Pyroclastic Flows Research Articles

Present sedimentation in the volcanic lakes of the Kurile-Kamchatka region (Russia) as a basis for paleoreconstructions

Abstract. The results of modern processes observations of the sedimentation in the volcanic lakes of the region are summarized with the use of material from other regions of the world. The available data on the drained volcanic lakes deposits, Uzon-Geysernaya caldera as example, was analysed. The main sources of material (volcanic and post-volcanic activity, gravitational displacements on slopes, and erosion-denudation processes) and the mechanisms of its entry into volcanic lakes, as well as the features of the subsequent deposits transformation as a result of hydrothermal, seismic, and volcano-tectonic activity, are identified. The results of the studies carried out allow us to conclude that the volcanic lakes deposits are complexly constructed polyfacial complexes with alternating fine-grained lacustrine and lacustrine-swamp deposits with pyroclastic horizons and interlayers of untreated or poorly processed coarse clastic material coming as a result of volcano-tectonic activity, gravitational and erosion processes. The irregularity of horizons along strike is typical; and is characterized by the large-scale sediments deformation under the influence of seismic activity, growth of effusive and extrusive domes, phreatic explosions, etc. Hydrothermal activity contributes to the weathering and cementation of the lake sediments. Lava outpourings and the high-temperature pyroclastic flows provokes sintering of contacting horizons sediments.

НОВЫЕ ДАННЫЕ ПО ВУЛКАНОГЕННО-ОСАДОЧНОМУ КОМПЛЕКСУ НЕОГЕНА ИЧИНСКОЙ ВПАДИНЫ: РАЗРЕЗ ТЫРКАЧИН (ЗАПАДНАЯ КАМЧАТКА)

In the 90-m river bank of the middle reaches of the Icha River, thick strata of volcano-sedimentary deposits represented by pyroclastic rocks and siliceous tuffites with layers of diatomites were studied. Representative diatom assemblages assigned to the Middle Miocene Denticulopsis hyalina Zone (14.9–13.1 Ma) indicate relatively deep-water conditions of the marine basin. Comparison with the data on diatom assemblages from reference sections of Western Kamchatka indicates that the deposits of the Tyrkachin section belong to the upper part of the Kakert Formation. The petrographic composition and lithological structure of volcanogenic formations (members 1, 2, and 4) suggest that they are the product of pyroclastic flows associated with volcanic activity in central Kamchatka. It is assumed that Members 1 and 2 were formed in subaerial conditions, as indicated by the complete absence of marine microfossils, signs of primary volcanic stratification, and the absence of layering. The sediments of Member 4 with eroded contacts and pronounced layering apparently accumulated in a subaqueous setting

Forensic Profiling Analogue Approach for the Investigation of Natural Hazards – A Case Study from Onokoba Elementary School, Unzen Volcano, Japan

Internal temperature variations of pyroclastic flows and their deposits are arguably the most challenging data to acquire. As a preliminary study of the temperature variation inside pyroclastic flows, the remains of Onokoba Elementary School (Shimabara, Japan) were investigated. The elementary school is located in the close vicinity of Unzen volcano and was hit by one of the largest pyroclastic flows during the latest active period of the volcano on 15th of September 1991. This present preliminary study aims to determine the temperature exposure of various portion of the school building using field-forensic and urban geology. Natural hazard methods applied to the damaged materials exposed to high temperature have generated a temperature fingerprint the maximum temperature distribution. Charred wooden parts and plastic gutters installed on the schoolyard-side faced of the building turns out to be the most useful temperature indicators. The various deformation and alterations of the studied materials show significant differences in the temperature exposed to. Such differences on the second-floor section (between 75-110°C and 120-150°C) and on the first-floor section (above 435-557°C) of the building do not simply imply significant temperature heterogeneity in short distance (some ten to ≤100 m) inside the pyroclastic flow, but also points toward the possible effects of the building architecture on some key dynamic parameter of the pyroclastic flow. Such information may be important for planning future hazard mitigation actions.

