Large Explosive Volcanic Eruptions Research Articles

Changes in Northern Hemisphere extra-tropical cyclone frequency following volcanic eruptions

Explosive volcanic eruptions are well known to influence Earth’s temperature. Changes in Earth’s temperature can affect temperature gradients which in turn could affect the isentropic slope and hence Northern Hemisphere high and mid-latitude weather. Yet, the possible influence of volcanic eruptions on these atmospheric circulation patterns and the potential spatial extent are not well understood. To address this issue, we pursue two independent lines of evidence. Firstly, we simulate volcanic eruptions with the MPI-ESM1.2 Earth System Model and use the TRACK algorithm to explore how extra-tropical cyclone (ETC) frequency is affected in the model experiments. Secondly, we query the Greenland ice core NEEM-2011-S1 for indications of increased Northern Hemisphere ETC frequency correlating with evidence for explosive volcanism by comparing the storm proxies sodium and calcium; with the eruption proxy sulphur. Both the model and proxy evidence suggest that large explosive volcanic eruptions increase storminess around the location of the ice core. Furthermore, the simulations indicate that the number of ETCs increases in the subtropics and at high latitudes, while they decrease in the mid-latitudes. A detailed interrogation of the simulated eruptions reveals that increases in cyclone frequency are linked to steepening of the isentropic slope due to a larger meridional temperature gradient and to a lower tropopause. The steepening is driven by a combination of warming of the tropical stratosphere from absorption of longwave radiation by volcanic aerosols and surface cooling due to the scattering of sunlight by the same aerosols, whereas the lower tropopause may be attributed to a warmer stratosphere.

Giant offshore pumice deposit records a shallow submarine explosive eruption of ancestral Santorini

Large explosive volcanic eruptions from island arcs pour pyroclastic currents into marine basins, impacting ecosystems and generating tsunamis that threaten coastal communities and infrastructures. Risk assessments require robust records of such highly hazardous events, which is challenging as most of the products lie buried under the sea. Here we report the discovery by IODP Expedition 398 of a giant rhyolitic pumice deposit emplaced 520 ± 10 ky ago at water depths of 200 to 1000 m during a high-intensity, shallow submarine eruption of ancestral Santorini Volcano. Pyroclastic currents discharged into the sea transformed into turbidity currents and slurries, forming a >89 ± 8 km3 volcaniclastic megaturbidite up to 150 m thick in the surrounding marine basins, while breaching of the sea surface by the eruption column laid down veneers of ignimbrite on three islands. The eruption is one of the largest recorded on the South Aegean Volcanic Arc, and highlights the hazards from submarine explosive eruptions.

Modeling Wind‐Blown Umbrella Clouds in Lagrangian Dispersion Models

AbstractThe ash and gas released by large explosive volcanic eruptions rises to its neutral buoyancy level in the atmosphere, then spreads laterally to form an umbrella cloud. Density stratification of the atmosphere generates buoyancy forces in the cloud, which drive the outward spread. Although umbrella clouds are often modeled as circular axisymmetric structures, in practice they are usually influenced quite strongly by the meteorological wind, with spread in the upwind direction halted by the oncoming wind, and different rates of spreading in the downwind and crosswind directions. In this work, we derive a simple parametrization of non‐axisymmetric umbrella cloud spreading from a much more complex physically based shallow‐layer intrusion model. The new parametrization is quick to evaluate and so is suitable for use in operational Volcanic Ash Transport and Dispersion Models (VATDMs). In contrast to previous parametrizations, in which there is assumed to be no interaction between a circular umbrella cloud and the meteorological wind, here the umbrella cloud is influenced by the wind and adopts a shape determined by the balance of buoyant spreading and downwind drag forces. We apply the new scheme to four historical case studies of eruptions at Puyehue 2011, Pinatubo 1991, Ulawun 2019, and Calbuco 2015. The results are compared with VATDM simulations using a conventional circular umbrella cloud parametrization. Using the new scheme, good descriptions of cloud spread are recovered and the prediction of horizontal ash distribution is improved relative to the axisymmetric parametrization.

