Caldera-forming Eruption Research Articles

Anatomy of the Xáltipan ignimbrite at Los Humeros Volcanic Complex; the largest eruption of the Trans-Mexican Volcanic Belt

The Xáltipan ignimbrite (ca. 290 km3 DRE) represents the major explosive event from Los Humeros Volcanic Complex (LHVC) that can be ranked as the largest caldera-forming eruption of the Trans-Mexican Volcanic Belt (TMVB). This eruption marked the onset of an explosive period of LHVC producing the collapse of the main Los Humeros caldera at 164 ± 4.2 ka. This paper provides a detailed characterization of the Xáltipan ignimbrite anatomy and eruptive chronology that records a multi-phase, single eruption. This eruption resulted in more complex architecture and lithofacies variations of the ignimbrite than previously reported. To support these findings, we present two representative composite Xáltipan stratigraphic columns based on two regional reference sections, together with a welding lithofacies map, digital mapping, energy line projections, geologic sections and high-resolution petrophysical characterization (micro-CT, He-pycnometry). Additionally, we have reevaluated the magnitude (DRE volume), and the total erupted mass of the deposit by taking into account critical primary and post-emplacement processes such as lithic concentration, welding grade, and bulk loss due to erosion. The internal architecture of the Xáltipan ignimbrite is composed of two flow units and two pumice fallout deposits (a basal one and an intercalated layer), which formed as a result of a voluminous and sequential eruption that can be resumed in four eruptive phases. The initial stage of the eruption produced a basal pumice fallout (BPF) (phase 1) derived from a buoyant plume that eventually collapsed, generating pyroclastic density current (PDC) deposits with intra-ignimbrite pumice fallout beds. This event was followed by the climactic, caldera-forming, boiling over-type eruptive phase (phase 2) that generated voluminous PDCs of the lower flow unit (LFU). As the eruption waned, a brief eruptive hiatus occurred, immediately followed by a second explosive event in the western rim of the caldera with a boiling-over type eruptive column depositing a discrete pumice fallout layer (IPF) (phase 3), which later collapsed depositing less voluminous PDC (UFU) in the western flank of the Los Humeros caldera (phase 4). The anatomy of the deposits shows that the Xáltipan ignimbrite was emplaced in different behaviors: radially in the proximal parts and channelized and confined in the distal parts, showing significant welding variations that range from non-to-highly welded conditions (welding ranks I-VI) controlled mainly by the paleotopography. The size calculations indicate that the deposits have an aerial extent of ca. 4600 km2, an ejected bulk volume of ca. 330 km3 with a DRE volume of ca. 290 km3, and an ejected mass of 1.1 × 1012 kg, which points out to a paroxysmal eruption (VEI 6-7).

Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography

AbstractDespite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.

Magma reservoir failure and the onset of caldera collapse at Kīlauea Volcano in 2018.

Caldera-forming eruptions are among Earth's most hazardous natural phenomena, yet the architecture of subcaldera magma reservoirs and the conditions that trigger collapse are poorly understood. Observations from the formation of a 0.8-cubic kilometer basaltic caldera at Kīlauea Volcano in 2018 included the draining of an active lava lake, which provided a window into pressure decrease in the reservoir. We show that failure began after <4% of magma was withdrawn from a shallow reservoir beneath the volcano's summit, reducing its internal pressure by ~17 megapascals. Several cubic kilometers of magma were stored in the reservoir, and only a fraction was withdrawn before the end of the eruption. Thus, caldera formation may begin after withdrawal of only small amounts of magma and may end before source reservoirs are completely evacuated.

