Fall Deposits Research Articles

Bimodal Holocene flank eruptions at Lanín composite volcano, Southern Volcanic Zone of the Andes: Characterization of their deposits and controls on the vents array

The Pleistocene to Holocene Lanín composite volcano, in the Southern Volcanic Zone of the Andes, has summit and flank eruptions of Holocene age. We present here a characterization of flank vents and their deposits produced during the most recent eruptions of the Lanín volcano. Volcanic facies analysis indicates four unreported Holocene eruptive units sourced from flank vents. The associated deposits have basaltic, basaltic andesitic and trachytic compositions, and were formed mainly by low-volume effusive and mainly Strombolian-like explosive eruptions. Pyroclastic deposits were identified at the NE, E and SW volcano flanks, and correspond to concentrated and diluted pyroclastic density current deposits, in addition to non-welded to welded fall deposits. Basaltic lava flows of pahoehoe morphology were produced in a purely effusive episode. An analysis of the distribution of Lanín's flank vents (dyke's orientation and vent alignments) along fault-kinematic analysis in basement rocks were carried out with the aim of assessing the basement anisotropies that may have controlled the location and orientation of dykes, eruptive fissures and vent alignments. Contrary to expectations, the Liquiñe-Ofqui Fault System does not constitute a first-order control on the orientation of dykes and vents alignments in Lanín. The vents arrangement may be partly influenced by pre-Andean crustal scale fault zones, such as the Mocha-Villarica Fault Zone, and local basement structures. The predominance of flank vents over summit eruptions strongly suggests that the load of the volcanic edifice is a controlling factor.

Hot Sintering of Melts, Glasses and Magmas

Research Article| May 01, 2022 Hot Sintering of Melts, Glasses and Magmas Fabian B. Wadsworth; Fabian B. Wadsworth Department of Earth Sciences, Durham University, Durham, United Kingdom fabian.b.wadsworth@durham.ac.uk Search for other works by this author on: GSW Google Scholar Jérémie Vasseur; Jérémie Vasseur Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany j.vasseur@lmu.de Search for other works by this author on: GSW Google Scholar Edward W. Llewellin; Edward W. Llewellin Department of Earth Sciences, Durham University, Durham, United Kingdom ed.llewellin@durham.ac.uk Search for other works by this author on: GSW Google Scholar Donald B. Dingwell Donald B. Dingwell Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany dingwell@lmu.de Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2022) 87 (1): 801–840. https://doi.org/10.2138/rmg.2022.87.17 Article history first online: 01 May 2022 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Fabian B. Wadsworth, Jérémie Vasseur, Edward W. Llewellin, Donald B. Dingwell; Hot Sintering of Melts, Glasses and Magmas. Reviews in Mineralogy and Geochemistry 2022;; 87 (1): 801–840. doi: https://doi.org/10.2138/rmg.2022.87.17 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Magma may fragment as it ascends through the volcanic plumbing system to the Earth surface, forming volcanic ash and pyroclasts (Scheu and Dingwell 2022, this issue). If the pyroclastic products of fragmentation are deposited hot, they can stick, coalesce, and weld together, re-amalgamating into coherent magma. Subaerial deposits include welded, lava-like, and rheomorphic ignimbrites (Branney et al. 1992; Andrews and Branney 2011), and welded fall deposits proximal to an eruptive vent (Houghton and Carey 2015). Welding occurs in the sub-surface at conduit margins (Gardner et al. 2017; Wadsworth et al. 2020... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Estimating tephra fall volume from point-referenced thickness measurements

SUMMARYWe describe an algorithm for producing an approximate 95 per cent confidence interval (CI) for the bulk volume of tephra fall deposits, based on point-referenced tephra thickness measurements, and expert judgement. Applied to the Aso-4 eruption, our algorithm gives a 95 per cent CI of $[220\, \mathrm{km^3}, 370\, \mathrm{km^3}]$. One component of our algorithm is an estimated thickness surface, from which is it also possible to produce isopach maps, thickness transects and ‘Pyleograms’. Our approach uses modern methods from statistics and machine learning to model and fit the thickness surface, and to produce the 95 per cent CI, and we contrast these with established approaches.

