Rhyolitic Eruptions Research Articles

Geochemical constraints on the role of tuffisite veins in degassing at the 2008–09 Chaitén and 2011–12 Cordón Caulle rhyolite eruptions

Hybrid activity during the rhyolitic eruptions of Chaitén (2008–09) and Cordón Caulle (2011–2012) in Chile has offered unprecedented insights into the enigmatic and complex degassing processes occurring during eruptions of silicic magma. Highly permeable, transient fracture networks within the conduit can act as outgassing channels. Their interaction with deeper volatile-rich melt can account for both punctuated explosive activity and large-scale degassing of the system, leading towards predominantly effusive behaviour. In this study we characterise trace element concentrations and 210Pb226Ra systematics within pyroclastic material from the recent eruptions at Chaitén and Cordón Caulle volcanoes. Results reveal how gas fluxing from deep, volatile-rich reservoirs to the surface, within magmatic conduits, can be recorded by trace elements and 210Pb226Ra disequilibria in tuffisite veins.Tuffisite veins (particle-filled fracture networks) are present in volcanic bombs from both eruptions. Trace element heterogeneity associated with tuffisites preserves evidence for degassing. At Chaitén, enrichments (e.g. Cu) and depletions (e.g. Mo, Li and Bi) are identified in vein material and clasts transported within veins, and record multiple degassing events. At Cordón Caulle, enrichments of volatiles in an early vein (e.g. Tl and Bi) and depletions in a later vein (e.g. Cd, In, Pb and Tl) reflect interactions between glassy clasts and the carrier gas phase that transported them. In contrast, 210Pb and 226Ra, which can be fractionated during degassing, are mostly in secular equilibrium. Modelling suggests that the disparity between the signals preserved in these two types of chemical signatures reflects the brevity of degassing events and the relative volumes of tuffisite veins and the bodies of degassing magma that they source gas from. The lack of preserved 210Pb enrichments in tuffisite veins at both volcanoes places an upper limit on the mass of deeper, bubble-rich magma outgassed via tuffisites during their lifetime.This study shows that both the presence, and absence, of sample-scale geochemical heterogeneity can be used to place constraints on syn-eruptive physical processes and underlines the value of analysing a wide suite of trace element species.

Accelerating exhumation in the Eocene North American Cordilleran hinterland: Implications from detrital zircon (U-Th)/(He-Pb) double dating

AbstractBasins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve records of deformation that are unavailable in footwall rocks. Following prolonged late Mesozoic–early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth’s best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermochronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105–740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (<4–6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to <100 m.y. in 45–43 Ma strata and to <10 m.y. in 43–41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1–20 m.y. lag times. These data reflect acceleration of basement exhumation rates by >1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere.

Halogen (Cl, F) release during explosive, effusive, and intrusive phases of the 2011 rhyolitic eruption at Cordón Caulle volcano (Chile)

We investigate sulphur, chlorine and fluorine release during explosive, effusive and intrusive phases of the 2011 Cordón Caulle eruption, with a focus on halogen devolatilization. Petrological analysis shows halogen release to have been promoted by isobaric crystallization in slowly-cooled magma that was emplaced in a lava flow and sub-vent intrusion. Fluorine in particular mobilized only after extensive groundmass crystallization and incipient devitrification. By 2017, the gas emitted from vent-proximal fumaroles had hydrothermal compositions, with HCl/HF ratios decreasing with decreasing temperature. We estimate that the eruption could eventually emit up to 0.84 Mt of SO2, 6.3 Mt of HCl, and 1.9 Mt of HF, but only ~7% and ~2% of total HCl and HF were emitted during explosive phases, and significant halogens are yet to be released from the intrusion. Halogen devolatilization and its associated hazards can persist long after the cessation of rhyolite eruptions with complex magma emplacement mechanisms.