A roadmap for amphibious drilling at the Campi Flegrei caldera: insights from a MagellanPlus workshop

Abstract. Large calderas are among the Earth's major volcanic features. They are associated with large magma reservoirs and elevated geothermal gradients. Caldera-forming eruptions result from the withdrawal and collapse of the magma chambers and produce large-volume pyroclastic deposits and later-stage deformation related to post-caldera resurgence and volcanism. Unrest episodes are not always followed by an eruption; however, every eruption is preceded by unrest. The Campi Flegrei caldera (CFc), located along the eastern Tyrrhenian coastline in southern Italy, is close to the densely populated area of Naples. It is one of the most dangerous volcanoes on Earth and represents a key example of an active, resurgent caldera. It has been traditionally interpreted as a nested caldera formed by collapses during the 100–200 km3 Campanian Ignimbrite (CI) eruption at ∼39 ka and the 40 km3 eruption of the Neapolitan Yellow Tuff (NYT) at ∼15 ka. Recent studies have suggested that the CI may instead have been fed by a fissure eruption from the Campanian Plain, north of Campi Flegrei. A MagellanPlus workshop was held in Naples, Italy, on 25–28 February 2017 to explore the potential of the CFc as target for an amphibious drilling project within the International Ocean Discovery Program (IODP) and the International Continental Drilling Program (ICDP). It was agreed that Campi Flegrei is an ideal site to investigate the mechanisms of caldera formation and associated post-caldera dynamics and to analyze the still poorly understood interplay between hydrothermal and magmatic processes. A coordinated onshore–offshore drilling strategy has been developed to reconstruct the structure and evolution of Campi Flegrei and to investigate volcanic precursors by examining (a) the succession of volcanic and hydrothermal products and related processes, (b) the inner structure of the caldera resurgence, (c) the physical, chemical, and biological characteristics of the hydrothermal system and offshore sediments, and (d) the geological expression of the phreatic and hydromagmatic eruptions, hydrothermal degassing, sedimentary structures, and other records of these phenomena. The deployment of a multiparametric in situ monitoring system at depth will enable near-real-time tracking of changes in the magma reservoir and hydrothermal system.

Stratigraphy and chronology of silicic tephras in the Shikotsu-Toya volcanic field, Japan: Evidence of a Late Pleistocene ignimbrite flare-up in southwestern Hokkaido

The Shikotsu-Toya volcanic field (STVF) in southwestern (SW) Hokkaido (Japan) comprises three caldera volcanoes (Toya, Kuttara, and Shikotsu calderas and their post-caldera volcanoes) and Yotei and Shiribetsu stratovolcanoes. The stratigraphy, chronology, petrography, and glass geochemistry of tephra layers in the Ishikari pyroclastic terrain are carefully examined and reassessed. On the basis of the presence of volcaniclastic soil horizons and common petrological features, individual eruptive episodes are defined and the source volcanoes of each tephra layer are identified by correlation with proximal deposits. 28 eruptive episodes are distinguished, including six that are newly discovered. Based on K-Ar ages of older effusive lavas around the STVF, effusive andesitic volcanism, which was active since the Middle Miocene in SW Hokkaido, would have converged in the STVF by 0.9–0.5 Ma. After a dormant period of 400 kyr, explosive eruptions began at Toya 122.5 ka, and at Shiribetsu before 120–115 ka. A catastrophic caldera-forming eruption ca. 106 ka resulted in the formation of Toya caldera. Explosive activity then migrated eastward, and Kuttara and Shikotsu volcanoes began erupting explosively before 90 ka and 85 ka, respectively. Subsequently, the large stratovolcano Yotei was constructed after ca. 75 ka in the back-arc side of the STVF. Following repeated explosive eruptions of these five volcanoes, the largest caldera-forming eruption occurred ca. 43.8 ka at Shikotsu volcano. The STVF has produced VEI* (volcanic explosivity index) 7 eruptions every 20–40 kyr since explosive eruptions began, and the total volume of magma erupted in the STVF is estimated to be ca. 700 km3 DRE (dense rock equivalent). These findings suggest that the STVF is one of the youngest ignimbrite flare-up suites in Japan, and that its eruption rate is up to ca. 5.6 km3 DRE/kyr since 125 ka.