High-temperature oxidation of proximal basaltic pyroclasts, 1886 Tarawera, New Zealand

Microlite crystallization in erupting basalt can occur in the conduit, in flight, or in situ after deposition. Distinguishing the products of primary versus secondary (post-fragmentation) crystallization can be challenging in near-vent environments, but is vital for interpreting shallow conduit conditions from pyroclast textures. Here, we examine pyroclasts of the 1886 basaltic Plinian eruption of Tarawera volcano, New Zealand, to assess the roles of primary versus secondary crystallization of microlites. Lapilli and ash were selected from (a) an ultra-proximal section (T47), < 100 m from vent, which is dominated by pyroclasts derived from the Plinian jet and column margin, and (b) a medial fall deposit section (T43), 2.5 km from the fissure, which contains products from the umbrella cloud. Strong contrasts in pyroclast groundmass crystallinities exist between sections, from near-holocrystalline (90–97% void-free corrected; VFC) in T47 pyroclasts to highly crystalline (77–83% VFC) in T43 pyroclasts. Subhedral-euhedral Fe–Ti microlites (< 3 μm) are ubiquitous and abundant in T47 pyroclasts, whereas they are virtually absent in T43 pyroclasts. Olivine is present in both T47 and T43 pyroclasts but evidence of its subsolidus transformation is only seen in T47 clasts, whereas in T43 clasts olivine is fresh. Near-complete crystallization of the groundmass and subsolidus transformation of olivine in the ultraproximal T47 clasts are evidence that post-depositional modification of primary pyroclast textures occurred in ultra-proximal environments at Tarawera as a response to high residual temperatures and oxidizing conditions, aided by short transport times of relatively coarse ejecta at high accumulation rates, likely supplemented by intense vent-derived heat. Ultra-proximal basaltic eruption products can continue to crystallize after deposition and are thus unlikely to be faithful indictors of shallow conduit magmatic processes; we recommend the use of medial and distal products instead.

Evaluating the Impact of Explosive Volcanic Eruptions on a Groundwater-Fed Water Supply System: An Exploratory Study in Ponta Delgada, São Miguel (Azores, Portugal)

Tephra fall is among the set of hazardous phenomena associated with volcanic activity that can impact water resources and services. The aim of this paper is to characterize the potential impacts of tephra fall on the groundwater-fed water supply system of Ponta Delgada (São Miguel, Azores) by comparing two scenarios of explosive eruptions. Vulnerability matrices were used to compute indexes, by multiplying the thickness of tephra fall deposits, corresponding to increasing hazard levels, by descriptors of the water supply system, representing the elements at risk. In a worst-case scenario, tephra covers a large area inland, severely constraining the abstraction of water to public supply, as 84.8% of the springs are affected. In 12 springs the expected hazard level is of perturbation (5157 m3/day; 12.1% of the daily abstraction) or damage (16,094 m3/day; 37.8%). The same trend is observed considering the storage capacity, as 75.4% of the reservoirs may be somehow affected. Moreover, 72.4% of the 68,392 inhabitants served by the water supply system will be in a zone where the damage level will be achieved. Results point out to the need of preparedness measures to mitigate consequences of a volcanic crisis over the water supply.

Analysis of the 2020 Taal Volcano tephra fall deposits from crowdsourced information and field data

After 43 years of dormancy, Taal Volcano violently erupted in January 2020 forming a towering eruption plume. The fall deposits covered an area of 8605 km2, which includes Metro Manila of the National Capital Region of the Philippines. The tephra fall caused damage to crops, traffic congestion, roof collapse, and changes in air quality in the affected areas. In a tropical region where heavy rains are frequent, immediate collection of data is crucial in order to preserve the tephra fall deposit record, which is readily washed away by surface water runoff and prevailing winds. Crowdsourcing, field surveys, and laboratory analysis of the tephra fall deposits were conducted to document and characterize the tephra fall deposits of the 2020 Taal Volcano eruption and their impacts. Results show that the tephra fall deposit thins downwind exponentially with a thickness half distance of about 1.40 km and 9.49 km for the proximal and distal exponential segments, respectively. The total calculated volume of erupted fallout deposit is 0.057 km3, 0.042 km3, or 0.090 km3 using the exponential, power-law, and Weibull models, respectively, and all translate to a VEI of 3. However, using a probabilistic approach (Weibull method) with 90% confidence interval, the volume estimate is as high as 0.097 km3. With the addition of the base surge deposits amounting to 0.019 km3, the volume translates to a VEI of 4, consistent with the classification for the observed height and umbrella radius of the 2020 main eruption plume. VEI 4 is also consistent with the calculated median eruption plume height of 17.8 km and sub-plinian classification based on combined analysis of isopleth and isopach data. Phreatomagmatic activity originated from a vent located in Taal Volcano’s Main Crater Lake (MCL), which contained 42 million m3 of water. This eruptive style is further supported by the characteristics of the ash grain components of the distal 12 January 2020 tephra fall deposits, consisting dominantly of andesitic vitric fragments (83–90%). Other components of the fall deposits are lithic (7–11%) and crystal (less than 6%) grains. Further textural and geochemical analysis of these tephra fall deposits contributes to better understand the volcanic processes that occurred at Taal Volcano, one of the 16 Decade Volcanoes identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) because of its destructive nature and proximity to densely populated areas. The crowdsourcing initiative provided a significant portion of the data used for this study while at the same time educating and empowering the community to build resilience.