La gran erupción de hace 4.2 ka cal en Cerro Blanco, Zona Volcánica Central, Andes: nuevos datos sobre los depósitos eruptivos holocenos en la Puna sur y regiones adyacentes

La erupción del Complejo Volcánico Cerro Blanco en el sur de la Puna, noroeste de Argentina (4410–4150 a BP) se investigó para obtener nueva información sobre estratigrafía, geomorfología, volcanología física, dataciones por radiocarbono, petrografía y geoquímica. La caracterización de los productos en relación a la evolución de la caldera de Cerro Blanco permitió estimar la distribución de los depósitos de ceniza de la fase paroxísmica Plineana de la erupción. Estos novedosos resultados evidencian una gran erupción explosiva riolítica que generó depósitos cineríticos en un área de aproximadamente 500.000 km2, acumulando > 100 km3 de tefra (volumen total). Este último valor supera el umbral inferior del Índice de Explosividad Volcánica (IEV) de 7. Los depósitos de caída de ceniza cubrieron la región, llegando a más de 400 km desde el Complejo Volcánico de Cerro Blanco, y los potentes depósitos de flujos piroclásticos rellenaron los valles vecinos alcanzando una distancia de 35 km. Esta erupción es la más grande documentada durante los últimos cinco milenios en la Zona Volcánica Central de los Andes y es probablemente una de las mayores erupciones explosivas holocenas del mundo. Además, se han identificado otras dos erupciones riolíticas en la región procedentes de otros dos centros eruptivos: una durante el Holoceno temprano y otra en el Holoceno tardío. La identificación y caracterización de estos grandes eventos volcánicos proporcionan nuevas guías para los registros geológicos y arqueológicos regionales del Holoceno, siendo marcadores cronostratigráficos de aplicación a una extensa área geográfica de América del Sur.

Isotopic Characteristics of Neogene‐Quaternary Tephra From IODP Site U1438: A Record of Explosive Volcanic Activity in the Kyushu‐Ryukyu Arc

AbstractInternational Ocean Discovery program (IODP) Site U1438 is located within the Amami‐Sankaku Basin, ~50 km SW of the Kyushu‐Palau Ridge and ~500 km SE of the present‐day volcanic front of the Kyushu‐Ryukyu arc. Thirty‐eight rhyolitic tephra layers (1–15 cm thick) deposited over the last 4 Myr were recovered at this site. Representative ash layers dominated by unaltered, colorless glass shards reaching 100–300 μm in size, in association with rare pyroxene, amphibole, biotite, and zircon, were sampled for isotopic analysis. The isotopic compositions of the tephra have a narrow range in 87Sr/86Sr (0.704 to 0.706), 206Pb/204Pb (18.32 to 18.46), 207Pb/204Pb (15.57 to 15.62), and 208Pb/204Pb (38.46 to 38.75) and are more variable in εNd (−3.5 to +4.8) and εHf (+2.0 to +13.3). This record indicates the tephra were not sourced from the Izu‐Bonin‐Mariana arc or from Central Japan but derived from felsic volcanism from the Kyushu‐Ryukyu arc. The 4.8‐km‐deep Amami‐Sankaku Basin was located up to 600–900 km from the Kyushu‐Ryukyu arc over the last 4 Ma, which is significantly more distal than other drill sites from which extensive tephra layers sourced from SW Japan have been recovered. Site U1438 tephra are thus likely related to widely distributed (M > 6) rhyolitic eruptions. We propose the unique tephra record and high‐precision isotope analysis of recovered tephra from Site U1438 can be used as an important marker to identify submerged calderas within the Ryukyu arc and/or constrain the history of activity (>1 Ma) of major calderas‐forming eruptions within the Kyushu‐Ryukyu arc.

Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions

Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. The dynamics of peralkaline rhyolite eruptions remain elusive with no direct observations recorded, significantly hindering the assessment of hazard and risk. Here we describe uniquely-preserved, fluidal-shaped pyroclasts found within pumice cone deposits at Aluto, a peralkaline rhyolite caldera in the Main Ethiopian Rift. We use a combination of field-observations, geochemistry, X-ray computed microtomography (XCT) and thermal-modelling to investigate how these pyroclasts are formed. We find that they deform during flight and, depending on size, quench prior to deposition or continue to inflate then quench in-situ. These findings reveal important characteristics of the eruptions that gave rise to them: that despite the relatively low viscosity of these magmas, and similarities to basaltic scoria-cone deposits, moderate to intense, unstable, eruption columns are developed; meaning that such eruptions can generate extensive tephra-fall and pyroclastic density currents.