Caldera-forming eruptions of mushy magma modulated by feedbacks between ascent rate, gas retention/loss and bubble/crystal framework interaction

Caldera-forming eruptions of mushy silicic magma are among the most catastrophic natural events on Earth. In such magmas, crystals form an interlocking framework when their content reaches critical thresholds, resulting in the dramatic increase in viscous resistance to flow. Here, we propose a new mechanism for the ascent of mushy magma based on microstructural observations of crystal-rich silicic pumices and lavas from the Central Andes and decompression experiments. Microstructural data include spherical vesicles and jigsaw-puzzle association of broken crystals in pumices, whereas there is limited breakage of crystals in lavas. These observations insinuate that shearing of magma during ascent was limited. Decompression experiments reveal contrasting interaction between growing gas bubbles and the crystal framework in crystal-rich magma. Under slow decompression typical of effusive eruptions, gas extraction is promoted, whereas under rapid decompression, bubbles are retained and the crystal framework collapses. This feedback between decompression rate, retention of gas bubbles, and integrity of the crystal framework leads to strong non-linearity between magma decompression rate and eruption explosivity. We extend these findings to caldera-forming eruptions of crystal-rich magma where large overpressures are induced by caldera-collapse, resulting in magma plug-flow, rapid decompression facilitated by shear-localization at conduit margins, and explosive eruption.

Oldest description of a caldera-forming eruption in Southeast Asia unveiled in forgotten written sources

As one of the most powerful volcanic events of the Holocene era with a Volcanic Explosivity Index of seven, the 1257 CE eruption of Samalas volcano in the Rinjani Volcanic Complex had large consequences for Lombok Island (Indonesia) and its surrounding area. This huge eruption with an enormous volume of ejected material (4435 ± 5.5 × 106 m3), was recorded in local written sources, called Babad, that were compiled around the sixteenth century and contain oral stories and myths. In this study, we present two written sources, namely Babad Lombok and Babad Suwung, which undoubtedly refer to the 1257 CE ultraplinian eruption of Samalas volcano. In addition to detailed analysis of the volcanic deposits, interviews with old Javanese linguists and museum employees were carried out to clarify the meanings of some words as well as the genesis of the Babads. The results from field observation and interviews were analyzed and compared with previous research. Babad Lombok describes the process of the Samalas eruption, the formation of the Segara Anak caldera, and the impacts of the voluminous ash fall and pyroclastic flows on Lombok Island. Meanwhile, Babad Suwung describes the impacts of the 1257 CE volcanic eruption on Talkuwang (Taliwang), on the western coast of Sumbawa Island. This description encompasses several volcanic processes including fallout and pyroclastic surges. The exegesis of Babad Lombok and Babad Suwung could be the oldest observations of a caldera-forming eruption in Southeast Asia. They also document the oldest pyroclastic surges in the world after those of Vesuvius as described by Pliny the Younger in 79 CE.

The analysis of spatial distributions, origins of caldera-forming eruptions with basaltic-andesitic magma compositions, and genesis of Miocene ignimbrites of the Eastern volcanic belt, Kamchatka

Based on the statistical data of the Global Volcanism Program of the Smithsonian Institution and published materials, we present a comparative analysis of caldera-forming eruptions on global scale. The geodynamic settings and genesis of the caldera-forming eruptions with basaltic-andesitic magma compositions are described. The origin of the majority of mafic ignimbrites was related with external water. Such ignimbrites were generated in a submarine environment or with a contact with water. The newly obtained data, paleogeodynamic reconstruction and geological mapping of Miocene mafic ignimbrites of the Eastern volcanic belt (EVB) of Kamchatka confirm their genesis in costal-marine environment. These new data show significance of paleoreconstructions in studies of paleo-volcanoes and relief-forming pyroclastic rocks.

The caldera-forming eruption of the quaternary Payún Matrú volcano, Andean back-arc of the southern volcanic zone

