Showing papers on "Pyroclastic rock published in 2012"

Volcanic ash impacts on critical infrastructure

Abstract: Volcanic eruptions can produce a wide range of hazards. Although phenomena such as pyroclastic flows and surges, sector collapses, lahars and ballistic blocks are the most destructive and dangerous, volcanic ash is by far the most widely distributed eruption product. Although ash falls rarely endanger human life directly, threats to public health and disruption to critical infrastructure services, aviation and primary production can lead to significant societal impacts. Even relatively small eruptions can cause widespread disruption, damage and economic loss. Volcanic eruptions are, in general, infrequent and somewhat exotic occurrences, and consequently in many parts of the world, the management of critical infrastructure during volcanic crises can be improved with greater knowledge of the likely impacts. This article presents an overview of volcanic ash impacts on critical infrastructure, other than aviation and fuel supply, illustrated by findings from impact assessment reconnaissance trips carried out to a wide range of locations worldwide by our international research group and local collaborators. ‘Critical infrastructure’ includes those assets, frequently taken for granted, which are essential for the functioning of a society and economy. Electricity networks are very vulnerable to disruption from volcanic ash falls. This is particularly the case when fine ash is erupted because it has a greater tendency to adhere to line and substation insulators, where it can cause flashover (unintended electrical discharge) which can in turn cause widespread and disruptive outages. Weather conditions are a major determinant of flashover risk. Dry ash is not conductive, and heavy rain will wash ash from insulators, but light rain/mist will mobilise readily-soluble salts on the surface of the ash grains and lower the ash layer’s resistivity. Wet ash is also heavier than dry ash, increasing the risk of line breakage or tower/pole collapse. Particular issues for water supply managers include: monitoring turbidity levels in raw water intakes, and if necessary increasing chlorination to compensate for higher turbidity; managing water demand; and communicating monitoring results with the public to allay fears of contamination. Ash can cause major damage to wastewater disposal systems. Ash deposited onto impervious surfaces such as roads and car parks is very easily washed into storm drains, where it can form intractable masses and lead to long-term flooding problems. It can also enter wastewater treatment plants (WWTPs), both through sewer lines and by direct fallout. Damage to modern WWTPs can run into millions of dollars. Ash falls reduce visibility creating hazards for ground transportation. Dry ash is also readily remobilised by vehicle traffic and wind, and dry and wet ash deposits will reduce traction on paved surfaces, including airport runways. Ash cleanup from road and airports is commonly necessary, but the large volumes make it logistically challenging. Vehicles are vulnerable to ash; it will clog filters and brake systems and abrade moving parts within engines. Lastly, modern telecommunications networks appear to be relatively resilient to volcanic ash fall. Signal attenuation and interference during ash falls has not been reported in eruptions over the past 20 years, with the exception of interference from ash plume-generated lightning. However, some telecommunications equipment is vulnerable to airborne ash, in particular heating, ventilation and air-conditioning (HVAC) systems which may become blocked from ash ingestion leading to overheating.

Global database on large magnitude explosive volcanic eruptions (LaMEVE)

Abstract: To facilitate the assessment of hazards and risk from volcanoes, we have created a comprehensive global database of Quaternary Large Magnitude Explosive Volcanic Eruptions (LaMEVE). This forms part of the larger Volcanic Global Risk Identification and Analysis Project (VOGRIPA), and also forms part of the Global Volcano Model (GVM) initiative (http://www.globalvolcanomodel.org). A flexible search tool allows users to select data on a global, regional or local scale; the selected data can be downloaded into a spreadsheet. The database is publically available online at http://www.bgs.ac.uk/vogripa and currently contains information on nearly 3,000 volcanoes and over 1,800 Quaternary eruption records. Not all volcanoes currently have eruptions associated with them but have been included to allow for easy expansion of the database as more data are found. Data fields include: magnitude, Volcanic Explosivity Index (VEI), deposit volumes, eruption dates, and rock type. The scientific community is invited to contribute new data and also alert the database manager to potentially incorrect data. Whilst the database currently focuses only on large magnitude eruptions, it will be expanded to include data specifically relating to the principal volcanic hazards (e.g. pyroclastic flows, tephra fall, lahars, debris avalanches, ballistics), as well as vulnerability (e.g. population figures, building type) to facilitate risk assessments of future eruptions.