Experimental constraints on the textures and origin of obsidian pyroclasts

Obsidian pyroclasts are commonly preserved in the fall deposits of explosive silicic eruptions. Recent work has suggested that they form by sintering of ash particles on the conduit walls above the fragmentation depth and are subsequently torn out and transported in the gas-particle dispersion. Although the sintering hypothesis is consistent with the general vesicle textures and dissolved volatiles in obsidian pyroclasts, previous sintering experiments do not capture all of the textural complexities observed in the natural pyroclasts. Here, we design experiments in which unimodal and bimodal distributions of rhyolitic ash are sintered at temperatures and H2O pressures relevant to shallow volcanic conduits and under variable cooling rates. The experiments produce dense, welded obsidian that have a range of textures similar to those observed in natural pyroclasts. We find that using a unimodal distribution of particles produces obsidian with evenly distributed trapped vesicles, while a bimodal initial particle distribution produces obsidian with domains of poorly vesicular glass among domains of more vesicle-rich glass. We also find that slow cooling leads to resorption of trapped vesicles, producing fully dense obsidian. These broad features match those found in obsidian pyroclasts from the North Mono (California, USA) rhyolite eruption, providing strong support to the hypothesis that obsidian can be produced by ash sintering above the fragmentation depth during explosive eruptions.

Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

AbstractDespite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.

Magma Reservoir Below Laguna del Maule Volcanic Field, Chile, Imaged With Surface‐Wave Tomography

AbstractThe Laguna del Maule (LdM) volcanic field comprises the greatest concentration of postglacial rhyolite in the Andes and includes the products of ~40 km3 of explosive and effusive eruptions. Recent observations at LdM by interferometric synthetic aperture radar and global navigation satellite system geodesy have revealed inflation at rates exceeding 20 cm/year since 2007, capturing an ongoing period of growth of a potentially large upper crustal magma reservoir. Moreover, magnetotelluric and gravity studies indicate the presence of fluids and/or partial melt in the upper crust near the center of inflation. Petrologic observations imply repeated, rapid extraction of rhyolitic melt from crystal mush stored at depths of 4–6 km during at least the past 26 ka. We utilize multiple types of surface‐wave observations to constrain the location and geometry of low‐velocity domains beneath LdM. We present a three‐dimensional shear‐wave velocity model that delineates a ~450‐km3 shallow magma reservoir ~2 to 8 km below surface with an average melt fraction of ~5%. Interpretation of the seismic tomography in light of existing gravity, magnetotelluric, and geodetic observations supports this model and reveals variations in melt content and a deeper magma system feeding the shallow reservoir in greater detail than any of the geophysical methods alone. Geophysical imaging of the LdM magma system today is consistent with the petrologic inferences of the reservoir structure and growth during the past 20–60 kyr. Taken together with the ongoing unrest, a future rhyolite eruption of at least the scale of those common during the Holocene is a reasonable possibility.

Generation of alkalic flood rhyolite: Insights from evolution of the Paisano volcano, Davis mountains, Trans-Pecos Texas, U.S.A.

Extensive silicic lava (flood rhyolite) constitutes part of the mid –Tertiary Paisano Volcano shield. This lava, borderline peralkaline quartz trachyte/rhyolite, forms part of a sequence of peralkaline rhyolite – quartz trachyte – trachyte produced as eruptions tapped deeper portions of underlying plutons. Flood rhyolite lava extended as far as 20 km from dikes of the eruptive center of the volcano, creating flows as thick as 100 m.Flood rhyolite was nearly aphyric to sparsely porphyritic, relatively hot (~911 °C) and erupted over a topographically subdued region, forming two lobes that extended north-northeast and southeast from a highland of previously-erupted rhyolite domes. Flood rhyolite extrusion followed eruption of a local ignimbrite, which filled valleys, flattening topography. Successive silicic flows off-lapped each other away from the volcano. Flood rhyolite eruption was followed by a second eruptive sequence that contained coarsely-porphyritic mafic trachyte lava.Silicic lava was most-likely generated by filter pressing from mush zones of mafic plutons. Glomeroporphyritic trachytes of the volcano are mixed rocks, silicic lava incompletely segregated from entrained fragments of mafic plutons. Continued evolution of quartz trachyte/low-silica rhyolite to peralkaline rhyolite was dominated by fractionation of alkali feldspar.