Subaerial-submarine morphological changes at Stromboli volcano (Italy) induced by the 2019–2020 eruptive activity

This study analyses the morphological changes induced by eruptive activity at Stromboli volcano (Italy) during and after events occurring during July–August 2019. This period was characterized by intense eruptive activity (two paroxysmal explosions, a two-month-long lava emission, and more intense and frequent “ordinary” explosive activity) that produced significant changes within the region known as Sciara del Fuoco, located on the most unstable, north-western flank of the volcano. Since September 2019, the eruptive activity waned but remained intense, and erosive phenomena continued to contribute to the re-shaping of the Sciara del Fuoco. The morphological changes described here were documented by integrating topographic (PLÉIADES satellite tri-stereo Digital Elevation Models) and multibeam bathymetric data, acquired before, during, and after the paroxysmal events. This allowed the study of the cumulative effect of the different processes and the characterization of the different phases of accumulation/emplacement, erosion, remobilization and re-sedimentation of the volcaniclastic materials.Data acquired at several periods between September 2018 and April 2020, allowed a comparison of the subaerial and submarine effects of the 2019 events. We find evidence of localized, significant erosion following the two pyroclastic density currents triggered by the paroxysmal explosion of the 3 July 2019. We interpret this erosion as being caused by submarine and subaerial landslides triggered by the propagation of pyroclastic density currents down the Sciara del Fuoco slope. Immediately after the explosion, a lava field accumulated on the sub-aerial slope, produced by effusive activity which lasted about two months. Subsequently, the newly emplaced lava, and in particular its breccia, was eroded, with the transfer of material onto the submarine slope. This work demonstrates how repeated topo-bathymetric surveys allowed identification of the slope processes that were triggered in response to the rapid geomorphological variations due to the eruptive activity. The surveys also allowed distinction of whether estimated volumetric losses were the result of single mass-flows or gradual erosive processes, with implications on the related geohazard. Furthermore, this work highlights how submarine slope failures can be triggered by the entry into the water of pyroclastic density currents, even of modest size. These results are important for the development and improvement of an early warning system for tsunami-induced by mass flows, both in Stromboli and for island-based and coastal volcanoes elsewhere, where landslides and pyroclastic density currents can trigger significant, potentially destructive, tsunami waves.

Estimating the velocity of pyroclastic density currents using an operational dual-PRF radar

Pyroclastic density currents are one of the deadliest hazards produced by a volcano. Understanding their dynamics and generation mechanisms is critical for developing better hazard mitigation strategies. This study presents a method for retrieving velocity profiles across a natural moving PDC, applied here to a PDC generated by collapsing column during the eruption of Sinabung Volcano, Indonesia, on 19 February 2018 at the onset time of 08:53. We used an operational dual Pulse Repetition Frequency (PRF) weather radar, located ~7.8 km to the SE of the volcano, to estimate the velocity profile components of the volcanic plume: updraft, fallout, and horizontal advection. Doppler radar data was post-processed by applying two different filters: median and Laplacian, to correct errors associated with dealiased Doppler velocities. The Laplacian filter method was more effective in correcting the dealiasing errors by producing a more continuous velocity field without over smoothing its values. Following the dealiasing process, the velocity profile components were retrieved according to radar parameters such as Doppler velocities, copolar correlation, and reflectivity intensity factor. Initially, the pyroclastic clast was released at a lower exit velocity of ~120 m/s (84 s after the onset). A maximum of ~190 m/s exit velocity was then observed at 08:57:52 (292 s after onset). Lower exit velocity in the initial phase and less than 10 m/s estimated wind speed are the main factors causing the partial collapse of the plume at ~2.5 km height above the vent. The part of the collapsing column was associated with a more than 50 dBZ reflectivity intensity factor of fallout velocity exceeding −50 m/s at 126 s after the onset. Dilute PDCs were observed until 09:09:34 (994 s after onset), moving downslope at SE sector at a maximum velocity of −84 m/s (i.e., in the direction of the radar) . The extracted velocity components are essential parameters in the numerical model of PDCs and tephra dispersal, enforcing the benefit of weather radar to complement the remote monitoring system of volcanic hazards.