Hydrovolcanic processes forming basaltic tuff rings and cones on

Abstract: Tuff rings and tuff cones are small volcanoes produced by explosive magma-water interactions and have been regarded as resulting from relatively dry and wet eruptions, respectively, which are related to low and high mixing ratios of water to magma. However, comparative work on four Pleistocene basaltic tuff rings and cones on Cheju Island, Korea, shows that there are dry and wet types in both tuff rings and tuff cones, and their variations are not satisfactorily explained by the prevailing model. Instead, it is inferred that the morphological variations are directly caused by depositional processes (pyroclastic surge‐dominated in tuff rings and fallout-dominated in tuff cones), irrespective of watermagma mixing ratios. The depositional processes are interpreted to be in turn controlled by a number of fundamental controls, which include depositional settings, type, level, and lithology of aquifers, strength of country rocks, ground-water behavior, and properties and behavior of magma. These controls determine the explosion depth, conduit geometry, mode of magma-water interaction, magnitude of explosion, eruption-column behavior, and subsequent depositional processes. The Suwolbong and Songaksan tuff rings, which formed almost entirely on land above fragile and permeable sediments and granites with some aquiclude beds, were produced by contact-surface steam explosivity at depth because of the fragility of country rocks, insufficient and inhibited supply of shallow-level external water into the vents, and interaction of nonvesiculated magma with interstitial water. These conditions led to generation of buoyancy-dominated eruption columns and pyroclastic surges, resulting in tuff rings. On the other hand, the Ilchulbong and Udo tuff cones formed in shallow seas above extremely permeable but rigid basalt lavas. The explosions occurred at shallow depths mainly by bulk-interaction steam explosivity because of the rigidity of country rocks, sustained supply of shallow-level external water into the vents, and interaction of vesiculated magma with free water. This process resulted in the generation of dense, inertia-dominated jets and the formation of tuff cones mainly by fallout processes. It is thought that the morphological and sedimentological variations of these volcanoes are more successfully explained by the fundamental controls rather than solely by the water-magma ratio. It is suggested that the water-magma ratio can explain the evolution of a single volcano or a group of volcanoes under otherwise identical conditions, but cannot explain the variability of tuff rings and cones in different hydrogeologic settings because the nature of hydroeruptions is governed by a number of fundamental controls.

Ash from the Eyjafjallajökull eruption (Iceland): Fragmentation processes and aerodynamic behavior

Abstract: [1] The fragmentation process and aerodynamic behavior of ash from the Eyjafjallajokull eruption of 2010 are investigated by combining grain-size, Scanning Electron Microscopy (SEM), and quantitative particle morphology. Ash samples were collected on land in Iceland at 3–55 km distance from the volcanic vent, and represent various phases of the pulsating eruption. The grain size is fine even for deposits close to the vent, suggesting that the parent particle population at fragmentation consisted of a substantial amount of fine ash. SEM investigation reveals that ash produced during the first phase of the eruption consists of juvenile glass particles showing key features of magma-water interaction, suggesting that phreatomagmatism played a major role in the fragmentation of a vesicle-poor magma. In the last phase of the eruption, fragmentation was purely magmatic and resulted from stress-induced reaction of a microvesicular, fragile melt. The shape of ash, as determined by quantitative morphology analysis, is highly irregular, rendering the settling velocity quite low. This makes transportation by wind much easier than for other more regularly shaped particles of sedimentary origin. We conclude that the combination of magma's fine brittle fragmentation and irregular particle shape was the main factor in the extensive atmospheric circulation of ash from the mildly energetic Eyjafjallajokull eruption.

Depositional processes and gas pore pressure in pyroclastic flows: an experimental perspective

Abstract: The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (U f) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U f 2 whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (t d) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously.

