Dense Clasts Research Articles

Facies architecture of a continental, below-wave-base volcaniclastic basin: The Ohanapecosh Formation, Ancestral Cascades arc (Washington, USA)

The >800-m-thick, Oligocene Ohanapecosh Formation records voluminous sedimentation of volcanic clasts in the Ancestral Cascades arc (Washington State, USA). Most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition, now entirely devitrified and altered. All beds are laterally continuous and have uniform thickness; fine sandstone and mudstone beds have features typical of deposits from low-density turbidity currents and suspension settling. Erosion surfaces, cross-beds, and evidence of bi-directional oscillatory currents (i.e., wave ripples and swaley and hummocky cross-stratification) are almost entirely absent. We infer that the setting was subaqueous and below wave base. The abundance of angular pumice clasts, crystals and dense volcanic clasts, and the extreme thickness of several facies, suggest they were derived from magmatic volatile-driven explosive eruptions. The extremely thick beds are ungraded or weakly graded, and lack evidence of hot emplacement, suggesting deposition from subaqueous, water-supported, high-concentration volcaniclastic density currents. Some of the thickest beds contain coarse, rounded, dense clasts at their base and are interbedded with accretionary lapilli–bearing mudstone; these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline. Shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra-basinal scoria cones that may have been partly subaerial. The range in facies in the Ohanapecosh Formation is typical of below-wave-base, continental (lacustrine) basins that form in proximity to active volcanic arcs, and includes eruption-fed and resedimented facies. Extreme instantaneous aggradation rates are related directly to explosive eruptions, and sediment pathways reflect the locations of active volcanoes, in contrast to conventional sedimentation processes acting in non-volcanic environments.

Biogeomorphology of a Mojave Desert landscape — Configurations and feedbacks of abiotic and biotic land surfaces during landform evolution

Terrestrial ecosystems can be more holistically understood by investigating the morphology of landscape mosaics, the assemblage of their ecological communities, and the linkages and feedbacks between the mosaics and communities. The overarching objectives of this study were to: (1) study the abiotic and biotic configurations of landform units as mosaics within a Mojave Desert chronosequence; and (2) elucidate their potential feedbacks, interactions, and dynamics during landform evolution. Seven landform units distributed over three geomorphic ages were identified, including: young bars and swales; intermediate-aged flattened bars, flattened swales, and bioturbation units; and old desert pavements and shrub zones. These landform units were characterized according to abiotic and biotic land surface properties. Landform units were statistically distinct and predictable based on a specific suite of abiotic and biotic properties. Vascular plant functional group and biological soil crust community diversity varied with geomorphology, with greatest diversity associated with bars and shrub zones and lowest diversity associated with desert pavements. Biological soil crust communities were controlled by geomorphic age, surface rock size, and protruding rocks with young bar units having the highest abundance and diversity. Perennial forbs were observed in old shrub zones with small rocks and few protruding rocks. A high clast density and a finer-sized clast distribution were found particularly in desert pavements and flattened swales, and generally inhibited biological soil crust and plant cover. Evolutionary trajectories for landforms of a lower piedmont landscape can be dominated by either abiotic and biotic landform processes. These two trajectories are distinctly different and are associated with their own unique linkages, feedbacks, and dynamics of abiotic and biotic land surface properties, producing a highly diverse desert landscape.

Facies architecture, emplacement mechanisms and eruption style of the submarine andesite El Barronal complex, Cabo de Gata, SE Spain

El Barronal complex consists of a succession of andesite lavas and andesite volcaniclastic facies interbedded with carbonate and siliciclastic sedimentary rocks. Carbonate and siliciclastic rocks were deposited in a shallow-marine environment during periods of volcanic quiescence. Lavas consist of an inner coherent core grading outward into hyaloclastite breccia made of dense clasts that in turn grade into hyaloclastite breccia made of vesicular clasts, in massive to layered zones. Volcaniclastic facies contain clasts produced during explosive eruptions and reworked clasts from sources above wave base. Volcaniclastic facies were deposited from cold granular flows with different grain size populations. Stratigraphy and facies architecture at El Barronal suggest that a succession of several discrete eruptive events occurred with a similar cyclic pattern made of an initial explosive phase followed by effusive emplacement of lavas, in turn followed by a period of quiescence of volcanic activity. Hyaloclastic fragmentation of magma took place in the final stages of lava emplacement, allowing only for local disorganization of the jigsaw-fit texture.