Intermittent Growth of a Newly-Born Volcanic Island and Its Feeding System Revealed by Geological and Geochemical Monitoring 2013–2020, Nishinoshima, Ogasawara, Japan

The island-forming Nishinoshima eruptions in the Ogasawara Islands, Japan, provide a rare opportunity to examine how the terrestrial part of Earth’s surface increases via volcanism. Here, the sequence of recent eruptive activity of Nishinoshima is described based on long-term geological and geochemical monitoring of eruptive products. Processes of island growth and temporal changes in the magma chemistry are discussed. The growth of Nishinoshima was sustained by the effusion of low-viscosity andesite lava flows since 2013. The lava flows spread radially with numerous branches, resulting in compound lava flows. Lava flows form the coherent base of the new volcanic edifice; however, pyroclastic eruptions further developed the subaerial volcanic edifice. The duration of three consecutive eruptive episodes decreased from 2 years to a week through the entire eruptive sequence, with a decreasing eruptive volume and discharge rate through time. However, the latest, fourth episode was the most intense and largest, with a magma discharge rate on the order of 106 m3/day. The temporal change in the chemical composition of the magma indicates that more mafic magma was involved in the later episodes. The initial andesite magma with ∼60 wt% SiO2 changed to basaltic andesite magma with ∼55 wt% SiO2, including olivine phenocryst, during the last episode. The eruptive behavior and geochemical characteristics suggest that the 2013–2020 Nishinoshima eruption was fueled by magma resulting from the mixing of silicic and mafic components in a shallow reservoir and by magma episodically supplied from deeper reservoirs. The lava effusion and the occasional explosive eruptions, sustained by the discharge of magma caused by the interactions of these multiple magma reservoirs at different depths, contributed to the formation and growth of the new Nishinoshima volcanic island since 2013. Comparisons with several examples of island-forming eruptions in shallow seas indicate that a long-lasting voluminous lava effusion with a discharge rate on the order of at least 104 m3/day (annual average) to 105 m3/day (monthly average) is required for the formation and growth of a new volcanic island with a diameter on km-scale that can survive sea-wave erosion over the years.

Hydrothermal Fluids-Rock Interactions in the Geothermal Area of the Ngebel Volcano Complex Ponorogo, East Java, Indonesia

The Ngebel geothermal area is morphologically in several volcanic cones arranged as lava domes, volcanic hills, and lake craters. Volcanic cones are composed of lava, pyroclastic falls, pyroclastic flows, volcanic cones such as Jeding Volcano, Kemlandingan Volcano, Manyutan Volcano, and Ngebel Volcano. The magmatism activity that occurs in this area is andesitic-dacitic and also forms a geothermal system. This geothermal system is estimated to produce 50 MWe. During the formation of a geothermal system, hydrothermal fluids interact with the surrounding rock. Based on the results of geological mapping, petrographic analysis, and analysis of rare earth elements. In general, the results of rock-fluid interactions show a decrease in the concentration of rare earth elements, both light and heavy REE elements, from andesite basaltic samples to cold water samples. However, an unexpected reduction from 10 times chondrite to 0.3 times chondrite from hot water to cold water. This research will contribute to the hydrothermal alteration process in the research area that previous researchers have never done and expect to provide new information about forming the geothermal system in the Ngebel Volcanic Complex area.