Causes and consequences of bimodal grain-size distribution of tephra fall deposited during the August 2006 Tungurahua eruption (Ecuador)

Abstract: The violent August 16–17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in the range 42–57 × 106 m3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16–18 km above the vent and making it the strongest eruptive phase since the volcano’s magmatic reactivation in 1999. Isopachs west of the volcano are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data processing may produce misleading determination of eruptive type.

Magmatic architecture of dome-building eruptions at Volcán de Colima, Mexico

Abstract: Changes in the physical, chemical and rheological properties of ascending magma regulate the style of volcanic eruptions. Volcan de Colima’s eruptive cycles of lava dome growth and explosions have been thoroughly monitored during the period 1998–2010 and provide a remarkable opportunity for deepening our understanding of the underlying processes responsible for the evolution of magma properties. Here, we integrate direct observation with analytical and experimental data to: (1) constrain the configuration of the shallow plumbing system and its influence on eruptive activity, (2) describe the rheological behaviour of the magma and (3) assess the conditions that lead to fragmentation and, ultimately, to explosive eruptions. The configuration of the shallow plumbing system was inferred from direct observation of extrusion sites and porosity of the erupted products. During the ongoing eruptive phase, magma was never extruded from a central vent: Both explosive and effusive activities were restricted to discrete vents inside the crater. Extensive field-based density measurements on 500 blocks in pyroclastic flow deposits reveal a bimodality of porosity at values of 12 and 26 vol.%. The least porous rocks tend to be altered, whereas the more porous rocks are pristine. This bimodal distribution, combined with the lack of a central vent, suggests the presence of a central, dense, altered plug, the fragments of which are entrained during explosive eruptions. During effusive periods, the plug appears to deflect the ascent of magma at a shallow depth and, consequently, the site of lava extrusion. The rheological properties and deformation-induced seismogenic behaviour of the magmas were investigated using a uniaxial deformation apparatus instrumented with acoustic sensors. The homogeneity in the physicochemical properties of the erupted magma permits the description of a flow law at eruptive temperature and strain rate conditions. The crystal-rich magma of Volcan de Colima exhibits a shear thinning rheology and becomes increasingly brittle at higher strain rates. Complete failure of magma can be predicted using the material failure forecast method, which integrates the acceleration of released acoustic energy throughout the deformation. Rapid decompression experiments of samples pressurised with argon were performed to assess the fragmentation conditions under which explosive eruptions progress. In the absence of gas loss due to permeable flow, the pore pressure required to fragment volcanic products is inversely proportional to the porosity. At Volcan de Colima, a rapid decompression of >6 MPa is required to fragment magma averaging 26 vol.% pores and to thereby instigate an explosive eruption. Analysis of ballistic impacts (4–6 km away from the vent) from recent explosive eruptions further requires decompression as great as 20 MPa, which is sufficient to disrupt the more porous material as well as a fraction of the denser plug. The continuing growth of a lava dome and of a larger, more impermeable plug could have consequences for the stability of the edifice and development of stronger and more erratic explosive activity at Volcan de Colima.

Morphology of the Faial Island shelf (Azores): The interplay between volcanic, erosional, depositional, tectonic and mass‐wasting processes

Abstract: [1] The extents of volcanic island shelves result from surf erosion, which enlarges them, and volcanic progradation, which reduces them However, mass-wasting, tectonics and sediment deposition also contribute to their morphology In order to assess the relative significance of these various processes, we have mapped in detail Faial Island's shelf in the Azores archipelago based on interpretation of geophysical and geological data The nearshore substrates of the island, down to 30–50 m depth, are rocky and covered by volcaniclastic boulder deposits formed by surf action on now-submerged lava flows Below those depths, sandy and gravel volcaniclastic beds dominate, building clinoforms up to the shelf edge In some sectors of the coast, prograding lava has narrowed the shelf, but, in contrast to nearby Pico Island, we find fewer submarine-emplaced lavas on the shelf In this island, we interpret the distance between the coastline and the shelf edge as almost entirely a result of a straightforward competition between surf erosion and lava progradation, in which erosion dominates Therefore shelf width can be used as a proxy for coastline age as well as for wave energy exposure The stratigraphy of shelf deposits in boomer seismic data is examined in detail to assess the roles of different sediment sources, accommodation space and wave exposure in creating these deposits We also show evidence of mass-wasting at the shelf edge and discuss the possible origins of slope instability Finally, we discuss the contributing role of tectonics for the development of the shelf