Dynamics of deep-submarine volcanic eruptions

Deposits from explosive submarine eruptions have been found in the deep sea, 1–4 km below the surface, with both flow and fall deposits extending several km’s over the seafloor. A model of a turbulent fountain suggests that after rising 10–20 m above the vent, the erupting particle-laden mixture entrains and mixes with sufficient seawater that it becomes denser than seawater. The momentum of the resulting negatively buoyant fountain is only sufficient to carry the material 50–200 m above the seafloor and much of the solid material then collapses to the seafloor; this will not produce the far-reaching fall deposits observed on the seabed. However, new laboratory experiments show that particle sedimentation at the top of the fountain enables some of the hot, buoyant water in the fountain to separate from the collapsing flow and continue rising as a buoyant plume until it forms a radially spreading intrusion higher in the water column. With eruption rates of 10^6–10^7hbox {kg s}^{-1}, we estimate that this warm water may rise a few 100’s m above the fountain. Some of the finer grained pyroclasts can be carried upwards by this flow and as they spread out in the radial intrusion, they gradually sediment to form a fall deposit which may extend 1000’s m from the source. Meanwhile, material collapsing from the dense fountain forms aqueous pyroclastic flows which may also spread 1000’s m from the vent forming a flow deposit on the seabed. Quantification of the controls on the concurrent fall and flow deposits, and comparison with field observations, including from the 2012 eruption of Havre Volcano in the South Pacific, open the way to new understanding of submarine eruptions.

Re-colonization of hostile environments by benthic foraminifera: an example from Montserrat, Lesser Antilles Volcanic Arc

The Soufrière Hills Volcano, on the island of Montserrat, has an eruption history spanning over 2 million years. During this time the volcano has undergone multiple eruptions with intervening periods of low activity or dormancy. The most recent activity began in 1995 and has seen a series of major eruptive events. One of the most recent of these, and the focus of this study, occurred on the 20th May 2006. This major dome collapse produced 90 million m3 of volcanic material in only 3 hours, propelling ash clouds through the air and pyroclastic flows down the side of the volcano and into the sea. In this study of benthic foraminifera, cores from four sites off-shore Montserrat are analysed, including those collected from a location within the path of the 2006 ash cloud, around 10 km west of the Montserrat coast. In this area, one core contained 6-7 cm of ash overlying hemipelagic sediments. Volcanic ash is present in two distinct layers, one 3 cm layer produced by the 2006 eruption and the other, 3-4 cm layer, from an earlier eruption in 2003. Other cores were collected from areas unaffected by recent ash fall deposits and provide a base line for comparisons within the affected areas. To the east and south-east of Montserrat there is a different situation as this is the direction of travel of massive pyroclastic flows down the Tar Valley and the impact on the sea floor is more dramatic. There are also two extinct volcanic centres that allow the investigation of sea floor re-colonization on different time scales. The sites to the west of Montserrat record rapid colonization by benthic foraminifera of opportunistic taxa, comparable to that seen near Mt Pinatubo in the South China Sea while the sites to the east record a slower pattern of re-colonization by a wider spectrum of taxa, similar to that recorded at Deception Island in the Southern Ocean, with different benthic taxa performing the pioneering role.