Atmosphere injection of sea salts during large explosive submarine volcanic eruptions

The 15 January 2022 submarine eruption at Hunga volcano was the most explosive volcanic eruption in 140 years. It involved exceptional magma and seawater interaction throughout the entire submarine caldera collapse. The submarine volcanic jet breached the sea surface and formed a subaerial eruptive plume that transported volcanic ash, gas, sea salts and seawater up to ~ 57 km, reaching into the mesosphere. We document high concentrations of sea salts in tephra (volcanic ash) collected shortly after deposition. We also discuss the potential climatic consequences of large-scale injection of salts into the upper atmosphere during submarine eruptions. Sodium chloride in these volcanic plumes can reach extreme concentrations, and dehalogenation of chlorides and bromides poses the risk of long-term atmospheric and weather impact. Salt content in rapidly collected tephra samples may also be used as a proxy to estimate the water:magma ratio during eruption, with implications for quantification of fragmentation efficiency in submarine breaching events. The balance between salt loading into the atmosphere versus deposition in ash aggregates is a key factor in understanding the atmospheric and climatic consequences of submarine eruptions.

Large-magnitude (VEI ≥ 7) ‘wet’ explosive silicic eruption preserved a Lower Miocene habitat at the Ipolytarnóc Fossil Site, North Hungary

During Earth’s history, geosphere-biosphere interactions were often determined by momentary, catastrophic changes such as large explosive volcanic eruptions. The Miocene ignimbrite flare-up in the Pannonian Basin, which is located along a complex convergent plate boundary between Europe and Africa, provides a superb example of this interaction. In North Hungary, the famous Ipolytarnóc Fossil Site, often referred to as “ancient Pompeii”, records a snapshot of rich Early Miocene life buried under thick ignimbrite cover. Here, we use a multi-technique approach to constrain the successive phases of a catastrophic silicic eruption (VEI ≥ 7) dated at 17.2 Ma. An event-scale reconstruction shows that the initial PDC phase was phreatomagmatic, affecting ≥ 1500 km2 and causing the destruction of an interfingering terrestrial–intertidal environment at Ipolytarnóc. This was followed by pumice fall, and finally the emplacement of up to 40 m-thick ignimbrite that completely buried the site. However, unlike the seemingly similar AD 79 Vesuvius eruption that buried Pompeii by hot pyroclastic density currents, the presence of fallen but uncharred tree trunks, branches, and intact leaves in the basal pyroclastic deposits at Ipolytarnóc as well as rock paleomagnetic properties indicate a low-temperature pyroclastic event, that superbly preserved the coastal habitat, including unique fossil tracks.

Decadal Timescale Correlations Between Global Earthquake Activity and Volcanic Eruption Rates

AbstractAt the global scale, large tectonic earthquakes and volcanic eruptions are believed to be random and independent events. Here, we compare global time‐series of large earthquakes ( 7) and explosive volcanic eruptions (VEI 2) spanning 1960–2019. Both time‐series exhibit decadal timescale trends, over which annual earthquake and eruption rates vary by a factor of 2. Moreover, global seismic moment release is positively correlated with global eruption rate, with Monte Carlo permutation tests showing that this correlation is significant with a ‐value of 0.05. Although large earthquakes can trigger eruptions at nearby volcanoes, the magnitude of this effect is insufficient to cause the observed global correlation. Other mechanisms, such as triggering of distant eruptions (>1000 km) by earthquake‐induced dynamic stress changes, modulation of global earthquake and eruption rates by variations in Earth's rotational velocity, or natural synchronization of events over repeating cycles, are therefore required to explain the correlation.

The radius of the umbrella cloud helps characterize large explosive volcanic eruptions

Eruption source parameters (in particular erupted volume and column height) are used by volcanologists to inform volcanic hazard assessments and to classify explosive volcanic eruptions. Estimations of source parameters are associated with large uncertainties due to various factors, including complex tephra sedimentation patterns from gravitationally spreading umbrella clouds. We modify an advection-diffusion model to investigate this effect. Using this model, source parameters for the climactic phase of the 2450 BP eruption of Pululagua, Ecuador, are different with respect to previous estimates (erupted mass: 1.5–5 × 1011 kg, umbrella cloud radius: 10–14 km, plume height: 20–30 km). We suggest large explosive eruptions are better classified by volume and umbrella cloud radius instead of volume or column height alone. Volume and umbrella cloud radius can be successfully estimated from deposit data using one numerical model when direct observations (e.g., satellite images) are not available.