Statistical means for identifying hunter–gatherer residential features in a lithic landscape

Techniques are described for extracting circular rock features from landscapes dominated by clasts of the same type from which cultural features are composed, using as a test case a large stone circle residential site in western Wyoming, USA. Methods consist of point plotting all relevantly-sized culturally and naturally-deposited clasts in the field and identifying potential cultural features using point density analyses tools in ArcGIS. Potential rings are either accepted or rejected as cultural features by comparing clast frequency, density and distribution in internal, feature-ring, and external spatial buffers to ethnoarchaeological data recording stone circle size and morphology and to similar data generated from a control sample of off-site, naturally-occurring clasts. The results of the analysis are used to discuss group size, mobility type, and duration of site occupation and to explore problems of assessing such at surface archaeological sites resulting from palimpsest-type site formation processes.

Local seismic and infrasound observations of the 2009 explosive eruptions of Redoubt Volcano, Alaska

Local seismic and infrasound data are used to characterize the March–April 2009 eruptions of Redoubt volcano, Alaska. Data are from a network of ten seismic stations ranging from 2.6 to 21km from the vent and a single infrasound sensor 12.2km from the vent. Measurements were made for more than 30 explosive eruptions including the 19 numbered events that were identified during the eruption response. More than 30 smaller explosions were also identified. Measured parameters include onset time, duration, reduced displacement (DR), pressure, seismic energy, and acoustic energy. Results were compared with complementary gas and lightning data. Based on ratios of pressure to DR and acoustic to seismic energy we find that the initial group of explosive eruptions on 23–24 March was relatively stronger seismically and included the four highest DR values. We infer that these represent vent enlargement or stronger coupling between the magma and wall rocks. One event, at 12:31 UT on 23 March had an extremely high pressure of 250Pa for the initial pulse and was likely weakly seismically coupled. A second group of explosive eruptions on 27–29 March was relatively stronger acoustically. Deposits were finer grained and of different lithology, with less scoria and more dense clasts. These two groups may represent different modes of gas release. Cumulative acoustic energy as a function of time correlates well with comparable lightning and SO2 data, suggesting that all three provide insight regarding different factors of gas release for the eruption as a whole. The number of events identified depends on criteria measured, and depends on the goals of the investigations. Overall the eruptions were substantially stronger than recent eruptions in Alaska, with some durations exceeding 1h and pressures an order of magnitude higher.

Transitions between fall phases and pyroclastic density currents during the AD 79 eruption at Vesuvius: building a transient conduit model from the textural and volatile record

The magmatic phase of the AD 79 eruption of Vesuvius produced alternations of fall and pyroclastic density current (PDC) deposits. A previous investigation demonstrated that the formation of several PDCs was linked with abrupt increases in the proportion of denser juvenile clasts within the eruptive column. Under the premise that juvenile clast density is controlled by vesiculation processes within the conduit, we investigate the processes responsible for these variations at or close to fragmentation levels. Pumice textures (vesicle sizes, numbers, and connectivity combined with crystal textures) from the AD 79 PDC deposits are compared to those from interbedded fall samples. Both PDC and fall deposits preserve textures that represent a full spectrum of degassing and outgassing processes, from bubble nucleation to collapse. Combining the textural and volatile (groundmass H2O) data, we derive a conduit model that satisfies all the textural and physical observations made for this phase of the eruption: lateral vesicularity/density stratifications are produced by maturing of bubble textures with superimposed localized shearing of bubble-rich magmas, which enhance outgassing of H2O. The incorporation of denser slower-moving magma from the conduit margins (“lateral magma density gradient”) is likely to be responsible for the higher abundances of dense juvenile pumice that triggered partial column collapses. We also illustrate how variations in the fragmentation depth (tapping a “vertical magma density gradient”) can be responsible for variations in erupted clast density distributions, and potentially in the extent of degassing/outgassing.

Column collapse and generation of pyroclastic density currents during the A.D. 79 eruption of Vesuvius: The role of pyroclast density

AbstractThe Plinian columns formed during the magmatic phase of the A.D. 79 eruption of Vesuvius alternated several times between fully stable, buoyantly rising regimes and unstable regimes of partial or total collapse. Six pyroclastic density currents (PDCs) were produced during unstable regimes, and ultimately caused the destruction of Roman towns around the volcano. Through new measurements of juvenile clast density and estimations of ascent parameters, we show that four partial collapses were likely triggered by increases in the abundance of dense juvenile clasts within the eruptive column. In contrast, the total collapse probably occurred in response to an increase in the wall-rock content injected into the plume during a progressive widening of the conduit. A sixth low-energy, small collapse resulted from high abundances in both dense juvenile clasts and wall-rock material. Simulations of eruption column behavior already account for the effects of variations in conduit radius, mass discharge rate, and particle size, but have yet to include variable clast density and wall-rock abundance that cause temporal variations in plume density. Our results suggest that both parameters can exert a significant control on the potential for generation of PDCs.