Ash Melting Behavior of Rice Straw and Calcium Additives

Rice straw is potentially an appropriate feedstock material for biofuel production, since a huge amount of this postharvest residue is generated every year. The transformation of such agricultural biomass into densified products with a higher energy value and their subsequent combustion is associated with several questions. One of them is that rice straw exhibits a large formation of ash during combustion; thus, it is essential to know the nature of its ash melting behavior. Generally, during the combustion of straw biomass, ash sintering occurs in relatively low temperatures, resulting in the damaging of heating equipment. This negative aspect can be overcome by the addition of calcium-based additives. This paper aimed to study the ash melting behavior at a laboratory scale and to determine the ash melting points of rice straw mixed with calcium carbonate (CaCO3) and calcium hydroxide (Ca(OH)2) in different proportional ratios. The standardly produced ash samples from the rice straw obtained from Cambodia were constantly heated up in a muffle furnace, and characteristic temperatures of ash melting, i.e., shrinkage, deformation, hemisphere, and flow temperature, were recorded. The results showed that increasing the additive ratio did not bring linear growth of the melting temperatures. The addition of 1% CaCO3 showed an optimal positive impact of higher ash melting temperatures, and thus a better ability to abate the sintering of the rice straw ash.

The Lahars of the South-East slope of Mount Cameroon: geological study, economic interest and impacts of their exploitation on the environment

Mount Cameroon is the site of explosive eruptions with the emission of ash, slag and lava flows. Mudflows, known as Lahars, can also be observed. These Lahars are mainly found in the lower part of the south-eastern slope. The outcrops observed on this flank show that these Lahars are very extensive and are avalanches of volcanic material, which flowed down the slopes of the volcano after the eruptions. The different outcrops observed are separated by pyroclastic products on which the lahars rest. The components of these lahars are generally black or grey basalts. The main granulometric characteristic of the deposits of these lahars is their heterometry. The size of the material varies from a few meters to the clay fraction. Large concentrations of mineable material are found in the watercourses. These materials are exploited in order to produce aggregates for civil engineering. This exploitation has consequences on the immediate environment. In addition to climatic factors, several parameters have influenced the mobilities and emplacement of the lahars on the south-eastern slope of Mount Cameroon: the slope, the volume and thickness of the deposits, the nature and physico-hydric characteristics of the deposits. The persistence of strombolian volcanic eruptions on Mount Cameroon and the particularly rainy context predispose it to other future episodes of lahar flow.

Numerical analysis of an 80,000 Nm3/h fly ash entrained-flow gasifier at various burner inclination angles.

The raw syngas effluent from a fluidized bed gasifier typically contains a large amount of fly ash having a high concentration of carbon, which is undesirable. The present work examined the newly developed entrained-flow gasification technology intended to gasify raw syngas. Simulation of gas-solid flow and reaction behavior in an industrial-scale entrained-flow gasifier applying this new technology was first performed to obtain a better understanding of the particle flow and gasification characteristics. In addition, the devolatilization and heterogeneous reactions of fly ash particles were characterized by thermogravimetric analysis and user-defined function. The predictions from the simulation showed good agreement with the results of in situ experimental measurements. The combustion reaction for raw syngas occurred in the burner jet zone. As the hot gaseous products diffused, gasification reactions dominated the other zones. When burner inclination angle was 0°, 8.5°, and 25.5°, the temperature at the bottom outlet of the gasifier was lower than the ash flow temperature with the value of 1360°C. Solid slag formed and blocked the outlet. By comparison, this gasifier with the burner inclination angle of 17° could discharge the liquid slag and function as a continuous operation. In this way, the carbon conversion in fly ash reached the maximum value of 87%.

