Explosive Volcanic Eruptions Research Articles

A new age model for the Ordovician (Sandbian) K-bentonites in Oslo, Norway

During the Late Ordovician, large explosive volcanic eruptions deposited worldwide K-bentonites, including the Millbrig and Deicke K-bentonites in North America and the Kinnekulle K-bentonite in Scandinavia. We have studied a classical locality in Oslo containing one of the most complete sections of K-bentonites in Europe.In a 53 m section of Sandbian age, we discovered 33 individual K-bentonite beds, the most notable beds being the Kinnekulle and the upper Grimstorp K-bentonite. Magnetic susceptibility (MS) measurements on two intervals show significant periodicity peaks interpreted as Milankovitch cycles and thus astronomically forced changes in sediment supply and composition. These cycles fit remarkably well with both the expected Milankovitch periodicities for the Ordovician as well as the radiometric ages presented in this study and may represent one of the most convincing demonstrations of Milankovitch cycles from the lower Paleozoic so far. Five of the K-bentonites have been dated by high-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon geochronology, where the Kinnekulle K-bentonite gives an age of 454.06 ± 0.43 Ma. We have integrated the new data and calculated an age model showing the sedimentation rates through the section and thereby the ages of each of the 33 K-bentonites. Using the age model, we further present a new age for the Sandbian-Katian stage boundary. The section in Oslo provides the highest resolution window into the Upper Ordovician K-bentonite succession so far and helps shed more light on the chronology of one of the most intense volcanic periods of the Paleozoic and the relationship with the global carbon cycle changes that followed.

The 2.0–1.88 Ga Paleoproterozoic evolution of the southern Amazonian Craton (Brazil): An interpretation inferred by lithofaciological, geochemical and geochronological data

The study of Paleoproterozoic rocks is crucial for understanding Earth's tectonic evolution during the time when most of the modern crust and ore deposits were formed. The rocks of the Brazilian Amazonian Craton record some of the most-complete and best-preserved Paleoproterozoic magmatic and volcanic episodes on Earth. Following previous investigations, we present new lithofaciological and stratigraphic records of the felsic rocks of the Tapajós Mineral Province (TMP) (~2–1.88 Ga) and the São Felix do Xingú region (SFX) (~1.88 Ga) which, combined with new petrological and geochronological data, help providing a more complete understanding of the tectonic, magmatic and volcanological evolution of the Amazonian Craton. This magmatism/volcanism is thought to be formed in a late-/post-orogenic to extentional regime confirmed by the new geochemical data presented here. The transition from late-convergent to extensional tectonic setting could register the beginning of the taphrogenesis that marked the Amazonian Craton throughout the Mesoproterozoic. The volcanological approach of this contribution can serve as a strategy for the modelling of the evolution of Precambrian volcano-sedimentary basins around the world. The large amount of rocks analyzed are divided into primary and secondary volcaniclastic products depending on if they resulted from a direct volcanic activity (pyroclastic) or processes that reworked pyroclastic fragments. Furthermore, the deposits are subdivided into massive and stratified, depending on their primary mechanisms of transport and emplacement. By confirming the results from previous studies, our study permits to depict a more precise paleo-environmental picture of the processes that occurred in the Amazonian Craton during the Late-Paleoproterozoic. In particular, the presence of large regional-scale fissural systems and caldera collapses produced large silicic explosive volcanic eruptions, also accompanied by the emission of large volume effusive products. Although studies on the Amazonian Craton are still scarce and controversial, the present study provides new evidence that this volcanism may have formed one of the largest Silicic Large Igneous Provinces (SLIP) on earth. Our data also confirm that at least two major Paleoproterozoic periods of formation of volcanic rocks exist in the Amazonian craton. This point is of great relevance for any future interpretation of the geological evolution of this craton.