The Middle to Late Pleistocene tephrostratigraphy around the Nasu Volcano Group, NE Japan: new description of the Kawatani Pyroclastic Fall Deposit and the horizon of the Yosasagawa and Higashi-iwazaki Debris Avalanche Deposits

那須火山群の東山麓に分布する後期～中期更新世の降下火砕物，岩屑なだれ堆積物の岩相と層序の再記載を行った．西郷村真船川谷では，16～12万年前に朝日岳・南月山火山から噴出した那須白河降下火砕物群の上位に未記載のデイサイト軽石火山礫が存在し，今回，これを川谷降下火砕物と新称する．その分布と火山ガラスの主成分化学組成は，この火砕物も那須火山群起源であることを示唆している．また，那須火山起源の余笹川岩屑なだれ堆積物直下の軽石を分析し，その火山ガラス主成分化学組成が32万年前の大田原火砕流堆積物と一致することを明らかにした．従って，余笹川岩屑なだれ堆積物のテフラ層序学的年代は32～23万年前である．一方，更に下位の真船降下火砕物群間から，那須火山群起源と考えられる東岩崎岩屑なだれ堆積物（新称）の存在を確認した．この堆積物はApmテフラ群に対比される根元13降下火砕物の下位にあることから，テフラ層序学的年代は35～34 万年前とみられる．

The influence of volcanic supply on the composition of modern river sands: the case study of the Ofanto River, southern Italy

Abstract The Ofanto River drains volcanic rocks from the Monte Vulture, lacustrine–fluviolacustrine deposits associated with the same volcano and sedimentary deposits of the Southern Apennines and the Bradanic foredeep sequences. Comparing the modal composition of river sands and the outcrop area of different lithologies in the different sub-basins, an over-concentration of the volcaniclastic fraction, mainly represented by loose crystals of clinopyroxene, garnet and amphibole, is shown. This has been related to the preferential erosion of pyroclastic deposits, characterized by high production of sand-sized loose minerals, together with the carbonate lability and the low sand-sized detritus production from claystones and marls. The occurrence of volcaniclastic components upstream of Monte Vulture can be explained as a contribution from the lacustrine–fluviolacustrine deposits cropping out in the upstream sector or from pyroclastic fall deposits of Monte Vulture and/or Campanian volcanoes. This research shows that the volcanic record in the fluvial sands of the Ofanto River comes from weathering and sorting processes of volcaniclastic deposits rather than of the lavas building the main edifice. Therefore, caution must be taken during palaeoenvironmental and palaeoclimatic reconstructions when relating the type and abundance of the volcanic component in sediments to the weathering stage and evolutionary history of the volcano.

Hybrid rhyolitic eruption at Big Glass Mountain, CA, USA

Eruptive dynamics of the 1060 CE rhyolitic eruption of Big Glass Mountain (BGM), USA, are investigated with field observations, hydrogen isotope and H2O content analysis of pyroclastic obsidian chips and lavas. Field relations at BGM reveal evidence for hybrid eruption, defined as synchronous explosive venting and effusive emplacement of vast obsidian lava flows. This activity is particularly well manifested by extensive breccia zones implanted within the BGM obsidian lavas, which may represent rafted tephra cones, in addition to remnants of airfall tephra on the lava. Rhyolitic obsidians collected from a 2.5-m-thick fall deposit and co-eruptive lava flow were studied by FTIR and TCEA methods to elucidate the eruption’s degassing history. The data, along with VolcDeGas program simulations, demonstrate a correlation between H2O content and H-isotopic composition (δD) that likely reflects ever-increasing amounts of volatile loss via repetitive close-system steps, best described as batched degassing.

The mafic Curacautín ignimbrite of Llaima volcano, Chile

Mafic volcanism accounts for 80% of magmas erupted on Earth. Although the majority of these eruptions are effusive to Strombolian and fountain-fed, large explosive mafic eruptions do occur. This work uses the deposits and pyroclast textures from the 12.6 ka Curacautín ignimbrite eruption of Llaima volcano to constrain the conditions that drove this mafic explosive eruption and extrapolate the findings to provide insights into the conditions that promote large-volume, mafic explosive volcanism elsewhere. The Curacautín ignimbrite (Ci) consists of four massive coarse ash to lapilli tuff flow units; Unit 1 is at least 30 m thick in proximal exposures, and Units 2–4 range from 1 to 4 m thick. New 14C dates and field observations suggest the Ci is the result of a single eruptive episode at ~12.6 ka. A lack of fall deposits and presence of abundant clast agglutination suggests the Ci eruption was a boil over event. We estimate the proximal Ci tephra volume to be between 6 and 9 km3 (equivalent to 3.5–4.5 km3 DRE), which is less than previous estimates. Even with our lower estimate, the Ci is still larger than the Masaya Triple Layer, Pucón ignimbrite, Tarawera 1886, and Etna 122 BCE mafic eruptions.Average vesicularities of pyroclasts range from 43 to 71%, and all but one exposure have vesicularities ≤56%. Average phenocryst content is ≤1–3%, but plagioclase microlite crystallinities are between 29 and 44%, with volumetric number densities between 8.21 × 106 and 1.84 × 107 mm−3. Such high microlite content suggests high disequilibrium resulting from rapid magma ascent and decompression. We interpret that the combination of rapid ascent and increased magma viscosity due to the crystallization of microlites caused gases to remain coupled with the Ci magma. This, in combination with ash textures, suggests the Ci eruption explosivity was driven by brittle fragmentation. Assuming that mass eruption rates exceeded 2.0 × 10 8 kg s − 1 to produce complete column collapse, we estimate an eruption duration of ~15–17 h. This study further supports the interpretation that extensive microlite nucleation from rapid ascent can lead to large mafic explosive eruptions.