Lava Dome Morphology and Viscosity Inferred From Data-Driven Numerical Modeling of Dome Growth at Volcán de Colima, Mexico During 2007-2009

Magma extrusion, lava dome growth, collapse of domes, and associated pyroclastic flow hazards are among important volcanological studies. In this paper, we analyze the influence of the magma viscosity and discharge rates on the lava dome morphology at Volcán de Colima in Mexico during a long dome-building episode lasting from early 2007 to fall 2009 without explosive dome destruction. Camera images of the lava dome growth together with recorded volumes of the erupted lava have been used to constrain numerical modeling and hence to match the history of the dome growth by nudging model forecasts to observations. Our viscosity model incorporates crystal growth kinetics and depends on the characteristic time of crystal content growth (or CCGT) and the crystal-free magma viscosity. Initially, we analyze how this viscosity, CCGT, and the rate of lava extrusion influence the morphology of the growing dome. Several model scenarios of lava dome growth are then considered depending on the crater geometry, the conduit location, the effective viscosity of dome carapace, and the extrusion rates. These rates are determined either empirically by optimizing the fit between the morphological shape of modeled domes and that of the observed dome or from the recorded lava dome volumes. The maximum height of the modeled lava dome and its horizontal extent are in a good agreement with observations in the case of the empirically-derived extrusion rates. It is shown that the topography of the crater at Volcán de Colima is likely to be inclined toward the west. The viscosity of the modeled lava dome (∼1012 Pa s) is in a good agreement with the effective viscosity estimated experimentally from lavas of Volcán de Colima. Due to the interplay between the lava extrusion and the gravity forces, the dome reaches a height threshold, and after that a horizontal gravity spreading starts to play an essential role in the lava dome evolution. The model forecasts that the dome carapace of higher viscosity (∼1014 Pa s) influences the dome growth and its morphology during long dome-building episodes by retarding horizontal advancement and developing steep-sided eastern edge of the dome at the volcano. The developed model can be used in assessments of future effusive eruptions and lava dome growth at Volcán de Colima or elsewhere. History matching modeling of lava dome growth sheds a light on dynamic processes inside the dome and may assist in assessing stress state in the dome carapace and in forecasting the dome failures.

The mineral transformation and molten behaviors of biomass waste ashes in gasification-melting process

Understanding the various ash transformation properties is vital for improving gasification behavior and optimizing industrial operation conditions during the gasification-melting process of biomass wastes. In this work, five typical biomass wastes were selected to investigate the slag property. The ash fusion process, mineral transformation and AAEMs solidification behaviors were performed and predicted by mineral transformation analysis and thermochemical calculation. The results indicated that ash fusion process contained sintering, swelling, bubbling and melting stages, which was related to ash chemical compositions. Besides, the ash flow temperature had a linear relationship with the basic-to-acid ratio of biomass waste ashes. High SiO2 content would lead to strong mineral crystallinity at high temperatures. The mineral transformation was also highly related to ash compositions. The complicated reactions of inorganic matters occurred when SiO2 content lied between 50 wt% and 70 wt%. For AAEMs solidification, the alkali was prone to react simultaneously with both SiO2 and Al2O3 to form eutectoid, and the range of 35–65 wt% for SiO2 + Al2O3 contents was beneficial for alkali solidification. All the obtained data was meaningful to provide a modifying reference for the efficient and clean utilization of various biomass wastes during gasification process.

Seismic velocity structure of Unzen Volcano, Japan, and relationship to the magma ascent route during eruptions in 1990\u20131995

Subsurface structures may control the migration of magma beneath a volcano. We used high-resolution seismic tomography to image a low- P-wave velocity (Vp) zone beneath Unzen Volcano, Japan, at depths of 3–16 km beneath sea level. The top of this low-Vp zone is located beneath Mt. Fugendake of Unzen volcano, which emitted 0.21 km3 of dacitic magma as lava domes and pyroclastic flows during eruptions in 1990–1995. Based on hypocenter migrations prior to the 1990–1995 eruptions and modeled pressure source locations for recorded crustal deformation, we conclude that the magma for the 1990–1995 eruptions migrated obliquely upward along the top of the low-Vp zone. As tectonic earthquakes occurred above the deeper part of the low-Vp zone, the deep low-Vp zone is interpreted to be a high-temperature region (> 400 °C) overlying the brittle–ductile transition. By further considering Vs and Vp/Vs structures, we suggest that the deeper part of the low-Vp zone constitutes a highly crystalized magma-mush reservoir, and the shallower part a volatile-rich zone.