Disproportionately strong climate forcing from extratropical explosive volcanic eruptions

Extratropical volcanic eruptions are commonly thought to be less effective at driving large-scale surface cooling than tropical eruptions. However, recent minor extratropical eruptions have produced a measurable climate impact, and proxy records suggest that the most extreme Northern Hemisphere cold period of the Common Era was initiated by an extratropical eruption in 536 ce. Using ice-core-derived volcanic stratospheric sulfur injections and Northern Hemisphere summer temperature reconstructions from tree rings, we show here that in proportion to their estimated stratospheric sulfur injection, extratropical explosive eruptions since 750 ce have produced stronger hemispheric cooling than tropical eruptions. Stratospheric aerosol simulations demonstrate that for eruptions with a sulfur injection magnitude and height equal to that of the 1991 Mount Pinatubo eruption, extratropical eruptions produce time-integrated radiative forcing anomalies over the Northern Hemisphere extratropics up to 80% greater than tropical eruptions, as decreases in aerosol lifetime are overwhelmed by the enhanced radiative impact associated with the relative confinement of aerosol to a single hemisphere. The model results are consistent with the temperature reconstructions, and elucidate how the radiative forcing produced by extratropical eruptions is strongly dependent on the eruption season and sulfur injection height within the stratosphere. Explosive volcanic eruptions in the extratropics have cooled the climate in their hemisphere more than tropical eruptions, suggests an analysis of reconstructions since ad 750 and simulations with an atmosphere–aerosol model.

Analyzing the role of interfacial water on sulfate formation on present Mars

This work investigates a realistic scenario for sulfate formation on Mars that may have occurred under conditions representative to the whole Amazonian period. The process, according to reaction kinetics, works without the necessity of elevated temperature and is due to volcanic eruptions. Theoretical modeling utilizing a heterogeneous pathway leading to sulfates in the presence of microscopic subzero-temperature liquid H2O layer is presented for the first time. The process takes place via the following steps: 1. explosive volcanic eruption with gas, vapor and ash particle release; 2. adsorption of volcanic SO2 and H2O on grains in the volcanic cloud, and formation of interfacial H2O layer between the ice–grain boundary; 3. chemical reactions in the microscopic liquid H2O film forming HSO3–(aq); 4. oxidation to HSO4–(aq); 5. acid-sulfate reaction yielding SO4–-bearing minerals. Steps 1 and 2 happen in the atmosphere, steps 3 and 4 may occur both in the atmosphere and after deposition on the surface, while step 5 takes place on the surface. According to the findings the heterogeneous pathway is an important one, therefore future models should consider them besides homogeneous gas-phase reactions. Furthermore, microscopic liquid H2O could produce specific zonal distribution of sulfate content on the surface.

Comparing the spatial patterns of climate change in the 9th and 5th millennia BP from TRACE-21 model simulations

Abstract. The spatial patterns of global temperature and precipitation changes, as well as corresponding large-scale circulation patterns during the latter part of the 9th and 5th millennia BP (4800–4500 versus 4500–4000 BP and 9200–8800 versus 8800–8000 BP) are compared through a group of transient simulations using the Community Climate System Model version 3 (CCSM3). Both periods are characterized by significant sea surface temperature (SST) decreases over the North Atlantic, south of Iceland. Temperatures were also colder across the Northern Hemisphere but warmer in the Southern Hemisphere. Significant precipitation decreases are seen over most of the Northern Hemisphere, especially over Eurasia and the Asian monsoon regions, indicating a weaker summer monsoon. Large precipitation anomalies over northern South America and adjacent ocean regions are related to a southward displacement of the Intertropical Convergence Zone (ITCZ) in that region. Climate changes in the late 9th millennium BP (the “8.2 ka event”) are widely considered to have been caused by a large freshwater discharge into the northern Atlantic, which is confirmed in a meltwater forcing sensitivity experiment, but this was not the cause of changes occurring between the early and latter halves of the 5th millennium BP. Model simulations suggest that a combination of factors, led by long-term changes in insolation, drove a steady decline in SSTs across the North Atlantic and a reduction in the North Atlantic Meridional Overturning Circulation (AMOC), over the past 4500 years, with associated teleconnections across the globe, leading to drought in some areas. Multi-century-scale fluctuations in SSTs and AMOC strength were superimposed on this decline. This helps explain the onset of neoglaciation around 5000–4500 BP, followed by a series of neoglacial advances and retreats during recent millennia. The “4.2 ka BP Event” appears to have been one of several late Holocene multi-century fluctuations that were embedded in the long-term, low-frequency change in climate that occurred after ∼4.8 ka. Whether these multi-century fluctuations were a response to internal centennial-scale ocean–atmosphere variability or external forcing (such as explosive volcanic eruptions and associated feedbacks) or a combination of such conditions is not known and requires further study.