40Ar/39Ar dating of the Thorsmork ignimbrite and Icelandic sub-glacial rhyolites

Robust correlation of paleoclimatic archives (e.g. marine sediments, ice, speleothems) necessitates an increase of the number of independent and accurately dated stratigraphic markers using numerical methods. Icelandic rhyolitic volcanism has a strong chronological interest because these products are found as tephra in the North Atlantic marine archives as well as in Greenland ice cores. In this study, we have dated, using the 40Ar/39Ar method, subglacial rhyolites of the Tindfjallajökull, Kerlingarfjöll and Torfajökull volcanoes which are the main sources of rhyolitic tephra identified in marine and glaciological records.The Thorsmork ignimbrite (Tindfjallajökull) which correlates with the rhyolitic component (II-RHY-1) of the North Atlantic Ash Zone II (NAAZ II) layer is dated at 55.6 ± 2.4 ka, an age that agrees with the ones given by the ice core chronologies (GICC05: 55.4 ± 1.2 ka). The eruptions of Rauðfossafjöll and Fannborg are respectively dated at 77.0 ± 3.0 ka and 149.3 ± 3.4 ka.These new ages along with the published ones confirm, as already concluded by Flude et al. (2010) that there is no clear evidence, at least at the latitude of Iceland, that climate warming is the main mechanism that triggered rhyolitic eruptions.Combining major and trace element compositions with our new age determinations, it appears that the Rauðfossafjöll and the Snaefell rhyolitic eruptions have not yet been discovered in marine sediment cores. Nevertheless, given the accuracy and precision of our dating, these rhyolitic eruptions have the potential to become chronostratigraphic markers, once tephra shards corresponding to these eruptions are discovered in ice and or marine sediment records. The Fannborg eruption might be recorded in the marine core LINK 16 during MIS 6 prior to the transition to MIS5e.

Mixing and Crystal Scavenging in the Main Ethiopian Rift Revealed by Trace Element Systematics in Feldspars and Glasses

AbstractFor many magmatic systems, crystal compositions preserve a complex and protracted history, which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. While whole‐rock data show strong fractional crystallization signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera‐forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal‐rich mush. The antecrysts crystallized from a Ba‐enriched (more primitive) melt, before later entrainment into a Ba‐depleted residual liquid. Crystal‐melt segregation can occur on fast timescales in these magma reservoirs, owing to the low‐viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal‐poor postcaldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300–400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body.

Magma degassing in the effusive-explosive subglacial rhyolitic eruption of Dalakvísl, Torfajökull, Iceland: insights into quenching pressures, palaeo-ice thickness, and edifice erosion

Dissolved volatile contents preserved in the matrix glass of subglacially erupted rocks offer important insights into quenching pressures. With careful interpretation, these data may yield information on eruption conditions. In this paper we present detailed edifice and glacier reconstructions for explosive and effusive subglacial rhyolitic deposits at Dalakvísl, Torfajökull, Iceland. When grouped by lithofacies, Dalakvísl glasses display trends of decreasing H2O with elevation, consistent with a subglacial setting. A number of solubility pressure curves (SPCs) have been used to model these quenching pressure–elevation trends in order to reconstruct the loading conditions. Effusively erupted glasses (e.g. lava lobes) have higher dissolved water contents than the more explosively produced material (e.g. obsidian sheets), indicating a systematic difference in subglacial pressure and/or degassing behaviour. Best model fits to data are achieved when loading is by a combination of erupted deposits (with a flat-topped morphology) and ice/meltwater. Our best estimate for the original edifice summit elevation is ∼810 m a.s.l., similar to its current elevation; however, as the edifice is now more conical this indicates significant post-eruptive erosion around the margins of the edifice. We propose that during the initial stages of the eruption, meltwater could not escape, thus maintaining high subglacial pressure under which effusive lava bodies were produced intrusively. Our best estimate is that the original palaeo-ice surface was ∼1,020 m a.s.l., suggesting a syn-eruptive glacier thickness of ∼350 m, assuming a similar base level to today (∼670 m a.s.l.). A sudden release of meltwater then led to a pressure drop, driving a transition to more explosive activity with an ice surface over the vent closer to 880 m a.s.l. This study demonstrates the uses of dissolved volatile contents in reconstructing past environments and shows how eruption dynamics can be tracked over the timeline of a pre-historic eruption, offering valuable insight into the complex coupling between pressure and the mechanisms of subglacial eruptions.