Natural drivers of multidecadal Arctic sea ice variability over the last millennium

The climate varies due to human activity, natural climate cycles, and natural events external to the climate system. Understanding the different roles played by these drivers of variability is fundamental to predicting near-term climate change and changing extremes, and to attributing observed change to anthropogenic or natural factors. Natural drivers such as large explosive volcanic eruptions or multidecadal cycles in ocean circulation occur infrequently and are therefore poorly represented within the observational record. Here we turn to the first high-latitude annually-resolved and absolutely dated marine record spanning the last millennium, and the Paleoclimate Modelling Intercomparison Project (PMIP) Phase 3 Last Millennium climate model ensemble spanning the same time period, to examine the influence of natural climate drivers on Arctic sea ice. We show that bivalve oxygen isotope data are recording multidecadal Arctic sea ice variability and through the climate model ensemble demonstrate that external natural drivers explain up to third of this variability. Natural external forcing causes changes in sea-ice mediated export of freshwater into areas of active deep convection, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC) and thereby northward heat transport to the Arctic. This in turn leads to sustained anomalies in sea ice extent. The models capture these positive feedbacks, giving us improved confidence in their ability to simulate future sea ice in in a rapidly evolving Arctic.

The Photochemistry of Gas Molecules in Earth’s Atmosphere Determines the Structure of the Atmosphere and the Average Temperature at Earth’s Surface

A molecule of oxygen absorbing solar ultraviolet-C radiation is photo-dissociated into two atoms of oxygen that fly apart at high velocity, converting kinetic energy of oscillation of the molecular bond directly and completely into kinetic energy of linear motion of the oxygen atoms. This increases air temperature. Two oxygen atoms can then collide forming a new oxygen molecule that can then be dissociated again as long as sufficient ultraviolet-C radiation exists. This continual dissociation of oxygen molecules is the primary reason for the stratopause being 30-40 degrees warmer than the tropopause and for all ultraviolet-C radiation being absorbed before reaching the lower stratosphere. Furthermore, an oxygen molecule and an oxygen atom can collide to form a molecule of ozone, which is photo-dissociated by solar ultraviolet-B radiation. Normally, 97-99 percent of ultraviolet-B radiation is absorbed in the ozone layer, warming the lower stratosphere. By 1970, however, humans manufacturing chlorofluorocarbon gases caused up to 70% depletion of ozone, cooling the ozone layer and allowing more ultraviolet-B to reach Earth where it photo-dissociates ground-level ozone pollution, raising air temperatures, especially in the most polluted areas. Ultraviolet-B also penetrates oceans tens of meters, efficiently raising ocean heat content. Earth’s surface warmed 0.6°C from 1970 to 1998 with warming twice as great in the northern hemisphere containing 90% of global population. In 2014, Bárðarbunga volcano in central Iceland extruded 85 km² of basaltic lavas in six months, depleting the ozone layer and warming Earth another 0.3°C by 2016. Throughout Earth history, basaltic lava flows covering areas of up to millions of square kilometers are contemporaneous with sudden global warming—the larger the lava flow, the greater the warming. Large explosive volcanic eruptions, on the other hand, typically form aerosols in the lower stratosphere that spread throughout the world, reflecting and scattering sunlight, cooling Earth approximately 0.5°C for two to four years. Computer modelling shows the effects of this global cooling can still be observed in ocean temperatures a century later. Several large explosive volcanic eruptions per century, continuing for millennia, cool oceans incrementally down into ice-age conditions. Detailed measurements of air temperatures in ice cores at Summit Greenland over the past 122,000 years show that the footprints of climate change are sudden warming within years, followed by slow, incremental cooling over millennia, in highly erratic sequences averaging only a few thousand years in length. Ozone depletion and aerosols are particularly effective because they occur worldwide.