Recurrent Quaternary magma generation at Baekdusan (Changbaishan) volcano: New zircon U-Th ages and Hf isotopic constraints from the Millennium Eruption

It is well known that Baekdusan (Changbaishan), an active 2744-m-high stratovolcano located on the border between North Korea and China, initiated trachytic/commenditic eruption in the Pleistocene but the petrogenetic and temporal evolutions of this alkaline magma system remain insufficiently constrained. The so-called “Millennium Eruption” (ME) in the mid-10th century, which resulted in the formation of the world's highest caldera lake (Cheonji, or Tianchi) atop the mountain, was one of the largest and most violently explosive volcanic eruptions to have occurred on Earth in the past 2000 years. Here, we present U–Th disequilibrium ages and Hf isotopic compositions of zircons from trachydacitic pumices ejected during the ME. Taken together with the previous U–Th isotope data, our laser ablation-multicollector ICPMS analyses of the zircons yielded multiple age components consisting of an earliest Holocene cluster (11.3 ± 1.3 ka) and Middle–Late Pleistocene populations (ca. 230 and 110 ka), which indicates that Baekdusan magma has been generated episodically at intervals of ca. 120–100 ky. If the eruption ages assigned for the samples analyzed in the previous isotopic studies are correct, the consistent age pattern of zircon may suggest the persistent existence of the same magmatic plumbing system beneath Baekdusan since the ME. The wide range of zircon εHf values (+5.8 to −3.5) reflects an interaction between the EM1-type primitive magma and pre-existing crustal materials, evidenced by the presence of a xenocrystic zircon core showing a significantly negative εHf value (=−21). A broad negative correlation observed between zircon εHf and the published δ18O values can be ascribed to the progressive assimilation of weathered supracrustal rocks, likely in the course of magma chamber expansion.

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

AbstractPyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s−1 (equivalent to pure wind velocity of 6 to 8 m.s−1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s−1.

An experimental study of SO2 reactions with silicate glasses and supercooled melts in the system anorthite–diopside–albite at high temperature

Sulfur dioxide [SO2(g)] is the most abundant sulfur-bearing volcanic gas species on Earth. From its magmatic origin at depth to expulsion at the surface via either persistent degassing or large explosive volcanic eruptions, SO2(g) interacts with silicate materials at elevated temperatures. Similar high-temperature reactions also occur in the volcanic systems and the atmospheres of Venus, the Galilean moon Io, and in Mars’ past, as well in industrial flue-gas processing. We present an experimental investigation of the reaction between SO2(g), glasses and supercooled melts in the system anorthite–diopside–albite (CaAl2Si2O8–CaMgSi2O6–NaAlSi3O8). The samples were exposed to SO2(g) at 600–800 °C for experimental durations of 10 min to 24 h. The reactions resulted in the formation of sulfate coatings and modified the near-surface composition of the silicate samples. The predominant sulfate reaction product is CaSO4, with hydrated MgSO4 or Na2SO4 also observed in some experiments. In the anorthite–diopside system, the reaction extent strongly depends on the temperature relative to the glass transition temperature (Tg). Above Tg, in reactions with supercooled melts, the reaction forms up to 20 times more sulfate. The overall rate of sulfate formation is controlled by the diffusive flux of Ca, Mg and Na from the increasingly depleted silicate to the surface where the reaction with SO2(g) occurs. The sulfate-forming reaction results in a volume increase relative to the unreacted silicate. When this reaction occurs in the subvolcanic environment it causes an increased molar volume that may close veins, reducing the permeability and decrease the SO2(g) flux at the surface. An increase in the SO2(g) flux would then result in the opening of new veins, which may be accompanied by seismic activity. Additionally, the change in molar volume may itself trigger seismicity. The strong preferential uptake of Ca into the sulfate reaction product results in a Si- and Al-enriched silicate. In the sulfate, the Ca component may be mobilized by secondary processes such as through the interaction with meteoric fluids. We recommend that the products of such gas–solid reactions should be the object of remote and robotic investigations of planetary environments with volcanic histories such as on Mars, Io, Venus and Mercury.