The hazards of unconfined pyroclastic density currents: A new synthesis and classification according to their deposits, dynamics, and thermal and impact characteristics

Pyroclastic density currents (PDCs) that escape their confining channels are among the most dangerous of volcanic hazards. These unconfined PDCs are capable of inundating inhabited areas that may be unprepared for these hazards, resulting in significant loss of life and damage to infrastructure. Despite their ability to cause serious impacts, unconfined PDCs have previously only been described for a limited number of specific case studies. Here, we carry out a broader comparative study that reviews the different types of unconfined PDCs, their deposits, dynamics and impacts, as well as the relationships between each element. Unconfined PDCs exist within a range of concentration, velocity and temperature: characteristics that are important in determining their impact. We define four end-member unconfined PDCs: 1. fast overspill flows, 2. slow overspill flows, 3. high-energy surges, and 4. low-energy detached surges (LEDS), and review characteristics and incidents of each from historical eruptions. These four end-members were all observed within the 2010 eruptive sequence of Merapi, Indonesia. We use this well-studied eruption as a case study, focusing on the villages of Bakalan (13 km south of the volcano) and Bronggang (14 km south of the volcano), which were impacted by slow overspill flows and LEDS, respectively. These two unconfined PDC types are the least described from previous volcanic eruptions, but during the 2010 Merapi eruption the overspill flows resulted in building destruction and the LEDS in significant loss of life. We discuss the dynamics and deposits of these unconfined PDCs, and the resultant impacts. We then use the lessons learned from the 2010 Merapi eruption to assess some of the impacts associated with the deadly 2018 Fuego, Guatemala eruption. Satellite imagery and media images supplementing fieldwork were used to determine the presence of both overspill flows and LEDS, which resulted in the loss of hundreds of lives and the destruction of hundreds of buildings in inundated areas within 9 km of the summit. By cataloguing unconfined PDC characteristics, dynamics and impacts, we aim to highlight the importance and value of accounting for such phenomena in emergency management and planning at active volcanoes.

Petrography And Field Study Of Andesitic Ignimbrite In Temangan, Kelantan

The volcanic rocks in Temangan, Kelantan consist of felsic to intermediate volcanic rocks, that is mainly of pyroclastic with rhyolitic, dacitic, and andesitic tuff. A geological review has been carried out in Kampung Bukit Besi, Temangan, in Kelantan, Malaysia, which is composed of schist, shale, andesite, and ignimbrite. In Temangan, andesite and ignimbrite show an intimate relationship, as the ignimbrite exists as the fragments and andesite as the matrix. The methodology used in this study are petrography and X-ray Diffraction (XRD) analyses, to determine the mineralogical composition of andesitic ignimbrite. Andesite shows aphenitic and porphyritic textures and consist of pyroxene, plagioclase, alkali feldspar, quartz, biotite and iron oxides. A range of plagioclase textures indicate imperfect equilibrium condition from andesite, reflecting plagioclase dissolution and regrowth. The structural analysis suggest that thrust fault occurred in the study area. Andesite and ignimbrite of the two stages are suggested to have derived from the andesite magma eruption which produced pyroclastic flow eruptions in small amount in most caldera volcanoes at high temperature gradient.