The Payún Matrú caldera is a Quaternary Andean back-arc caldera (36° 25′ S, 69° 12′ 30” W), and represents the main volcanic edifice of the homonym Payún Matrú volcanic field, located within the Payenia Basaltic province. The Payún Matrú volcanic field is also characterized by the Payún Liso volcano, and 220 monogenetic basaltic cones and associated lava flows. The Pleistocene – Holocene Payún Matrú caldera presents a long lived pre-caldera stage, which built a multi-vent and shield-like volcanic edifice, mainly trachytic in composition. The syn-caldera stage is represented by the trachytic Portezuelo Ignimbrite (the caldera-forming eruption deposits) constrained between 148 and 82 ka, and the resulting caldera depression of 8.5 km in diameter. Afterwards the caldera event, volcanism continued mostly along the caldera rim.Stratigraphic sections of the Portezuelo Ignimbrite and its correlations are presented, in addition to mineral compositions and previous whole-rock chemical analyses. Only the extra-caldera deposits can be observed, while the intra-caldera ignimbrite is hidden by post-caldera volcanic products and unconsolidated modern sediments. Different facies within the ignimbrite were recognized, being most of them massive or eutaxitic and massive lapilli tuff facies, deposited by dense pyroclastic density currents. Fall deposits are found only proximally and restrained to the caldera margin. The absence of a widespread fall deposit suggests the lack of a sustained eruptive column at the onset of eruption. Massive tuff facies at the topmost deposits indicates the development of a co-ignimbritic plume as the pyroclastic density currents wanned.On the basis of color, size and shape, vesicularity and crystallinity, four distinct juvenile clasts types were defined: gray fiamme, black juvenile, dark gray and light gray pumice. Available chemical analysis of gray fiamme and black juvenile clasts indicate a similar trachytic composition, although their mineralogical composition presents some differences.Several eruptive units for the northern and southern outer slopes of the caldera were defined, on the basis of the juvenile types present and discontinuity surfaces. The onset of caldera collapse is not marked by the presence of a lithic breccia, but it is suggested by the appearance of dark gray and light gray pumice along with black juveniles in the topmost eruptive units, together with an increase in lithic content relative to previous units. The sequential appearance of different juvenile clasts types suggests a zoned magma chamber, in terms of crystal content and composition.

Evidence for a large-magnitude eruption from Campi Flegrei caldera (Italy) at 29 ka

The 40 ka caldera-forming eruption of Campi Flegrei (Italy) is the largest known eruption in Europe during the last 200 k.y., but little is known about other large eruptions at the volcano prior to a more recent caldera-forming event at 15 ka. At 29 ka a widespread volcanic ash layer, termed the Y-3 tephra, covered >150,000 km2 of the Mediterranean. The glass compositions of the layer are consistent with Campi Flegrei being the source, but no prominent proximal equivalent in the appropriate chrono-stratigraphic position had been previously identified. Here we report new glass chemistry data and 40Ar/39Ar ages (29.3 ± 0.7 ka [2σ]) that reveal the near-source Y-3 eruption deposit in a sequence at Ponti Rossi and a nearby borehole (S-19) in Naples. The dispersal and thickness of the deposits associated with this eruption, herein named the Masseria del Monte Tuff, were simulated using a tephra sedimentation model. The model indicates that ∼16 km3 dense rock equivalent of the magma erupted was deposited as fall. This volume and the areal distribution suggest that the Masseria del Monte Tuff resulted from a magnitude (M) 6.6 eruption (corresponding to volcanic explosivity index [VEI] 6), similar to the 15 ka caldera-forming Neapolitan Yellow Tuff (M 6.8) eruption at Campi Flegrei. However, the lack of coarse, thick, traceable, near-vent deposit suggests peculiar eruption dynamics. Our reconstruction and modeling of the eruption show the fundamental role that distal tephrostratigraphy can play in constraining the scale and tempo of past activity, especially at highly productive volcanoes.

Magma interactions, crystal mush formation, timescales, and unrest during caldera collapse and lateral eruption at ocean island basaltic volcanoes (Piton de la Fournaise, La Réunion)