The effects of volcanic eruptions on the frequency of particulate matter suspension events in Iceland

Large quantities of natural particulate matter are generated in Iceland every year. Glaciers, rivers, and explosive volcanic eruptions contribute to the production of suspended material in the air. With frequent high winds and sparse vegetation cover, fine and coarse particles are suspended and transported over land and out to sea. Observations of particulate matter during the period of 1966–2016 were used to determine the impact of explosive volcanic eruptions on the number of observed particulate matter events. These observations were made by weather observers at synoptic weather stations distributed around Iceland. Deposits from some explosive volcanic eruptions that produce new source material were found to increase the number of observations of particulate matter at stations relatively close to the volcano (within 125 km) for at least several months after the end of the eruption. This signal is only observed for eruptions that produced enough material to be classified into Volcano Explosive Index (VEI) of 3, or greater, and did not end during the winter, and is only rarely seen at the national scale. Eruptions starting shortly before or during the winter season and ending before the spring melt did not have an impact on the observations of suspended particulate matter due to snow covering the fresh material, reducing the potential for resuspension. The data set is insufficient to explain why only some eruptions fulfilling the criteria produce local effects persisting over multiple months. Seasonality, weather, and wind conditions are much stronger factors for increasing the number of particulate matter (PM) events than the creation of new material provided by explosive volcanic eruptions. A PM event is defined to occur when any type of PM is observed in the atmosphere.

Forecasting resuspended ash clouds in Iceland at the London VAAC

Explosive volcanic eruptions can leave behind widespread ash deposits that can subsequently be remobilised during strong winds. The London Volcanic Ash Advisory Centre (VAAC), based at the Met Office UK, provides forecasts for resuspended ash clouds in southern Iceland. In this article we consider a significant resuspended ash event on 24 April 2017, which was captured by the London VAAC forecast. We examine the meteorological observations and NWP data from the event and identify specific synoptic conditions conducive to significant remobilisation of volcanic ash in southern Iceland. These criteria will help forecasters to predict resuspended ash events in the future.

Automated Detection of Explosive Volcanic Eruptions Using Satellite‐Derived Cloud Vertical Growth Rates

AbstractAsh rich clouds, produced by explosive volcanic eruptions, are a major hazard to aviation. Unfortunately, explosive volcanic eruptions are not always detected in a timely manner in satellite data. The large optical depth of emergent volcanic clouds greatly limits the effectiveness of multispectral infrared‐based techniques for distinguishing between volcanic and nonvolcanic clouds. Shortwave radiation‐based techniques require sufficient sunlight and large amounts of volcanic ash, relative to hydrometeors, to be effective. Given these fundamental limitations, a new automated technique for detecting emergent clouds, produced by explosive volcanic eruptions, has been developed. The Cloud Growth Anomaly (CGA) technique utilizes geostationary satellite data to identify cloud objects, near volcanoes, that are growing rapidly in the vertical relative to clouds that formed through meteorological processes. Explosive volcanic events are shown to frequently be a source of rapidly developing clouds that, at a minimum, reach the upper troposphere. As such, the CGA algorithm is effective at determining when a recently formed cloud is possibly the result of an explosive eruptive event. While the CGA technique can be applied to any geostationary satellite sensor, it is most effective when applied to latest generation of meteorological satellites, which provide more frequent images with improved spatial resolution. Using a large collection of geographically diverse explosive eruptions, and several geostationary satellites, the CGA technique is described and demonstrated. A CGA‐based eruption alerting tool, which is designed to improve the timeliness of volcanic ash advisories, is also described.

The effects of simulating volcanic aerosol radiative feedbacks with WRF-Chem during the Eyjafjallajökull eruption, April and May 2010