Influence of pre-eruptive degassing and crystallization on the juvenile products of laterally directed volcanic explosions

Strikingly similar examples of edifice collapse and directed blast are the 18 May 1980 eruption of Mount St. Helens (MSH), Washington, USA, and the 30 March 1956 eruption of Bezymianny Volcano (BZ), Kamchatka, Russia. In these cases, flank failures led to near-instantaneous decompression and fragmentation of intra-edifice cryptodome magma, which produced catastrophic, laterally directed blasts. In both instances, the blast products consisted of juvenile material with bimodal density/vesicularity distributions: low- and high-density modes at 1900 and 2400 kg m − 3 for BZ, 1600 and 2300 kg m − 3 for MSH, although the proportion of high-density material is greater at BZ. Blast materials also exhibit striking variety in groundmass crystallinity (< 40 to > 90 vol.%) despite having fairly uniform pheno-crystallinities, suggesting that degassing-driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes. New bulk-rock H 2O and ∂D measurements confirm that progressive open-system outgassing occurred prior to both blasts. The correlations between crystallinity, clast density, and bulk H 2O contents suggest that syn-blast magma expansion was modulated both by non-uniform volatile distribution within the cryptodome and rheological controls associated with non-uniform crystal content. Spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin, which probably correlates with timing of magma injection. The greater proportion of high-density material at BZ is speculatively related to lower blast energy compared with MSH.

Constraints on eruption dynamics of basaltic explosive activity derived from chemical and microtextural study: The example of the Fontana Lapilli Plinian eruption, Nicaragua

The Fontana Lapilli deposit is one of very few examples of basaltic Plinian eruptions discovered so far. Juvenile clasts have uniform chemical composition and moderate ranges of density and bulk vesicularity. However, clast populations include two textural varieties which are microlite-poor and microlite-rich respectively. These two clast types have the same clast density range, making a distinction impossible on that base alone. The high bubble number density (∼ 10 7 cm − 3 ) and small bubble population of the Fontana clasts suggest that the magma underwent coupled degassing following rapid decompression and fast ascent rate, leading to non-equilibrium degassing with continuous nucleation as it is common for silicic analogues. The Fontana products have lower microlite contents (10–60 vol.%) with respect to the other documented basaltic Plinian eruptions suggesting that the brittle fragmentation, implied for the other basaltic Plinian deposits, does not apply to the Fontana products and another fragmentation mechanism led the basaltic magma to erupt in a Plinian fashion.

Wind-driven reorganization of coarse clasts on the surface of Mars

Coarse (pebble to cobble sized) clasts on the intercrater plains of the Mars Exploration Rover Spirit landing site exhibit a nonrandom (i.e., uniformly spaced) distribution. This pattern has been attributed to the entrainment and redistribution of coarse clasts during extreme wind events. Here we propose an alternative mechanism readily observable in wind tunnels and numerical models at modest wind speeds. In this process, coarse clasts modify the air fl ow around them, causing erosion of the underlying substrate on the windward side and deposition on the leeward side until a threshold bed-slope condition is reached, after which the clast rolls into the windward trough. Clasts can migrate across an erodible substrate in repeated cycles of trough formation and clast rolling, “attracting” or “repelling” one another through feedbacks between the local clast density, substrate erosion and/or deposition rate, and substrate elevation. The substrate beneath areas of locally high clast densities aggrades, building up a topographic high that can cause clasts to repel one another to form a more uniform distribution of clasts through time. This self-organized process likely plays a signifi cant role in the evolution of mixed grain size eolian surfaces on Earth and Mars.