The dynamics of magmatic processes at large mafic ocean island volcanoes control the likely locations (central caldera versus flanks) and timing of their eruptions. Crystals and their melt inclusions are key witnesses of these processes but are rarely studied in detail and in the same samples. Here we report the crystal and melt inclusion compositions of the April 2007 caldera-forming eruption of Piton de la Fournaise volcano and discuss how they relate to geophysical unrest monitoring data. Olivine crystals show mainly normal zoning (decrease in Mg/Fe) towards the rims, and also around some melt inclusions. Many crystals also show fine-scale skeletal structures defined by high phosphorus concentrations. Melt inclusions contain 53–205 ppm CO2 and 0.25–1.1 wt% H2O, and δD (δD values expressed as δDVSMOW) ranges from −135 to 62‰. Monitoring data show that inflation of the edifice started about a month before the first 2007 eruption: magma intrusion occurred at ≈3 km below sea level, and quickly migrated towards shallower depths (about 1 km above sea level). Such a time frame of magma movement is recorded in the chemical zoning of the olivine crystals that massively and quickly crystallized when reaching shallow depth, without significant interactions between resident and intruding magmas. The intrusion was followed by lateral flank eruption and caldera collapse. The chemical zoning of the olivine crystal rims and around the melt inclusions indicates that the newly created crystal-mush moved laterally towards the surface in matter of days to 3 months. Post-caldera samples show significant H+ loss, likely due to the depressurization of the magmatic system stored at shallow level. Our findings are different from other mafic ocean island volcanoes or stratovolcanoes (e.g. Kilauea, Canary Islands, and Etna), where crystals commonly record magma mixing between evolved and shallow melts and intruding mafic melts. We speculate that the difference between our findings and those of similar mafic ocean island volcanoes is due to the variety of magma supply rates from depth.

The Ilopango caldera complex, El Salvador: Stratigraphic revision of the complete eruptive sequence and recurrence of large explosive eruptions

Ilopango caldera erupted episodically at least 13 tuff-forming eruptions with a minimum estimate volume of 1–5 km3 DRE per eruption, reaching up to 150 km3 DRE for the first caldera-forming eruption. All tuffs are of dacitic-rhyolitic composition. The complete pyroclastic sequence spans a range in time from 1.785 to 0.0015 Ma, and based on stratigraphy and geochronology constraints can be divided into three formations: the Comalapa, Altavista and Tierras Blancas formations. In this work, we focus on the members of the newly described Altavista Formation (middle part of Ilopango caldera volcanic sequence), which consist of six consolidated pyroclastic deposits or tuffs. Each tuff corresponds to a specific eruption followed by a period of quiescence during which soil beds were developed on the deposits. The ages of the Altavista Formation ranges from 918 to 257 ka, based on new 40Ar/39Ar, U/Pb-zircon, and U/Th-zircon analyses. The tuffs of this formation show similar characteristics in mineralogy and composition. They are calcalkaline, rhyodacitic tuffs, with plagioclase, clinopyroxene, and hornblende. From field mapping and descriptions of the deposits, we have inferred the eruptive styles that include pumice fallouts, pyroclastic density currents and also hydromagmatic explosions. The common vent in all tuffs was the Ilopango caldera and each member of the Altavista Formation could correspond to a caldera collapse event, except for one of the six eruptions. The volume of each member was estimated to be >30 km3 DRE, which is the same order of magnitude than that estimated for the Tierra Blanca Joven (TBJ) eruption at about 1,500 B. P, and smaller than those of the ignimbrites of the Comalapa Formation, the first three members of the Ilopango caldera reported previously. The tuffs of the Altavista Formation are visible up to 15–20 km away from the caldera's topographic margin. The recurrence interval of large explosive events at the Ilopango caldera was established by integrating the stratigraphic and geochronologic data of all 13 ignimbrites and pumice fallouts erupted from Ilopango caldera since the first one at 1.78 Ma to the last explosive event (TBJ).

Dynamics of caldera collapse during the Coranzulí eruption (6.6 Ma) (Central Andes, Argentina)

The Coranzulí caldera (23°00′ S – 66°15′ W) is one of the least known caldera complexes in the eastern part of the Argentinian Altiplano-Puna plateau (Central Andes). It lies at the intersection of N-S, NW-SE and NE-SW fault systems and was formed about 6.6 Ma ago; during the eruption, four main crystal-rich dacitic ignimbrites were emplaced in different directions around the caldera. Caldera collapse was not homogeneous, rather it occurred along different sectors of the ring fault as subsidence progressed. The location of co-ignimbrite lag breccias and the composition of the dominant lithic fragments within the different ignimbrite flow units reveal how the caldera collapse developed. According to the succession of deposits, in particular the lack of an initial fallout and the presence of co-ignimbrite lag breccias associated with the different ignimbrite units, we interpret this caldera-forming eruption as a pulsating boiling-over event in which the caldera collapse developed immediately after the onset of the eruption, favored by a transtensive tectonic system. Within the central part of the caldera, there is Cerro Coranzulí, a resurgent dome that exposes a thick intracaldera ignimbrite succession covered by ~ 100-m-thick dacite lava flows that erupted at the end of caldera formation.