Explosive volcanic eruptions can inject large amounts of ash and gases into the atmosphere. Such volcanic aerosols can have a significant impact on the surrounding environment, and there is the need to closely investigate their effects on meteorology on local, regional, and even continental scale. This work presents a study of the 2010 Eyjafjallajökull volcanic eruption the resulting ash dispersion and its radiative feedback effects on the meteorological conditions with the Weather Research Forecasting model with on-line Chemistry (WRF-Chem). Two model runs, one meteorology-only simulation (without chemistry) and one that considers gas- and aerosol chemistry as well as direct- and semidirect aerosol feedbacks were performed and compared. Results for daily values show that aerosol radiative feedback effects can cool the atmosphere close to the surface on average by 1 °C with maximum cooling exceeding even 2 °C for the considered episode. Near-surface atmospheric wind speed changed on average by 0.5 m/s with maximum values above 2 m/s. Furthermore, the presence of ash aerosols affected the vertical shape of the profiles of wind speed and temperature and resulted in a better agreement with radiosonde measurements when radiative feedback effects were considered. Although the modeling of the dispersion of volcanic ash clouds is subject to large uncertainties, we have demonstrated that the WRF-Chem model can reproduce observations at surface levels and vertical profiles more realistically when radiative feedback effects are considered in the simulations.

Detection of the dispersion and residence of volcanic SO2 and sulfate aerosol from Nabro in 2011

Continuous detection of dispersion and residence of volcanic plumes in troposphere and lower stratosphere is vitally important for improving the understanding on the role of volcano eruptions in climate change. We report a 3-month continuous detection of dispersion and residence of volcanic plumes in the troposphere and stratosphere generated from the volcanic SO2 erupted by Nabro in Eritrea on June 12th, 2011 observed by the OMI sensor. The background SO2 concentration of 3 different height layers in troposphere and lower stratosphere were estimated by the 3-year-average daily concentration of monthly SO2 in 2005, 2007 and 2013, when there were no large explosive volcanic eruptions occurring. We also traced the diffusion path and the concentration of volcanic SO2 for the first 3 months after Nabro's eruption, and detected the appearance and dissipation of sulfate aerosols, which is a product converted from volcanic SO2. The results show that after Nabro erupted on June 12th, the volcanic plumes spread to middle latitudes (30°N-60°N) of Northern Hemisphere and loading by westerly jet. The volcanic SO2 in middle troposphere layer (TRM) and lower troposphere layer (TRL) stopped eastward spreading, and dissipated over the western Pacific Ocean on June 23rd. On June 26th, the volcanic SO2 in upper troposphere and lower stratosphere (STL) reached Mexico in Central America, and almost encircled the low latitudes and parts of middle latitudes. On June 28th, the volcanic SO2 plume showed an even distribution in STL. 37 days after the eruption, the volcanic SO2 in STL encircled the Northern Hemisphere evenly, sulfate aerosols in STL largely covered the low and middle latitudes and the daily concentration of SO2 was still higher than the background value. One month after the eruption, the global mean daily concentration of SO2 dropped to the normal value, but the daily concentration of SO2 and sulfate aerosols in low latitudes remained high, and dissipated not earlier than 3 months after the eruption of Nabro.

Formation of massive iron deposits linked to explosive volcanic eruptions

The genetic link between magmas and ore deposit formation is well documented by studies of fossil hydrothermal systems associated with magmatic intrusions at depth. However, the role of explosive volcanic processes as active agents of mineralization remains unexplored owing to the fact that metals and volatiles are released into the atmosphere during the eruption of arc volcanoes. Here, we draw on observations of the uniquely preserved El Laco iron deposit in the Central Andes to shed new light on the metallogenic role of explosive volcanism that operates on a global scale. The massive magnetite (Fe3O4) ore bodies at El Laco have surface structures remarkably similar to basaltic lava flows, stimulating controversy about their origin. A long-standing debate has endured because all proposed models were constructed based exclusively on samples collected from surface outcrops representing the uppermost and most altered portion of the deposit. We overcome this sampling bias by studying samples retrieved from several drill cores and surface outcrops. Our results reveal complex lithological, textural and geochemical variations characterized by magmatic-like features and, most notably, a systematic increase in titanium concentration of magnetite with depth that account for an evolving system transitioning from purely magmatic to magmatic-hydrothermal conditions. We conclude that El Laco, and similar deposits worldwide, formed by a synergistic combination of common magmatic processes enhanced during the evolution of caldera-related explosive volcanic systems.