Post-eruptive lahars related to the 1913 eruption in La Lumbre Ravine, Volcán de Colima, Mexico: The influence of ravine morphometry on flow dynamics

A detailed historic record on Volcán de Colima, Mexico, of lahar activity and related phenomena dating back to the sixteenth century has proven that debris flows have caused more human casualties in the area than any other volcanic activity including pyroclastic flows. In this paper we present a multi-approach study of a relevant lahar (a non-cohesive debris flow with the largest runout and one of the most voluminous of the volcano) at Volcán de Colima, Mexico. Observations and information relevant to the lahar deposit and event including componentry, textural, sedimentological, stratigraphic, morphometric and geochronologic analyses and oral testimonies from eyewitnesses enable us not only to confirm a relationship between the studied outcrops and the 1913 Plinian eruption of the Colima Volcano (the most intense and large in volume eruption of this volcano of the last century), but also to find a clear and direct correlation among morphometric features such as slope, ravine width, curves and kinematics, flow patterns, flow transport mechanisms, volume, shape of clasts, granulometry and sub-unit formations. In particular, we maintain the slope has a marked control on the thickness of the deposits, deposited volume, longitudinal changes, and patterns of downstream changes in particle morphology and granulometry. All this information will be useful for generating or updating hazard maps, taking these lahars as a worst-case scenario.

Sedimentological analysis of ash-rich pyroclastic density currents, with special emphasis on sin-depositional erosion and clast incorporation: The Brown Tuff eruptions (Vulcano, Italy)

The sedimentological, lithological and textural characteristics of the Brown Tuffs (BT) pyroclastic deposits, combined with their grain-size, componentry and geochemical glass compositions, are here investigated to obtain information on the transport and depositional mechanisms of the corresponding pyroclastic density currents (PDCs). The BT are widespread reddish-brown to grey, ash-rich pyroclastic deposits generated by pulsating hydromagmatic explosive activity from the La Fossa Caldera on Vulcano island during the c. 80–6 ka time-stratigraphic interval, and then distributed on most of the Aeolian Islands and Capo Milazzo peninsula (Sicily) and in the Tyrrhenian and Adriatic Sea regions. Near the source area on Vulcano, the BT are characterised by alternating massive and planar to cross stratified lithofacies that result from the stepwise, repeating aggradation of discrete PDC pulses. This alternance is regulated by either fluid escape or granular flow depositional regimes at high clast concentration or grain by grain traction deposition in the waning diluted stages of the PDCs. Most of the BT on Vulcano show intermittently stratified and massive ash deposits resulting from a pervasive post-depositional disruption of the primary structures. This is induced by upward fluid expulsion associated with dissipation of pore pressure between layers at different grain size (fine to coarse ash) and porosity, as outlined by distinctive upwards bends and pillar-type escape structures through the fluid-filled cracks and rupture points. Massive BT deposits with a faint colour and grain-size banding are widely recognised on Lipari, the nearby island of Vulcano. Based on the presence, at the base of BT depositional units, of cm-thick amalgamation bands containing pumice lapilli, scoria and lithic clasts ripped-up and embedded from the loose underlying pyroclastic units, they are interpreted as deposited by ash-rich PDCs laterally spreading from La Fossa Caldera and moving to Lipari. During their motion to Lipari these currents (likely) crossed a narrow and shallow sea-water inlet which did not stop their advancement but influenced the grain size distribution of those spreading on the Lipari mainland. In this paper, the mechanism of clast erosion and incorporation is outlined across the whole island of Lipari by means of field study, grain-size, and geochemical glass analyses on the different components of the mixed basal bands of the BT. This suggests that the BT PDCs maintained enough flow power as to erode the substratum, hence likely impacting the territory, over a distance up to at least 16–17 km from the volcanic source. Evidence that the BT PDCs exerted a high shear-stress over the loose substratum is also provided by undulated, recumbent flame and rip-up structures at the base of some depositional units in southern and central Lipari. In order to form such bed granular instabilities between the BT and the underlying deposits we calculate that the currents had at least a shear velocity of ca. 2 m s−1 and a shear stress in the range of 1‐4.5 kPa. These results add new insights on the large-scale hazard at the Aeolian Islands and shed new lights on the widespread transport and depositional dynamics of ash flows spreading over the sea and reaching nearby islands, and their interactions with the substratum and the pre-depositional topography.