Postglacial faulting near Crater Lake, Oregon, and its possible association with the Mazama caldera-forming eruption

Abstract Volcanoes of subduction-related magmatic arcs occur in a variety of crustal tectonic regimes, including where active faults indicate arc-normal extension. The Cascades arc volcano Mount Mazama overlaps on its west an ∼10-km-wide zone of ∼north-south–trending normal faults. A lidar (light detection and ranging) survey of Crater Lake National Park, reveals several previously unrecognized faults west of the caldera. Postglacial vertical separations measured from profiles across scarps range from ∼2 m to as much as 12 m. Scarp profiles commonly suggest two or more postglacial surface-rupturing events. Ignimbrite of the ca. 7.6 ka climactic eruption of Mount Mazama, during which Crater Lake caldera formed, appears to bury fault strands where they project into thick, valley-filling ignimbrite. Lack of lateral offset of linear features suggests principally normal displacement, although predominant left stepping of scarp strands implies a component of dextral slip. West-northwest–east-southeast and north-northwest–south-southeast linear topographic elements, such as low scarps or ridges, shallow troughs, and straight reaches of streams, suggest that erosion was influenced by distributed shear, consistent with GPS vectors and clockwise rotation of the Oregon forearc block. Surface rupture lengths (SRL) of faults suggest earthquakes of (moment magnitude) Mw6.5 from empirical scaling relationships. If several faults slipped in one event, a combined SRL of 44 km suggests an earthquake of Mw7.0. Postglacial scarps as high as 12 m imply maximum vertical slip rates of 1.5 mm/yr for the zone west of Crater Lake, considerably higher than the ∼0.3 mm/yr long-term rate for the nearby West Klamath Lake fault zone. An unanswered question is the timing of surface-rupturing earthquakes relative to the Mazama climactic eruption. The eruption may have been preceded by a large earthquake. Alternatively, large surface-rupturing earthquakes may have occurred during the eruption, a result of decrease in east-west compressive stress during ejection of ∼50 km3 of magma and concurrent caldera collapse.

Maturation and rejuvenation of a silicic magma reservoir: High-resolution chronology of the Kneeling Nun Tuff

Knowledge of the conditions of magma storage prior to volcanic eruptions is key to their forecasting, yet little is known about how melt compositions, crystallinity and intensive parameters within individual magma reservoirs evolve over time. To address this, we studied the Kneeling Nun Tuff, a voluminous (>900 km3) deposit of an Eocene caldera-forming eruption from the Mogollon–Datil volcanic field in New Mexico, USA. Whole-rock, feldspar and amphibole compositions were combined with zircon trace-element geochemistry and precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) U–Pb zircon crystallisation ages to arrive at a detailed, time-resolved record of chemical and physical changes within the voluminous, upper-crustal (∼2.2 kbar) magma reservoir. Chemical compositions and zircon ages from the Kneeling Nun Tuff and from co-magmatic clasts hosted within it reveal prolonged (>1.5 million years) growth and maturation of the magma reservoir that was heterogeneous in terms of temperature, melt composition and crystallinity. This protracted storage at a dominant crystallinity in excess of 50% culminated in a period of ca. 50 ky of increase in recharge heat supply and related homogenisation, decrease in crystallinity to 40–50%, and potential increase in average melt temperature, leading up to eruption at 35.299 ± 0.039 Ma. Sampling of co-magmatic lithic clasts derived from early-cooled domains of the reservoir shows that the long, million year-scale maturation time is shared across all erupted domains of the magmatic system, irrespective of their final cooling history. This study provides key observations from a natural system against which thermal and mechanical models of upper-crustal magma reservoir construction can be validated.