The weathering of volcanic tephra and how they impact histosol development. An example from South East Iceland

Little is known about the impact of tephra deposits from explosive volcanic eruptions on soil formation and weathering processes in organic soils. The weathering of tephra of basaltic and rhyolitic origin and their impact on Icelandic histosols were studied through a combination of physical, chemical and mineralogical analyses. Two sampling sites were selected according to the presence of the light coloured rhyolitic tephra from the Öræfajökull eruption in 1362 CE and a dark-coloured basaltic Veiðivötn tephra from 1477 CE in the volcanically active area south of Vatnajökull in South East Iceland.The determining factor of pedogenesis in the investigated histosols is the OM, but the influence of tephra and aeolian material from external sources must be taken into consideration. The soils are the result of altered plant residues and volcanic material (tephra and aeolian material). Plant remnants, as well as the soil itself, are protected from decomposition by the prevailing anaerobic conditions, a low soil pH and the repeated addition of inorganic matter. Clay formation is low, while metal-humus complexes are predominant. Feo/Fed ratios indicated a generally low degree of weathering, being higher close to the basaltic tephra.The mineralogy was dominated by plagioclase and pyroxene, with quartz and zeolite as minor components. In contrast to previous research on Icelandic soils, our investigations revealed layer silicates at both sites. While we found evidence of smectite in the soils at Kálfafell, hydroxy interlayered minerals were found at Reynivellir. In the basaltic tephra, traces of layer silicates could be verified. In contrast, the rhyolitic tephra did not show any pedogenic minerals, suggesting that it had hardly altered since its deposition in 1362 CE. It is not the chemical composition of the inorganic parent material, but the location that may be an influencing factor on the formation of clay minerals in the investigated histosols.

Trashcano: Developing a quantitative teaching tool to understand ballistics accelerated by explosive volcanic eruptions

Accurate predictions of volcanological phenomena, such as the trajectory of blocks accelerated by volcanic explosions, require quantitative skills training. Large outdoor experiments can be useful to convey concepts of volcanic processes to students in an exciting way. Beyond the fun aspects, these experiments provide an opportunity to engage with the physics of projectile flight and help promote mathematical learning within the Earth Sciences. We present a quantitative framework required to interpret ballistic trajectories and the outdoor experiment known commonly as “trashcano”, taking a step-by-step approach to the physics of this problem, and deriving a range of mathematical solutions involving different levels of complexity. Our solutions are consistent with the predictions from established computer programs for volcanic ballistic trajectory modelling, but we additionally provide a nested set of simplified solutions for a range of teaching scenarios as well as simulated datasets for use where the full experiment may not be possible.

Message in a bottle: Spontaneous phase separation of hydrous Vesuvius melt even at low decompression rates

Violent explosive volcanic eruptions are destructive and threaten millions of people and infrastructure. Ejected ash, pumice and gases are end-products of volcanic factories sourced deep within magma chambers and conduits. The different production stages are not directly observable. However, experimental simulation reveals different stages of dynamic volcanic processes. The starting point of explosive volcanic eruptions is determined by the phase separation of an H2O fluid from a supersaturated hydrous silicate melt. The number of formed H2O fluid vesicles per unit volume of silicate melt (VND) is a basic property that controls the efficiency of fluid-melt separation, ascent velocity and finally explosive volcanism. We performed decompression experiments at superliquidus temperatures to simulate phase separation of a single phase hydrous silicate melt during ascent with AD79 Vesuvius white pumice composition using decompression rates of 0.024–1.7 MPa⋅s−1. The white pumice buried Herculaneum and Pompeii and is representative of other catastrophic phonolitic and trachytic explosive eruptions like the violent 39 ka Campi Flegrei and the 1815 AD Tambora eruption. Here we report a high log⁡VND of 5.2 (in mm−3) that is independent from decompression rate within the investigated range. Even at a decompression rate of 0.024 MPa⋅s−1 the formation of a high VND inevitably causes rapid degassing due to short H2O diffusion distances from the melt into fluid vesicles. A decompression rate meter based on nucleation theory, which is commonly used to estimate magma ascent velocity during volcanic eruptions using VND of volcanic ejecta, cannot be adapted to explain our experimentally determined decompression rate independent VND. Alternatively, decompression induced H2O–silicate melt phase separation may be described by diffusion controlled spinodal decomposition where maximum supersaturation is reached. This process occurs spontaneously and free of activation energy if hydrous melt is driven into thermodynamic instability where the second derivative of free energy of mixing to the H2O content is ≤0. However, the decompression rate independent VND has profound consequences for the dynamics of natural polyphase hydrous magma phase separation. Even at low ascent rates spontaneous hydrous melt phase separation facilitates rapid density decrease accompanied by sudden increase of magma buoyancy triggering explosive eruptions.

