and ambient pressure of this site had no apparent effect on the processes of lava extrusion and emplacement. This study reveals that some segments of the Mariana back-arc have active magmatic systems despite the relatively low spreading rate, and that other eruptions are possible in the near future.

The nature and origin of magnetic anomalies over the Gölcük caldera; Isparta; South-Western Turkey

For the first time; ground magnetic survey was performed which data were inverted by means of total horizontal derivative; horizontal gradient analytic signal; and hyperbolic tilt angle techniques to identify subsurface volcanic structures around Isparta city (South-Western Turkey). Here; Golcuk volcanism took place at the apex of the Isparta Angle at the intersection of the Lycian and Antalya nappes. It initiated between 4.0—4.7 my ago mainly as lava extrusions and ended with phreatoplinian eruptions during Quaternary time. The study area is covered by authochtonous and allochthonous units that are intruded by Pliocene and Quaternary Golcuk volcanics and also overlain by pyroclastic fall and flow deposits. The boundaries were revealed for the buried volcanic structure from the edge detection methods. The geometry of the trachytic dome southwestern of the Golcuk Lake and its downward continuation were studied by 2D modelling with the control of the power spectrum depth results applied to the focused anomaly. The azimuthally-averaged logarithmic power spectra plot indicates that the downward continuation of source depth of the trachytic dome reaches up to 850 m. The forward inversion results indicate that the horizontal size of the model for this trachytic dome is 1250 m beneath the surface while it’s surface extension is only about 400 m.

Geodetic evidence for interconnectivity between Aira and Kirishima magmatic systems, Japan

It is not known whether clustered or aligned volcanic edifices at the Earth surface have connected magmatic systems at depth. Previously suggested by geological records of paired eruptions, volcano interconnectivity still lacks proper geodetic evidence. Here we use GPS time-series and deformation modeling to show how Aira caldera and Kirishima, two adjacent volcanic centers in Kagoshima graben (southern Japan), interacted during Kirishima unrest in 2011. Whereas Aira caldera had been inflating steadily for two decades, it deflated during the eruption of Kirishima which started with a large-volume lava extrusion. This deflation, which cannot be explained by stress changes, is interpreted as the result of magma withdrawal from the Aira system during the Kirishima replenishment phase. This study highlights the behavior of connected neighboring volcanic systems before and after a large eruption, and the importance of taking into account volcano interactions in eruption probability models.

Using a discrete element approach to model lava dome emplacement and collapse

Lava dome collapses can lead to explosive activity and pyroclastic flow generation which makes them one of the most deadly consequences of volcanic activity. The mechanisms linked to a collapse are however still poorly understood and very few numerical models exist that investigate the actual collapse of a lava dome after emplacement. We use a discrete element method implemented in the modelling software Particle Flow Code to investigate lava dome growth, but also go further to test the stability of the dome under the following conditions: increased internal pressure; switch in extrusion direction caused by partial cooling of the dome; and extrusion of lava onto variable underlying topography. We initially show the morphology development of a growing lava dome, and how the rheological boundary between core and talus evolves throughout the lifetime of a dome and with varied solidus pressures. Through visualisation of strain accumulation within the lava dome we show superficial rockfall development due to interaction with topography, whereas large deep-seated failures occur when the dome is exposed to internal overpressures. We find that a switch in extrusion direction promotes a transition from endogenous to exogenous dome growth and leads to lava lobe formation. We demonstrate that lava dome collapse exhibits many features similar to common landslides and by investigating strain patterns within the dome, we can use numerical modelling to understand features that elude field observations.

Thickness of extrusive basalts dominating the magnetic structure along the ultraslow-spreading Mohns Ridge axis (71.8°–73.7°N)

The equivalent magnetization (EM) and mantle Bouguer anomaly (MBA) were calculated along the ultraslow-spreading Mohns Ridge axis in the Norwegian-Greenland Sea. The magnetic anomaly and the associated EM were compared with the bathymetry, MBA, seismically determined crustal structure and geochemical data at both the inter-segment scale (>60 km) and the intra-segment scale (20–60 km). At the inter-segment scale, the magnetic highs at the segment centers are independent of the MBA. Of the 13 segments, 9 with magnetic anomalies >700 nT coincide with axial volcanic ridges identified from multibeam bathymetry maps, which suggests that the magnetic highs at the segment centers may be more associated with the extrusive lavas rather than the amount of magma supply. With few exceptions, the magnetic anomaly lows associated with MBA highs at the segment ends increase from south to north. This trend might be explained by thickened extrusive basalts and/or more serpentinized peridotites at the segment ends in the north. At the intra-segment scale, the most prominent features are the decreases in the magnetic anomalies and associated EMs from the segment centers to the ends. The intra-segment magnetic anomalies have positive and negative correlations with the bathymetry and MBA, respectively. The magnetic signal modeled by the seismically determined layer 2A with an assumed constant magnetization is remarkably consistent with the observed magnetic anomaly, which strongly suggests that the thickness of the extrusive basalts dominates the magnetic structure in each segment along the Mohns Ridge. In general, the thickness of the extrusive basalts dominates the magnetic structure along the Mohns Ridge, whereas the contributions from serpentinized peridotites may be significant at the segment ends and may produce long-wavelength magnetic variations. The magnetic data can be used as an indicator of the thickness of the extrusive basalts within segments along the ultraslow-spreading Mohns Ridge.

Towards reconstruction of the lost Late Bronze Age intra-caldera island of Santorini, Greece

During the Late Bronze Age, the island of Santorini had a semi-closed caldera harbour inherited from the 22 ka Cape Riva Plinian eruption, and a central island referred to as ‘Pre-Kameni’ after the present-day Kameni Islands. Here, the size and age of the intracaldera island prior to the Late Bronze Age (Minoan) eruption are constrained using a photo-statistical method, complemented by granulometry and high-precision K-Ar dating. Furthermore, the topography of Late Bronze Age Santorini is reconstructed by creating a new digital elevation model (DEM). Pre-Kameni and other parts of Santorini were destroyed during the 3.6 ka Minoan eruption, and their fragments were incorporated as lithic clasts in the Minoan pyroclastic deposits. Photo-statistical analysis and granulometry of these lithics, differentiated by lithology, constrain the volume of Pre-Kameni to 2.2–2.5 km3. Applying the Cassignol-Gillot K-Ar dating technique to the most characteristic black glassy andesite lithics, we propose that the island started to grow at 20.2 ± 1.0 ka soon after the Cape Riva eruption. This implies a minimum long-term lava extrusion rate of ~0.13–0.14 km3/ky during the growth of Pre-Kameni.