The role of H2O on the extraction of melt from crystallising magmas

The segregation and accumulation of felsic melts, from crystallising crustal magma reservoirs, is essential for the chemical evolution of the crust and is a phenomenon preceding some of the largest eruptions on Earth. The physical properties of residual melt and magma and the time over which the conditions remain appropriate for melt extraction are important factors controlling the efficiency of melt extraction and the distribution of melt in magma reservoirs. Here we focus on the initial water content (H2Oi) of magma as it affects both the physical properties of the residual melt and the timescales at which conditions remain appropriate for melt extraction during progressive magma crystallisation. We use rhyolite-MELTS simulations to evaluate the physical evolution of crystallising granodioritic (or dacitic) hydrous magma (i.e. ≥1 wt.% H2Oi) at shallow depth at 200 MPa. To constrain the solidification timescales of reservoirs containing magmas with initially different water content, we perform 2.5D thermal modelling. We combine these results with calculations of melt extraction velocity by compaction and hindered settling to identify the optimal conditions at which melt segregation occurs. These calculations suggest that hydrous felsic magmas that attain water saturation after 40 wt.% crystallisation (rheological locking point) are best suited for melt extraction. Once water-saturation is achieved, the rate of release of latent heat of crystallisation and with it the time magma spends within a given temperature interval increases while the viscosity of the residual liquid and crystal–liquid density contrast remain favourable for melt segregation. We first test our findings on the Takidani pluton (Japan) because it shows evidence of residual melt segregation from crystallising magma, and is associated with caldera-forming eruptions. We finally generalise our results to crustal magma reservoirs containing hydrous felsic magmas. Our results suggest that if segregation starts at rheological locking (i.e. crystallinity of 40–50 wt.%) upper crustal reservoirs of ≥100 km3 granodioritic (i.e. dacitic) magma with >2 wt.% H2Oi can produce large melt-rich caps at the top of partially crystallised magma reservoirs in few hundreds to few thousands of years. The formation of separate melt lenses becomes more likely when segregation of melt starts at crystallinities >0.6. Our results suggest that H2Oi plays an important role in modulating the distribution of eruptible melt in upper crustal reservoirs. Reservoirs of felsic and water-poor magma (<2 wt.% H2Oi) tend to be associated with the formation of isolated pockets of crystal-poor and eruptible magma, which could account for the often-observed geochemical heterogeneity of the products of large caldera-forming eruptions in the Snake River Plane. The limited dimensions of these eruptible magma pockets make their detection by geophysical methods challenging.

The lateral growth and coalesence of magma systems.

Thermal and mechanical models of magma reservoir growth need to be reconciled with deformation patterns and structural relationships observed at active magma systems. Geophysical observations provide a series of short time-scale snap-shots (100–102 years) of the long-term growth of magmatic bodies (103–106 years). In this paper, we first review evidence for the growth of magmatic systems along structural features and the associated deformation patterns. We then define three distinct growth stages, (1) aligned melt pockets, (2) coalesced reservoirs, (3) highly evolved systems, which can be distinguished using short-term surface observations. We use two-dimensional thermal models to provide first-order constraints on the time scales and conditions associated with coalescence of individual magma bodies into large-scale reservoirs. We find that closely spaced intrusions (less than 1 km apart) can develop combined viscoelastic shells over time scales of 10s kyr and form laterally extensive mush systems over time scales of 10–100 kyr. The highest temperatures and melt fractions occur during a period of thermal relaxation after melt injection has ceased, suggesting that caldera-forming eruptions may preferentially occur long after the main intrusive activity. The coalescence of eruptible melt-rich chambers only occurs for the highest melt supply rates and deepest systems. Thus, these models indicate that, in most cases, conductive heat transfer alone is not sufficient for a full coalescence of magma chambers and that other processes involving mechanical ruptures and mush mobilization are necessary; individual melt lenses can remain isolated for long periods within growing mush systems, and will only mix during eruption or other catastrophic events. The long-term history of the magmatic system is therefore critical in determining rheological structure and hence short-term behaviour. This framework for the development of magmatic systems in the continental crust provides a mechanical basis for the interpretation of unrest at the world's largest volcanoes.This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics'.