Understanding the isotopic and chemical evolution of Yellowstone hot spot magmatism using magmatic-thermomechanical modeling

Large-scale melting and reworking of the crust and the upper mantle occurs in areas of mantle plume-crust interaction, generating voluminous and bimodal basaltic and silicic magmatism. Continental hot spot track magmas preserve geochemical and temporal records of these processes. Here, we apply and further develop the large scale (crust and the upper mantle) I2VIS magmatic-thermomechanical modeling code to investigate the potential origins of chemical and isotopic trends in the Yellowstone hot spot track, perhaps the best studied and constrained subcontinental mantle plume system on Earth. We model the propagation of melt via dikes in solid crust and via percolation in partially molten crust by using Lagrangian markers which also track the chemical and isotopic compositions of the melts that eventually erupt at the surface. Confirming the results of earlier geophysical and geochemical studies, we show that the eruptive activity at Yellowstone is best explained in terms of the formation of a ~15 km-thick mid-crustal mafic sill complex with its top at a depth of ~8 km that develops over a period of ~3 Myr. This sill complex releases rhyolitic fractionates and creates additional rhyolites by melting the surrounding crust, driving the voluminous rhyolitic volcanism which characterizes the Yellowstone system.We recognize three key trends in the evolution of a continental hot spot volcanic center such as Yellowstone: (1) frequent, small eruptions results in in larger erupted rhyolite volumes compared to rare and large eruptions, (2) erupted magmas tend to be initially comprised of a large degree of crustal melt, and the relative contribution of fractionates of basalts from the mantle increases with time, and (3), the initial depth of the crust which melts to produce eruptible rhyolites becomes shallower with time. The first of these trends is simply a function of the continuous supply of new basalt to the crust from the mantle. The latter trend, which is responsible for low-δ18O rhyolites, is produced by a combination of progressive melting of shallower crust as the system becomes hotter, repeated caldera collapses advecting shallow crust to a depth where it can melt, and the overplating and burial of shallow crust by repeated intrusions of basalt. The mid- and lower crust is not a significant source of erupted rhyolitic melts, as both the crustal melting and basalt fractionation takes place in the sill complex which forms near the bottom of the upper crust. The resulting modeled isotopic evolution of erupted magmas are a good match with the actual stable and radiogenic isotopic record of Yellowstone hot spot track volcanism preserved in zircon phenocrysts, and further replicates the results of recent geophysical imaging campaigns, giving us confidence that the assumptions and results of these thermomechanical models are broadly correct.

Inferring magma ascent timescales and reconstructing conduit processes in explosive rhyolitic eruptions using diffusive losses of hydrogen from melt inclusions