The Condition of Eruption Column Collapse: 2. Three‐Dimensional Numerical Simulations of Eruption Column Dynamics

AbstractThe scenarios leading to column collapse during explosive volcanic eruptions are diverse because of their complex dependence on geological and petrological factors. In a companion paper, we proposed a reference model to predict column collapse condition on the basis of analytical solutions of one‐dimensional eruption models. Here we perform three‐dimensional (3‐D) simulations of eruption column dynamics to confirm the reference model. In the 3‐D simulations, the velocity and pressure boundary conditions at the crater top are determined from quasi‐1‐D conduit flow. When a gas‐pyroclast mixture erupts from a conduit‐crater system that satisfies the conditions of choked flow at the crater base or top, the mixture exits as either an underexpanded (overpressured) or overexpanded (underpressured) jet. The 3‐D simulations show that the underexpanded jets are accelerated and the overexpanded jets are decelerated due to decompression‐compression processes just above the vent. The total momentum flux after these decompression‐compression processes measured in the 3‐D simulations agrees well with the values estimated using the reference model. The column collapse condition determined by the 3‐D simulations is consistent with the reference model, with an effective entrainment coefficient of k = 0.05. We also find that complex flow patterns in the transitional state of column collapse result from multidimensional compressible fluid dynamical and buoyancy effects.

On ash dispersal from moderately explosive volcanic eruptions: Examples from Holocene and Late Pleistocene eruptions of Italian volcanoes

New tephrostratigraphic data from three marine cores sampled in different depositional settings around southern Italy's coastline allowed correlating each of them with the regional tephrostratigraphy and to gain insights on ash dispersal from explosive eruptions of various intensities. Volume magnetic susceptibility curves, lithological analysis and geochemistry allowed the recognition of 38 tephras or cryptotephras in the three cores. The lithology of each tephra/cryptotephra was described, and the major element composition of volcanic glasses determined. Major element compositions of mineral phases and trace element compositions of bulk rock and volcanic glasses were also determined for selected samples. The majority of tephras/cryptotephras were correlated with the activity of Neapolitan volcanoes (16 from Somma-Vesuvius, 13 from Campi Flegrei, 1 from Ischia Island), whereas the others were correlated with deposits from to the volcanic activity of Lipari Island (6) and Mount Etna (1). The different tephras/cryptotephras were correlated to nine Plinian/Subplinian eruptions that occurred in the central Mediterranean region in the last 15,000 years. In addition, correlation with proximal counterparts highlighted the occurrence in marine core successions of several tephra layers related with moderately explosive eruptions from Neapolitan volcanoes that, in some cases, show dispersal areas comparable to those of Subplinian products. This finding means that the amount of fine-grained particles produced during explosive eruptions is independent with respect to the intensity of the eruption itself.

Positive Platinum anomalies at three late Holocene high magnitude volcanic events in Western Hemisphere sediments

Changes in the global atmospheric budget of platinum reportedly correspond to explosive volcanic eruptions. Using inductively coupled plasma mass spectrometry (ICP-MS) elemental analysis we examined eight widely separated stratified sites to evaluate the geographic extent of three late Holocene high magnitude volcanic events. We found characteristic Pt anomalies across the Western Hemisphere dating to the Laki, Iceland (CE 1783–1784), Kuwae, Vanuatu (CE 1452–1453), and Eldgjá, Iceland (CE 934) explosive volcanic eruptions. Pt anomalies in sediments over a broad geographic area indicate distinctive time-correlative atmospheric deposition rates of platinum-rich volcanic ash. These anomalies provide new chronostratigraphic markers for these late Holocene high magnitude volcanic eruptions, which are especially valuable in the Western Hemisphere in strata with limited chronometric control. Pt anomalies provide an important tracer for the age of these volcanic events and ultimately a new chronostratigraphic marker in archaeological, geological, palynological, and paleontological sediments.