The timing and widespread effects of the largest Holocene volcanic eruption in Antarctica

The caldera collapse of Deception Island Volcano, Antarctica, was comparable in scale to some of the largest eruptions on Earth over the last several millennia. Despite its magnitude and potential for far-reaching environmental effects, the age of this event has never been established, with estimates ranging from the late Pleistocene to 3370 years before present. Here we analyse nearby lake sediments in which we identify a singular event produced by Deception Island’s caldera collapse that occurred 3980 ± 125 calibrated years before present. The erupted tephra record the distinct geochemical composition of ejecta from the caldera-forming eruption, whilst an extreme seismic episode is recorded by lake sediments immediately overlying the collapse tephra. The newly constrained caldera collapse is now the largest volcanic eruption confirmed in Antarctica during the Holocene. An examination of palaeorecords reveals evidence in marine and lacustrine sediments for contemporaneous seismicity around the Antarctic Peninsula; synchronous glaciochemical volcanic signatures also record the eruption in ice cores spread around Antarctica, reaching >4600 km from source. The widespread footprint suggests that this eruption would have had significant climatic and ecological effects across a vast area of the south polar region.

Long-term magmatic evolution reveals the beginning of a new caldera cycle at Campi Flegrei.

Understanding the mechanisms that control the accumulation of large silicic magma bodies in the upper crust is key to determining the potential of volcanoes to form caldera-forming eruptions. Located in one of the most populated regions on Earth, Camp Flegrei is an active and restless volcano that has produced two cataclysmic caldera-forming eruptions and numerous smaller eruptive events over the past 60,000 years. Here, we combine the results of an extensive petrological survey with a thermomechanical model to investigate how the magmatic system shifts from frequent, small eruptions to large caldera-forming events. Our data reveal that the most recent eruption of Monte Nuovo is characterized by highly differentiated magmas akin to those that fed the pre-caldera activity and the initial phases of the caldera-forming eruptions. We suggest that this eruption is an expression of a state shift in magma storage conditions, whereby substantial amounts of volatiles start to exsolve in the shallow reservoir. The presence of an exsolved gas phase has fundamental consequences for the physical properties of the reservoir and may indicate that a large magma body is currently accumulating underneath Campi Flegrei.

Widespread tephra layers in the Bering Sea sediments: distal clues to large explosive eruptions from the Aleutian volcanic arc

Tephra layers within marine sediments provide information on past explosive eruptions, which is especially important in the case of remote island arcs where data on proximal pyroclastic deposits can be scarce. Three Alaska-Aleutian tephras (labeled Br2, SR2, and SR4) were found in the late Pleistocene-Holocene sediments of the Bering Sea (north Pacific). We fingerprint glass from these tephras with the help of single-shard electron microprobe and LA-ICP-MS analyses and provide microprobe data on minerals from two of these tephras. The large compositional variability of the Alaska-Aleutian volcanoes permits the use of ratios of highly incompatible trace elements (Ba/Nb, Th/Nb, Th/La, La/Nb) for identification of distal tephra sources by comparison of these ratios in tephra glass and proximal bulk rock analyses. This method, along with mapped tephra dispersal, has allowed us to link tephras under study to Aniakchak, Semisopochnoi, and Okmok volcanoes, respectively. Our results indicate that tephra Br2 was derived from the ~ 3.6 ka Aniakchak II caldera-forming eruption (Alaska, USA). This is the first ever finding of the Aniakchak II tephra in Bering Sea sediments, which permits enlargement of its tephra volume and eruption magnitude to ~ 100 km3 and 6.8, respectively. Tephra SR2, dated at ~ 12.2 ka, is likely associated with a post-glacial caldera on the Semisopochnoi Island, Aleutians (USA). Tephra SR4 (dated at ~ 64.5 ka), likely was derived from an earlier undocumented eruption from Okmok volcano (Aleutians). All three regionally spread tephra layers are valuable isochrones, which can be used for correlating and dating of Bering Sea sediments.