We integrate field observations with geochemical data from quartz-hosted melt inclusions to develop a hydrogen diffusion-based method to shed light on timescales and conduit processes during the opening stages of two supereruptions. We focus on the initial explosive phases (first ~5% of the total magma volume erupted) from: (1) the episodic Oruanui eruption, Taupo NZ (530 km3, 25.4 ka) and (2) the continuous Bishop Tuff eruption, Long Valley (650 km3, 0.765 Ma). Quartz-hosted melt inclusions for both eruptions exhibit coherent trends for major and trace elements (e.g. Al2O3, SiO2, Nb, U, Rb). However, in the first three phases (of ten) in the Oruanui eruption there is significant variability in measured H2O values (2.6–5.8 wt%), particularly in phases 1 and 3, each of which reflects the opening stages of two respective inferred vent sources. Phase 3 inclusions also record greater scatter in Li concentrations. In contrast, melt inclusions from the first two Bishop Tuff fall layers yield a narrower range of H2O concentrations (4.0–5.8%, with 90% between 4.5 and 5.8 wt%). We interpret the scatter in measured H2O concentrations to represent changes during slow magma ascent or partial stalling within the conduit at lower pressures. Using a diffusion model, we estimate that 61% of the Oruanui melt inclusions experienced modification of original H2O concentrations due to slow ascent in the conduit system prior to eruptive quenching, with the longest timescales (>5 days) mostly in phase 3 inclusions. In contrast, only 33% of melt inclusions from the initial Bishop fall layer (F1) experienced diffusive losses from their starting concentrations, implying that most magma spent less than a day ascending from the storage region at the eruption onset. Similarities between the Oruanui and the previously studied Huckleberry Ridge Tuff include comparable extents and distribution of diffusive losses (seen as H and Li scatter), and field evidence for episodic activity. Our observations imply that in these two systems, the first-erupted magmas rose from reservoirs with low degrees of overpressure and that magma mobilization was controlled by external factors (e.g. tectonics) rather than by strong volatile over-pressures. In contrast the Bishop eruption may well have been initiated by overpressure, leading to continuous, escalating activity.

H2O-content and temperature limit the explosive potential of rhyolite magma during Plinian eruptions

Recent rhyolite eruptions on Earth have demonstrated their capacity to produce a multitude of hazards, including ash formation lasting months and impacting the large reaches of the southern hemisphere. Nevertheless, the underlying mechanisms driving these eruptions are not yet fully understood. Magmatic volatiles, especially H2O, dictate whether volcanic eruptions proceed explosively or effusively. Experimental evidence for the role played by H2O in driving explosive fragmentation is rare, in particular in the eruption of rhyolitic magma. Here we show that when hydrous rhyolitic obsidians from Chaitén Volcano (Chile) are experimentally heated above their glass transition temperatures at ambient 1 atm-conditions, two different behaviors result, depending on starting H2O concentration and temperature: obsidians vesiculate to stable or quasi-steady state foams when H2O is ≤1 wt.% for a wide range of temperatures (728–1032°C), but will explode within just tens of seconds (<30 s) of the foaming process when H2O is 1.4 wt.% and T is >874°C. Explosive activity occurs above Chaitén's estimated eruption temperature but, for lower temperatures, only foaming occurs. Whether a foaming sample remains coherent or explodes depends on two interrelated factors, the Peclet number (Pe), a dimensionless ratio of diffusive and viscous timescales, and the timescale or rate of decompression, which is dictated in part by the H2O-vapor pressure gradient between bubbles and atmosphere. At or above 1.4 wt.% H2O, and for a range of permissible Chaitén eruption temperatures (∼780–825°C), Pe is large (>10), meaning viscous deformation aiding vapor expansion is the dominant mode of growing bubbles. Consequently, vesiculation can proceed rapidly due to high initial overpressure and low melt viscosity, until the point is reached that the melt deforms at a rate greater than its relaxation rate, resulting in fragmentation. Below 1.4 wt.% H2O and for temperatures equal to or lower than Chaitén's eruption, decompression and deformation rates, inferred from experimental clast expansion and bubble growth behavior, are at least an order of magnitude smaller than the explosive cases, which is insufficient for critical melt-rupturing. Pe estimations highlight the first order influence of temperature and H2O content on physical degassing behavior. Since the starting H2O content and temperature dictate many the parameters that govern bubble growth—pressure gradient, melt viscosity, and chemical diffusivity—our experiments shed light on what limits fragmentation in natural eruptions and offer an alternative explanation for H2O contents measured in Plinian fall deposits from Chaitén and other rhyolite centers. In essence, pyroclastic obsidian H2O contents could reflect temperature and P-H2O limitations on fragmentation. Furthermore, since foam expansion under low-P, high-T-H2O conditions can foster strain rates in excess of the melt's relaxation rate and thus drive fragmentation, we contend that, in addition to rapid magma ascent (and high magma supply rates), rapid clast expansion under low pressure conditions may also be important for explosive magma fragmentation.