Fingerprinting the Late Pleistocene tephras of Ciomadul volcano, eastern–central Europe

ABSTRACTLate Pleistocene tephras derived by large explosive volcanic eruptions are widespread in the Mediterranean and surrounding areas. They are important isochronous markers in stratigraphic sections and therefore it is important to constrain their sources. We report here tephrochronology results using multiple criteria to characterize the volcanic products of the Late Pleistocene Ciomadul volcano in eastern–central Europe. This dacitic volcano had an explosive eruption stage between 57 and 30 ka. The specific petrological character (ash texture, occurrence of plagioclase and amphibole phenocrysts and their compositions), the high‐K calc‐alkaline major element composition and particularly the distinct trace element characteristics provide a strong fingerprint of the Ciomadul volcano. This can be used for correlating tephra and cryptotephra occurrences within this timeframe. Remarkably, during this period several volcanic eruptions produced tephras with similar glass major element composition. However, they differ from Ciomadul tephras by glass trace element abundances, ratios of strongly incompatible trace elements and their mineral cargo that serve as discrimination tools. We used (U‐Th)/He zircon dates combined with U‐Th in situ rim dates along with luminescence and radiocarbon dating to constrain the age of the explosive eruptions of Ciomadul that yielded distal tephra layers but lack of identified proximal deposits.

Global ozone depletion and increase of UV radiation caused by pre-industrial tropical volcanic eruptions

Large explosive tropical volcanic eruptions inject high amounts of gases into the stratosphere, where they disperse globally through the large-scale meridional circulation. There is now increasing observational evidence that volcanic halogens can reach the upper troposphere and lower stratosphere. Here, we present the first study that combines measurement-based data of sulfur, chlorine and bromine releases from tropical volcanic eruptions with complex coupled chemistry climate model simulations taking radiative-dynamical-chemical feedbacks into account. Halogen model input parameters represent a size-time-region-wide average for the Central American eruptions over the last 200 ka ensuring a comprehensive perspective. The simulations reveal global, long-lasting impact on the ozone layer affecting atmospheric composition and circulation for a decade. Column ozone drops below 220 DU (ozone hole conditions) in the tropics, Arctic and Antarctica, increasing biologically active UV by 80 to 400%. Our model results could potentially be validated using high-resolution proxies from ice cores and pollen records.

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.

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.

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.

Evidence of Influence of Human Activities and Volcanic Eruptions on Environmental Perchlorate from a 300-Year Greenland Ice Core Record.

A 300-year (1700-2007) chronological record of environmental perchlorate, reconstructed from high-resolution analysis of a central Greenland ice core, shows that perchlorate levels in the post-1980 atm were two-to-three times those of the pre-1980 environment. While this confirms recent reports of increased perchlorate in Arctic snow since 1980 compared with the levels for the prior decades (1930-1980), the longer Greenland record demonstrates that the Industrial Revolution and other human activities, which emitted large quantities of pollutants and contaminants, did not significantly impact environmental perchlorate, as perchlorate levels remained stable throughout the 18th, 19th, and much of the 20th centuries. The increased levels since 1980 likely result from enhanced atmospheric perchlorate production, rather than from direct release from perchlorate manufacturing and applications. The enhancement is probably influenced by the emission of organic chlorine compounds in the last several decades. Prior to 1980, no significant long-term temporal trends in perchlorate concentration are observed. Brief (a few years) high-concentration episodes appear frequently over an apparently stable and low background (∼1 ng kg-1). Several such episodes coincide in time with large explosive volcanic eruptions including the 1912 Novarupta/Katmai eruption in Alaska. It appears that atmospheric perchlorate production is impacted by large eruptions in both high- and low-latitudes, but not by small eruptions and nonexplosive degassing.