Incursion of a large-volume, spatter-bearing pyroclastic density current into a caldera lake: Pavey Ark ignimbrite, Scafell caldera, England

The Scafell caldera-lake volcaniclastic succession is exceptionally well exposed. At the eastern margin of the caldera, a large andesitic explosive eruption (>5 km3) generated a high-mass-flux pyroclastic density current that flowed into the caldera lake for several hours and deposited the extensive Pavey Ark ignimbrite. The ignimbrite comprises a thick (≤125 m), proximal, spatter- and scoria-rich breccia that grades laterally and upwards into massive lapilli-tuff, which, in turn, is gradationally overlain by massive and normal-graded tuff showing evidence of soft-state disruption. The subaqueous pyroclastic current carried juvenile clasts ranging from fine ash to metre-scale blocks and from dense andesite through variably vesicular scoria to pumice (<103 kg m−3). Extreme ignimbrite lithofacies diversity resulted via particle segregation and selective deposition from the current. The lacustrine proximal ignimbrite breccia mainly comprises clast- to matrix-supported blocks and lapilli of vesicular andesite, but includes several layers rich in spatter (≤1.7 m diameter) that was emplaced in a ductile, hot state. In proximal locations, rapid deposition of the large and dense clasts caused displacement of interstitial fluid with elutriation of low-density lapilli and ash upwards, so that these particles were retained in the current and thus overpassed to medial and distal reaches. Medially, the lithofacies architecture records partial blocking, channelling and reflection of the depletive current by substantial basin-floor topography that included a lava dome and developing fault scarps. Diffuse layers reflect surging of the sustained current, and the overall normal grading reflects gradually waning flow with, finally, a transition to suspension sedimentation from an ash-choked water column. Fine to extremely fine tuff overlying the ignimbrite forms ∼25% of the whole and is the water-settled equivalent of co-ignimbrite ash; its great thickness (≤55 m) formed because the suspended ash was trapped within an enclosed basin and could not drift away. The ignimbrite architecture records widespread caldera subsidence during the eruption, involving volcanotectonic faulting of the lake floor. The eruption was partly driven by explosive disruption of a groundwater-hydrothermal system adjacent to the magma reservoir.

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufrière Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition. The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5, 2.5, and 0.05 km3. The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase, and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern 3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC. This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly viscous, phenocryst-rich, variable microlite content) determined the bimodal distributions of juvenile clast density and vesicularity. Thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater, and laterally from the axial region. The proximal stratigraphy of the blast deposits comprises four layers named A, B, C, D from bottom to top. Layer A is represented by very poorly sorted debris with admixtures of vegetation and soil, with a strongly erosive ground contact; its appearance varies at different sites due to different ground conditions at the time of the blasts. The layer reflects intense turbulent boundary shear between the basal part of the energetic head of the PDC and the substrate. Layer B exhibits relatively well-sorted fines-depleted debris with some charred plant fragments; its deposition occurred by rapid suspension sedimentation in rapidly waning, high-concentration conditions. Layer C is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated, created by rapid sedimentation under moderate-concentration, weakly tractive conditions, with the uppermost laminated part reflecting a dilute depositional regime with grain-by-grain traction deposition. By analogy to laboratory experiments, mixing at the flow head of the PDC created a turbulent dilute wake above the body of a gravity current, with layer B deposited by the flow body and layer C by the wake. The uppermost layer D of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high-rising buoyant thermal plume derived from the sediment-depleted pyroclastic density current. The strong similarity among these eruptions and their deposits suggests that these cases represent similar source, transport and depositional phenomena.

Distribution patterns of siltstone clasts in turbidites and their relation to grain fabric of host turbidites

Siltstone clasts are commonly known from turbidites in deep-water successions and have been interpreted to document information regarding depositional processes of host sandstones. Here we studied formative processes of siltstone clasts on the basis of size and fabric of siltstone clasts and of host sandstones.Although siltstone clasts generally exhibit imbrication dipping to upslope directions regardless of depositional features of host sandstones, their size, density, and position in a turbidite bed are variable in response to vertical variations in grain size and grain fabric of host sediments. Therefore, the variation in size, density, and position of siltstone clasts in a single turbidite bed is interpreted to be controlled by temporal variations in the relative rates of suspended-load fallout during a single depositional process of sandstone beds from turbidity currents.

Vertical segregation in granular mass flows: A shear cell study

Non‐fluidised, dry granular mass flows are obtained with rock fragments located on a rough rotating disk. In these flows that develop a quasi‐rigid upper layer and a basal layer of colliding particles, dense clasts sink whereas light ones rise when surrounded by particles with intermediate density. Our experiments demonstrate that the presence of a quasi‐rigid upper layer in granular mass flows does not prevent vertical segregation and that the formation of coarse‐tail grading in pyroclastic flows does not require fluidising gases. High‐speed videos reveal that vertical segregation in granular mass flow of rock fragments is generated by inertia differences between segregating clasts and matrix when they are both pushed upward by collisions with the basal layer. Coarse‐tail grading occurs because the average segregation velocity of smaller clasts is smaller than that of larger clasts.