The volcanism of the Jurassic Chon Aike Silicic LIP influenced by Paleozoic inherited crustal structures in the northeastern Deseado Massif, Patagonia

An area of ∼40 km2 located in the northeastern sector of the Deseado Massif is detailed studied, in which Paleozoic metamorphic and Jurassic volcanic rocks crop out. Two genetically linked and compositionally different Jurassic volcanic events of the Chon Aike Silicic LIP are documented by geological mapping and lithofacies and petrographic analyses. The first event was only represented by andesitic lava flows assignable to the intermediate portion of the Bahía Laura Volcanic Complex (Bajo Pobre Formation). The second was represented by the acidic rocks of the Bahía Laura Volcanic Complex (Chon Aike and La Matilde formations) and it was constituted by low-grade and moderate grade ignimbrites, pyroclastic fall deposits (ash tuffs) and several lava bodies (rhyolitic lava domes and lava flows). Firstly, the effusive eruptions dominated during the intermediate magmatic eruptions, whereas the explosives mechanisms dominated during the first stage of the acidic magma extrusion, to end with effusive eruptions of previously degassed magma. According to the morphology of the NNW-elongated lava domes (feeder conduits plugs) and internal measurements carried out on these rhyolitic bodies, fissure conduits are interpreted, which were directly controlled by the Paleozoic inherited structures during the Jurassic extensional tectonic reactivation ascribed to the initial stages of the Gondwana breakup.

Investigation of Rawa Dano Volcanic Deposits and Its Paleotopography Using Ground-Penetrating Radar

The detailed mechanisms of volcanic eruptions happened around Rawa Dano, Banten, Indonesia, remain undiscovered. One of the key features to this geological event is the presence of a 13.7 km × 6.5 km caldera-like morphology in the middle of Banten tuff deposits. Surface geological investigation in the area indicates that the eruptions are massive and occurred in several periods. Low-frequency ground-penetrating radar (GPR) signals are used as an aid to identify the unexposed part of the deposits in this volcanological study. Common-offset GPR surveys were carried out along three measurement lines traversing over the deposit outcrops. An outcrop which is exposed after sand mining activities at one of the survey locations shows dipping interfaces between the upper pyroclastic flow deposits, pumice-rich deposits, paleosol, and the lower pyroclastic fall deposits. These stratigraphic contacts are detected as well under the surface which are clearly recognizable in radar images. The GPR cross-section also shows some other reflections due to different deposit types. The overall results of the GPR profiles give the idea about the thickness of each type of volcanic deposits and the paleotopography in the surrounding area.

Insights into the mobility characteristics of seismic earthflows related to the Palu and Eastern Iburi earthquakes

As an important indicator for landslide hazard and risk assessments, landslide mobility has attracted considerable attention in the literature, yet few studies have investigated seismic earthflows. Therefore, the earthflows induced by the 2018 Mw 7.5 Palu earthquake (Indonesia) and the 2018 Mw 6.5 Eastern Iburi earthquake (Japan) and their mobility characteristics were analyzed. Long-term irrigation for cultivation of wet rice and heavy rainfall produced by Typhoon Jebi introduced substantial moisture into alluvial fans (Palu earthquake) and pyroclastic fall deposits (Eastern Iburi earthquake), respectively, and seismic shaking liquefied these moist deposits, which consequently traveled long distances. Four earthflows that were triggered on very gentle slopes of 1–3° by the Palu event exhibited very high mobility levels (H/L) of 0.0129, 0.01125, 0.0209 and 0.0271 (average H/L = 0.018), whereas 7058 earthflows triggered by the Eastern Iburi event exhibited an average mobility of 0.33. Different liquefied layers were located below or above the low permeability layer, and movement restrictions and complex final flow disturbances could be responsible for obvious mobility differences triggered by the two seismic events. However, the earthflow mobility induced by the Eastern Iburi earthquake was obviously higher than that of coseismic rock landslides worldwide and is described by the following relationship: H/L = 1.46–0.24 × log10 (earthflow_area). Furthermore, the earthflow area, peak ground acceleration (PGA) and slope angle all have increase influences on mobility.