The global magnitude–frequency relationship for large explosive volcanic eruptions

For volcanoes, as for other natural hazards, the frequency of large events diminishes with their magnitude, as captured by the magnitude–frequency relationship. Assessing this relationship is valuable both for the insights it provides about volcanism, and for the practical challenge of risk management. We derive a global magnitude–frequency relationship for explosive volcanic eruptions of at least 300Mt of erupted mass (or M4.5). Our approach is essentially empirical, based on the eruptions recorded in the LaMEVE database. It differs from previous approaches mainly in our conservative treatment of magnitude-rounding and under-recording. Our estimate for the return period of ‘super-eruptions’ (1000Gt, or M8) is 17ka (95% CI: 5.2ka, 48ka), which is substantially shorter than previous estimates, indicating that volcanoes pose a larger risk to human civilisation than previously thought.

Tropical explosive volcanic eruptions can trigger El Ni\xf1o by cooling tropical Africa

Stratospheric aerosols from large tropical explosive volcanic eruptions backscatter shortwave radiation and reduce the global mean surface temperature. Observations suggest that they also favour an El Niño within 2 years following the eruption. Modelling studies have, however, so far reached no consensus on either the sign or physical mechanism of El Niño response to volcanism. Here we show that an El Niño tends to peak during the year following large eruptions in simulations of the Fifth Coupled Model Intercomparison Project (CMIP5). Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Niñas, lengthen El Niños and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air–sea interactions in the Pacific, favouring an El Niño-like response.

Impact zone dynamics of dilute mono- and polydisperse jets and their implications for the initial conditions of pyroclastic density currents

The fluid dynamics at the impact zone of an impinging jet dictate the initial conditions for the resulting downstream flow. Current understanding of this impact zone is limited to small scale, single phase, or monodisperse impinging jet experiments with a focus on industrial and engineering applications. Here, we present multifield numerical modeling results of mono- and polydisperse impinging jets with length scales and physical parameters relevant to large explosive volcanic eruptions. Our modeling shows that particle behavior is sensitive to whether a jet is monodisperse or polydisperse. For a monodisperse jet, the downstream flow behavior can be predicted by the coupling between the particles and gas, which we characterize by a form of the Stokes number (Stimp) where the time scale of changes in fluid motion is defined by the length scale and velocity change associated with vertical deceleration as a mixture approaches an impact surface. We find that the length scale of deceleration is sensitive to the mixture Mach number, as high Mach number flows produce standing shocks upstream of the impact site. For low Stimp monodisperse cases, the particles make the transition from axial to radial flow easily, and the flow continues downstream relatively unimpeded and well mixed. In contrast, in situations where the monodisperse particles are sufficiently poorly coupled (high Stimp), the particles rebound and/or concentrate at the impact zone, which results in a radial acceleration of gas as it is expelled from the concentrating mixture. In polydisperse jets, larger particles become better coupled to the gas in the free jet zone when in the presence of smaller, well-coupled particles due to particle-particle drag. However, the larger particles still lose significant momentum in the impact zone, which results in a lag effect where the smaller particles and gas are expelled and advance radially at a greater velocity. Although the simulations are simplified compared with real volcanic eruption scenarios, the results suggest that processes in the impact zone contribute directly to the formation of different types of gas-particle flows (concentrated versus dilute) that move outward across the ground.

Ancient Turks and Climate Disasters in Central Asia (534–679 AD)

This paper is the first to analyse the effect climate disasters (AD 536, 581, 626, 679) had on the historical processes in the east of Central Asia in the Early Middle Ages. Climate disasters are abrupt, severe, and sometimes protracted periods of cooling and drought that produce a negative effect on the economy of a nomadic society, the reason for this being large explosive volcanic eruptions. Cooling and drought brought famine to the nomads and caused illnesses. In addition to that, all the natural catastrophes were accompanied by plague pandemics among the eastern Turks. The Turkic Empire can be split into several chronological periods characterised by significant events that changed the course of history of the nomadic state, namely, AD 534–545 — the rise of the Turkic Empire; AD 581–583 — the division of the Turkic Empire into the Western and the Eastern Empires; AD 627–630 — the Eastern Turks are conquered by China; AD 679–687 — the second rise of the Eastern Turkic Empire. The research carried out shows that there is a certain connection between the important historical events and climate disasters in the history of the Turkic Empire (AD 534–745), this factor making it possible for the author to come forth with the proposition that the climatic factor did have an effect on the historical processes in the east of Central Asia, especially on the territories that had a nomadic economy.