Field-based density measurements as tool to identify preeruption dome structure: set-up and first results from Unzen volcano, Japan

For an improvement in the quality of conduit flow and dome-related explosive eruption models, knowledge of the preeruption or precollapse density of the rocks involved is necessary. As close investigation is impossible during eruption, the best substitute comes from quantitative investigation of the eruption deposits. The porosity of volcanic rocks is of primary importance for the eruptive behaviour and, accordingly, a key-parameter for realistic models of dome stability and conduit flow. Fortunately, this physical property may be accurately determined via density measurements. We developed a robust, battery-powered device for rapid and reliable density measurements of dry rock samples in the field. The density of the samples (sealed in plastic bags at 250 mbar) is determined using the Archimedean principle. We have tested the device on the deposits of the 1990–1995 eruption of Unzen volcano, Japan. Short setup and operation times allow up to 60 measurements per day under fieldwork conditions. The rapid accumulation of correspondingly large data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block-and-ash flow deposits. More than 1100 samples with a total weight of 2.2 tons were measured. The data set demonstrates that the deposits of the last eruptive episode at Unzen display a bimodal density distribution, with peaks at 2.0±0.1 and 2.3±0.1 g/cm 3, corresponding to open porosity values of 20 and 8 vol.%, respectively. We use this data set to link the results of laboratory-based fragmentation experiments to field studies at recently active lava domes.

Eruptive dynamics and caldera collapse during the Onano eruption, Vulsini, Italy

The Onano explosive eruption of the Latera Volcanic Complex (Vulsini Volcanoes, Quaternary potassic Roman Comagmatic Region, Italy) provides an interesting example of multiple changes of eruptive style that were concomitant with a late phase of collapse of the polygenetic Latera Caldera. This paper reports a reconstruction of the event based on field analysis, laboratory studies of grain size and density of juvenile clasts, and re-interpretation of available subsurface geology data. The Onano eruption took place in a structurally weak area, corresponding to a carbonate substrate high bordered by the pre-existing Latera caldera and Bolsena volcano-tectonic depression, which controlled the ascent and eruption of a shoshonitic-phonotephritic magma through intersecting rim fault systems. Temporal changes of magma vesiculation, fragmentation and discharge rate, and consequent eruptive dynamics, were strongly controlled by pressure evolution in the magma chamber and changing vent geometry. Initially, pumice-rich pyroclastic flows were emplaced, followed by spatter- and lithic-rich flows and fallout from energetic fire-fountaining. The decline of magma pressure due to the partial evacuation of the magma chamber induced trapdoor collapse of the chamber roof, which involved part of the pre-existing caldera and external volcano slopes and eventually led to the present-day caldera. The widening of the vent system and the emplacement of the main pyroclastic flow and associated co-ignimbrite lag breccia marked the eruption climax. A sudden drop of the confining pressure, which is attributed to a pseudo-rigid behaviour of the magma chamber wall rocks during a phase of rapid magma drainage, led to extensive magma vesiculation and fragmentation. The disruption of the magma chamber roof and waning magma pressure in the late eruption stage favoured the explosive interaction of residual magma with groundwater from the confined carbonate aquifer. Pulsating hydrostatic and magma pressures produced alternating hydromagmatic pyroclastic surges, strombolian fallout and spatter flows.

Landslide Preconditions and Collapse of the San Francisco Mountain Composite Volcano, Arizona, into Cold Debris Avalanches in Late Pleistocene

Abstract Large composite volcanoes can collapse into debris avalanches even if alteration is absent and an eruption is not in progress or imminent. At San Francisco Mountain, Arizona, a large valley that penetrates the core of the volcano is at the apex of a large fan of bouldery volcaniclastic deposits. A 115‐m‐thick layer sampled in a borehole that penetrates the fan is composed of dense and unaltered lithic clasts in proportions similar to the bulk composition of the andesitic volcano; pumiceous clasts are absent. Within this layer, a 20‐m‐thick deposit consists mostly of clasts from two dacite extrusions that predate the valley: a flank flow and an inferred summit dome. This dacite‐rich deposit may be from a debris avalanche that originated by failure of the summit and northeast flank of the volcano in a repose interval during the last eruptive stage, which was about 0.43 to 0.22 Ma. Preconditions for slope failure include top loaded by massive lava flows and summit dome and cone inflated by intrusion...