Apparently ‘dry’ littoral rootless cones in Hawaiʻi formed by sustained, “confined” mixing of lava and sea water

Hydrovolcanic rootless cones form through the explosive interaction between lava and external water at locations remote from primary vents. Littoral rootless cones such as those on the coast of Hawaiʻi form at the ocean entries of basaltic lava flows and most consist dominantly of thinly bedded ash and lapilli. In contrast, the littoral rootless cones at ʻAuʻau Point and South Puʻu Kī Cone on the southwestern flank of Mauna Loa, Hawaiʻi consist of alternating layers of clastogenic lavas and thermally oxidised, tack- to moderately welded lapilli and coarse bombs with little to no ash. The pyroclastic layers have characteristics traditionally thought to be indicative of ‘dry’ explosive magmatic activity at primary vents, including the high degree of sorting in fall deposits, the paucity of ash and quench textures, extensive thermal oxidation, a high degree of welding, dominantly fluidal pyroclastic morphologies and association with clastogenic lavas. We propose that littoral rootless activity at both cones was driven by the “confined mixing” of tube-fed lava and sea water. In this configuration, the mingling of water and lava is complicated, resulting in relatively inefficient Molten Fuel Coolant Interactions (MFCIs). Cycles of more vigorous (littoral lava fountains) and less vigorous (lava bubble bursts) activity produced the alternating layers of clastogenic lava and welded bombs and lapilli. These cycles probably reflect the local depletion and replenishment of sea water injected into the lava tube. Similar apparently ‘dry’ facies found in the upper sequences of Icelandic rootless cones formed when the water content of the underlying wet sediment was depleted and MFCIs were relatively inefficient. This study suggests that both “confined mixing” of tube-fed lava and external water, and lava–water interactions with unfavourable lava–water mass ratios, such as during the late stages of formation of Icelandic rootless cones, can produce seemingly ‘dry’ deposits that nevertheless are fundamentally hydrovolcanic.

Quantitative chemical mapping of plagioclase as a tool for the interpretation of volcanic stratigraphy: an example from Saint Kitts, Lesser Antilles

Establishing a quantitative link between magmatic processes occurring at depth and volcanic eruption dynamics is essential to forecast the future behaviour of volcanoes, and to correctly interpret monitoring signals at active centres. Chemical zoning in minerals, which captures successive events or states within a magmatic system, can be exploited for such a purpose. However, to develop a quantitative understanding of magmatic systems requires an unbiased, reproducible method for characterising zoned crystals. We use image segmentation on thin section scale chemical maps to segment textural zones in plagioclase phenocrysts. These zones are then correlated throughout a stratigraphic sequence from Saint Kitts (Lesser Antilles), composed of a basal pyroclastic flow deposit and a series of fall deposits. Both segmented phenocrysts and unsegmented matrix plagioclase are chemically decoupled from whole rock geochemical trends, with the latter showing a systematic temporal progression towards less chemically evolved magma (more anorthitic plagioclase). By working on a stratigraphic sequence, it is possible to track the chemical and textural complexity of segmented plagioclase in time, in this case on the order of millennia. In doing so, we find a relationship between the number of crystal populations, deposit thickness and time. Thicker deposits contain a larger number of crystal populations, alongside an overall reduction in this number towards the top of the deposit. Our approach provides quantitative textural parameters for volcanic and plutonic rocks, including the ability to measure the amount of crystal fracturing. In combination with mineral chemistry, these parameters can strengthen the link between petrology and volcanology, paving the way towards a deeper understanding of the magmatic processes controlling eruptive dynamics.