Finite element formulation for compressible multiphase flows and its application to pyroclastic gravity currents

The numerical modeling of compressible multiphase flows is of high interest for several engineering applications. In this work, we focus on the study of pyroclastic flows arising from volcanic eruptive events. An accurate evaluation of the effects of this multiphase flow is of crucial importance for the Civil Protection for the preservation of urban settlements in volcanic areas. In this work, we propose a Finite Element formulation for the simulation of pyroclastic flows at conditions of thermal and kinetic equilibrium. This analysis belongs to the class of advection dominated problems, which are known to suffer from numerical instabilities. The required stabilization is provided by using a Variational Multiscale Method, typically used for monophase compressible flows and extended here for the first time to multiphase flows. The stabilized formulation is validated with benchmark problems for compressible flows, which are solved for both monophase and multiphase cases. A throughout comparison of the numerical results of the two different flows is also presented. Moreover, the numerical formulation is applied to the simulation of representative cases of pyroclastic flows considering large-scale computational domains and realistic material properties and initial thermal-kinematic conditions. The numerical analyses presented show the accuracy of the proposed method for the simulation of compressible multiphase flows and its suitability for risk assessment studies of urban settlements prone to be affected by pyroclastic gravity currents.

Tectono-Paleogeographic Impact on the Permian Depositional Environment and Provenance around the Chaiwopu Depression in the Southern Junggar Basin, NW China

The Chaiwopu Depression in the southern Junggar Basin is located between the West Bogda Mountains and the northeastern Tian Shan Mountains in northwest China. The intracontinental basin–mountain system was formed in the Central Asian Orogenic Belt during the Late Paleozoic. The Permian strata around the depression exhibits distinct variations, which provide significant information to understand its tectonic and depositional evolution. This study investigated six outcrop sites using lithological, sedimentological, and geochemical analyses. The representative lithology of the Lower Permian is submarine lava and pyroclastic flows on the northern margins and alluvial deposits near the southern margins. In the Middle Permian sequence, the extensive distribution of alternating shale and silt/sandstone with oil shale and carbonate indicates a lacustrine setting. The sediments are composed of felsic rock-forming minerals derived mainly from island arc settings. The source rock properties correspond to the Carboniferous volcanic terrain of northeastern Tian Shan. The Lower to Middle Permian source-to-sink system occurred in an incipient level of weathering and maturation, a simple recycling process, and arid to semi-arid climatic conditions. The characteristics and changes of the depositional environment and provenance can be understood in terms of implications of tectono-paleogeographic evolution associated with the West Bogda rifting and uplift.

Twenty-Five Years of Geomorphological Evolution in the Gokurakudani Gully (Unzen Volcano): Topography, Subsurface Geophysics and Sediment Analysis

In the aftermath of pyroclastic density current-dominated eruptions, lahars are the main geomorphic agent, but at the decadal scale, different sets of processes take place in the volcanic sediment cascade. At Unzen volcano, in the Gokurakudani gully, we investigated the geomorphologic evolution and how the topographic change and the sediment change over time is controlling this transition. For this purpose, a combination of LiDAR data, aerial photography and photogrammetry, ground penetrating radar and sediment grain size analysis was done. The results show choking zones and zones of enlargement of the gully, partly controlled by pre-eruption topography, but also by the overlapping patterns of the pyroclastic flow deposits of 1990–1995. The ground penetrating radar revealed that on top of the typical lahar structure at the bottom of the gully, side wall collapses were trapping finer sandy sediments formed in a relatively low-energy deposition environment. This shows that secondary processes are taking place in the sediment transport process, on top of lahar activity, but also that these temporary dams may be a source of sudden sediment and water release, leading to lahars. Finally, the sediments from the gully walls are being preferentially oozed out of the pyroclastic flow deposit, meaning that over longer period of time, there may be a lack of fines, increasing permeability and reducing internal pore pressure needed for lahar triggering. It also poses the important question of how much of a past event one can understand from outcrops in coarse heterometric material, as the deposit structure can remain, even after losing part of its fine material.