Renewed Posteruptive Uplift Following the 2011–2012 Rhyolitic Eruption of Cordón Caulle (Southern Andes, Chile): Evidence for Transient Episodes of Magma Reservoir Recharge During 2012–2018

AbstractThe VEI 4 rhyolitic eruption of Cordón Caulle volcano in 2011–2012 was immediately followed by ∼0.77 m of exponentially decaying uplift during 2012–2015. In this study, we present evidence of additional transient pulses of inflation with interferometric synthetic aperture radar (InSAR) time series during 2016–2018. We also assess whether the 2012–2015 uplift can be explained by viscoelastic relaxation or not. InSAR detected ∼12 cm of uplift during 2016–2017 and ∼5 cm during 2017–2018. The three inflation episodes have very similar spatial scales and can be modeled with the same inflating sill (z ∼ 6 km), but their time evolution is significantly different. Numerical models of a pressurized reservoir surrounded by a viscoelastic shell do not have a better fit than a magma injection model to the 2012–2015 InSAR time series, indicating that magma injection is the most likely mechanism to explain this uplift signal. The spatial similarities of the three pulses suggest that they are produced by episodic magma injection. These magma injection pulses provide the heat to remobilize the crystal mush beneath the volcano on timescales of a few months, but the end of the uplift is not predicted by existing models. None of these uplift pulses were related to abnormal seismicity, and we speculate that they are mostly aseismic because they caused stresses of lower magnitude than the coeruptive stresses. The meter scale displacement observed at Cordón Caulle between 2007 and 2018 suggests that the volcano undergoes episodic cycles of inflation like those observed in silicic calderas.

The origin and evolution of breakouts in a cooling-limited rhyolite lava flow

Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in cooling limited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011–2012 rhyolite eruption of Puyehue-Cordon Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multiscale satellite, field, and textural data from Cordon Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordon Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core.

Geomorphic expression of rapid Holocene silicic magma reservoir growth beneath Laguna del Maule, Chile.

Large rhyolitic volcanoes pose a hazard, yet the processes and signals foretelling an eruption are obscure. Satellite geodesy has revealed surface inflation signaling unrest within magma reservoirs underlying a few rhyolitic volcanoes. Although seismic, electrical, and potential field methods may illuminate the current configuration and state of these reservoirs, they cannot fully address the processes by which they grow and evolve on geologic time scales. We combine measurement of a deformed paleoshore surface, isotopic dating of volcanism and surface exposure, and modeling to determine the rate of growth of a rhyolite-producing magma reservoir. The numerical approach builds on a magma intrusion model developed to explain the current, decade-long, surface inflation at >20 cm/year. Assuming that the observed 62-m uplift reflects several non-eruptive intrusions of magma, each similar to the unrest over the past decade, we find that ~13 km3 of magma recharged the reservoir at a depth of ~7 km during the Holocene, accompanied by the eruption of ~9 km3 of rhyolite. The long-term rate of magma input is consistent with reservoir freezing and pluton formation. Yet, the unique set of observations considered here implies that large reservoirs can be incubated and grow at shallow depth via episodic high-flux magma injections. These replenishment episodes likely drive rapid inflation, destabilize cooling systems, propel rhyolitic eruptions, and thus should be carefully monitored.

Thermomechanical Modeling of the Formation of a Multilevel, Crustal‐Scale Magmatic System by the Yellowstone Plume

AbstractGeophysical imaging of the Yellowstone supervolcano shows a broad zone of partial melt interrupted by an amagmatic gap at depths of 15–20 km. We reproduce this structure through a series of regional‐scale magmatic‐thermomechanical forward models which assume that magmatic dikes stall at rheologic discontinuities in the crust. We find that basaltic magmas accumulate at the Moho and at the brittle‐ductile transition, which naturally forms at depths of 5–10 km. This leads to the development of a 10‐ to 15‐km thick midcrustal sill complex with a top at a depth of approximately 10 km, consistent with geophysical observations of the pre‐Yellowstone hot spot track. We show a linear relationship between melting rates in the mantle and rhyolite eruption rates along the hot spot track. Finally, melt production rates from our models suggest that the Yellowstone plume is ~175°C hotter than the surrounding mantle and that the thickness of the overlying lithosphere is ~80 km.