Comparison of Bubble Shape Model Results With Textural Analysis: Implications for the Velocity Profile Across a Volcanic Conduit

AbstractTube pumice, which is characterized by aligned elongated bubbles, is valuable for our understanding of the shear field in a volcanic conduit. Most previous studies interpreted that tube pumice forms through simple shear, assuming a parabolic velocity profile across a volcanic conduit. However, simple shear cannot account for the observation that tube pumice is rare in plinian fall deposits but frequent in ignimbrites at the Taupō volcano, if the pyroclastic flows that formed them erupted through a wider vent and had a lower average shear rate. Here, we analyze the dynamics of bubble deformation that occurred during the 232 CE Taupō eruption by combining a bubble deformation model with a conduit flow model. The results indicate that bubble deformation is largely controlled by the velocity profile across a conduit. If the conduit flow has a parabolic velocity profile, the amount of bubble elongation is unrealistic compared to the Taupō fall deposits. It is more likely that the conduit flow has a plug‐like velocity profile due to viscous heating. We numerically confirmed that bubbles in the plug‐like flow are subject mainly to pure shear and accumulate less strain for elongation. We also calculated a viscous heating flow of the ignimbrite phase. We modeled a wider conduit, which suppresses viscous heating around the conduit walls and causes a higher proportion of the conduit to experience parabolic flow, compared to the plinian phase. The increased proportion of a parabolic profile may explain the high frequency of elongated bubbles in the ignimbrite phase pumice.

How rainfall influences tephra fall loading \u2014 an experimental approach

The load a tephra fall deposit applies to an underlying surface is a key factor controlling its potential to damage a wide range of assets including buildings, trees, crops and powerlines. Though it has long been recognised that loading can increase when deposits absorb rainfall, few efforts have been made to quantify likely load increases. This study builds on previous theoretical work, using an experimental approach to quantify change in load as a function of grainsize distribution, rainfall intensity and duration. A total of 20 laboratory experiments were carried out for ~ 10-cm thick, dry tephra deposits of varying grainsize and grading, taken to represent different eruptive scenarios (e.g. stable, waxing or waning plume). Tephra was deposited onto a 15° impermeable slope (representing a low pitch roof) and exposed to simulated heavy rainfalls of 35 and 70 mm h−1 for durations of up to 2 h. Across all experiments, the maximum load increases ranged from 18 to 30%. Larger increases occurred in fine-grained to medium-grained deposits or in inversely graded deposits, as these retained water more efficiently. The lowest increases occurred in normally graded deposits as rain was unable to infiltrate to the deposit’s base. In deposits composed entirely of coarse tephra, high drainage rates meant the amount of water absorbed was controlled by the deposit’s capillary porosity, rather than its total porosity, resulting in load increases that were smaller than expected. These results suggest that, for low pitch roofs, the maximum deposit load increase due to rainfall is around 30%, significantly lower than the oft-referenced 100%. To complement our experimental results, field measurements of tephra thickness should be supplemented with tephra loading measurements, wherever possible, especially when measurements are made at or near the site of observed damage.

The Independent Volcanic Eruption Source Parameter Archive (IVESPA, version 1.0): A new observational database to support explosive eruptive column model validation and development

Eruptive column models are powerful tools for investigating the transport of volcanic gas and ash, reconstructing past explosive eruptions, and simulating future hazards. However, the evaluation of these models is challenging as it requires independent estimates of the main model inputs (e.g. mass eruption rate) and outputs (e.g. column height). There exists no database of independently estimated eruption source parameters (ESPs) that is extensive, standardized, maintained, and consensus-based. This paper introduces the Independent Volcanic Eruption Source Parameter Archive (IVESPA, ivespa.co.uk), a community effort endorsed by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Commission on Tephra Hazard Modelling. We compiled data for 134 explosive eruptive events, spanning the 1902-2016 period, with independent estimates of: i) total erupted mass of fall deposits; ii) duration; iii) eruption column height; and iv) atmospheric conditions. Crucially, we distinguish plume top versus umbrella spreading height, and the height of ash versus sulphur dioxide injection. All parameter values provided have been vetted independently by at least two experts. Uncertainties are quantified systematically, including flags to describe the degree of interpretation of the literature required for each estimate. IVESPA also includes a range of additional parameters such as total grain size distribution, eruption style, morphology of the plume (weak versus strong), and mass contribution from pyroclastic density currents, where available. We discuss the future developments and potential applications of IVESPA and make recommendations for reporting ESPs to maximize their usability across different applications. IVESPA covers an unprecedented range of ESPs and can therefore be used to evaluate and develop eruptive column models across a wide range of conditions using a standardized dataset.