Transport and Emplacement of the 15.4 Ma Rhyolitic Ignimbrites from Gutai Mts., Eastern Carpathians

The 15.4 Ma rhyolitic ignimbrites are the first volcanic products from Gutâi Mts. The study of their sedimentary structures reflects the mechanisms involved in the transport and emplacement of the parental flows, essential to inffering the style of eruption and the source evolution and location. Primary sedimentary structures show massive deposits emplaced from mass flows, sequence of units showing the normal coarse-tail grading of the dense clasts and the reverse coarse-tail grading of the pumice clasts. They reflect concentrated laminar flows or dilute, subcritical flows emplaced by progressive aggradation. Secondary sedimentary structures are represented by the eutaxitic texture or welding, cooling textures (columnar jointings, spherulitic textures) and gas-escape pipes. They assess the hot state deposition with low degree of fluidization, low cooling rates and gas-retention regime. Secondary sedimentary structures are represented by the eutaxitic texture or welding, cooling textures (columnar jointings, spherulitic textures) and gas-escape pipes. They assess the hot state deposition with low degree of fluidization, low cooling rates and gas-retention regime.

Textural heterogeneities in pumices from the climactic eruption of Mount Pinatubo, 15 June 1991, and implications for magma ascent dynamics

The climactic event of Mount Pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism. We have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits, and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions. White pumice has higher vesicularity, deformed and highly coalesced vesicles with thin walls, euhedral phenocrysts and microlite-free groundmass. Gray pumice shows lower vesicularity, wider ranges in vesicle number density, limited coalescence, vesicles with thick walls that are less deformed, phenocrysts and microphenocrysts with abundant solution pitting, and groundmass containing ubiquitous microlites and crystal fragments. The presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors. In particular, the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls, a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety.

Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece

The Kos Plateau Tuff (KPT) is dominated by a sequence of nonwelded rhyolitic ignimbrites, some of which were deposited more than 60 km from source on islands in the eastern Aegean sea, Greece. Palaeo-shoreline reconstruction at the time of the KPT eruption (161 ka) suggests that northward-directed pyroclastic flows travelled over an extensive landmass, although the proximal zone may have been sea. In comparison, flows directed to the southeast and east most likely crossed wide expanses of sea, depositing ignimbrite on islands and peninsulas. The ignimbrites include basal coarse lithic-rich breccias overlain by thick, massive pumiceous zones and generally have low abundances of fine ash. They are interpreted to have been deposited from pyroclastic flows that were relatively dilute and stratified, with lower, denser, lithic-rich zones and upper, expanded, pumice-rich zones. High-velocity inherited from the intensity of the eruption, combined with turbulence and low bulk density, favoured transport across water. The pyroclastic flows remained hot even after crossing 35 km of the sea. Heat capacity equations indicate that a depth of ~1 cm of water ingested over the outflow path would have cooled the flows by only a few degrees. The ignimbrites deposited prior to the eruption climax have distributions restricted by 300-m-high topographic barriers and they thicken in topographic depressions. These ignimbrites show significant decreases in grain size, proportion of lithic clasts and thickness with distance from source. In comparison, during the eruption climax, the coarsest, most voluminous and widespread KPT ignimbrite was deposited. This ignimbrite has relatively even thickness and uniform componentry and texture regardless of topographic relief or distance from source. Textural analysis indicates that crossing 35 km water to the southeast had little effect on the pyroclastic flow generated during the eruption climax, except for the loss of the coarsest (>20 cm) vent- and conduit-derived lithic clasts. Preferential loss of lithic clasts is related to the mechanism by which they were transported. On land, large, dense clasts could be saltated over the ground surface at the base of the pyroclastic flow. In the case of transport over water, these large, dense clasts sank and were permanently removed from the transport system. Outflow across water had a greater effect on the pre-climax pyroclastic flows. The ignimbrites formed at this stage are significantly thinner and slightly finer grained, and lack basal lithic breccias and lithic clasts >5 cm where the pyroclastic flows crossed the sea, reflecting the lower velocities of the flows. All KPT ignimbrites to the east of the source were deposited from pyroclastic flows that crossed >25 km of water. Ignimbrites in this sector are markedly finer grained, possibly as a result of blocking of coarser pyroclasts by a topographic barrier close to source.