The geological evolution of Merapi volcano, Central Java, Indonesia

Abstract: Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuees ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of 40K–40Ar and 40Ar–39Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170 ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109 ± 60 ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138 ± 3 ka; 135 ± 3 ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30 ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8 ± 1.5 ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792 ± 90 14C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 14C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuees ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872 AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi.

High levels of particulate matter in Iceland due to direct ash emissions by the Eyjafjallajökull eruption and resuspension of deposited ash

Abstract: [1] The dangers to people living near a volcano due to lava and pyroclastic flows, and, on glacier- or snow-covered volcanoes, jokulhlaups, are well known The level of risk to human health due to high concentrations of ash from direct emission and resuspension from the ground is, however, not as well known The eruption at Eyjafjallajokull, 14 April to 20 May 2010, produced abundant particulate matter due to its explosive eruption style Even after the volcanic activity ceased, high particulate matter (PM) concentrations were still measured on several occasions, due to resuspended ash The 24 hour mean concentration of PM10 in the small town of Vik, 38 km SE of the volcano, reached 1230 μg m−3, which is about 25 times the health limit, on 7 May 2010, with 10 min average values over 13,000 μg m−3 Even after the eruption ceased, values as high as 8000 μg m−3 (10 min), and 900 μg m−3 (24 h), were measured because of resuspension of freshly deposited fine ash In Reykjavik, 125 km WNW of the volcano, the PM10 concentration reached over 2000 μg m−3 (10 min) during an ash storm on 4 June 2010, which should have warranted airport closure Summarizing, our study reveals the importance of ash resuspension compared to direct volcanic ash emissions This likely has implications for air quality but could also have detrimental effects on the quality of ash dispersion model predictions, which so far generally do not include this secondary source of volcanic ash

A 3.51 Ga bimodal volcanics‐BIF‐ultramafic succession from Singhbhum Craton: implications for Palaeoarchaean geodynamic processes from the oldest greenstone succession of the Indian subcontinent

Abstract: The Palaeoarchaean greenstone belt of the southern Iron Ore Group (SIOG) (3.51 Ga) in the Singhbhum Craton, eastern India, includes low-strained and low-greenstone grade bimodal volcanics, Banded Iron Formation (BIF) and chromiferous ultramafics as enclaves within tonalite–trondjhemite–granodiorite (TTG) granitoids, collectively referred to as the Singhbhum Granite (3.4 Ga to 3.1 Ga). The succession comprises, from base to top, a lower unit of massive and pillowed basalt conformably overlain by dacitic lava and pyroclastics which in turn is overlain by a major BIF unit. The ultramafics are juxtaposed with the volcanics-BIF succession along a thrust fault. The lithological association of pillow lava, subaqueous dacitic lava and pyroclastic rocks and BIF collectively, suggests that the entire succession was deposited in a deep-marine depositional setting. The ash-poor dacitic volcanic rock succession with evidences of a transition from suppressed-volatile deep-water lava flow and pyroclastics to more evolved mass-flow deposits with increasing trend of subaqueous flow transformation, records a transition from a deep-water low-height volcanic chain to a shallower subaqueous eruption in an aggradational volcanic chain. Geochemical proxies from the bimodal volcanics and ultramafics showing enrichment of La/Nb, Th/Nb, Th/La, Ba/La, Pb/Ce, depletion in Nb–Ta relative to neighbouring REE, together with tectonic discrimination criteria using Nb, Y, Zr, Ti compositions, suggest an extending oceanic arc–forearc geodynamic setting similar to many of the Phanerozoic supra-subduction zone ophiolites where ophiolite development in the extending upper plate in a relatively short time span is facilitated by slab rollback processes. The positive Eu-anomaly together with high Y/Ho values from the BIFs also suggests their deposition in close proximity to spreading centres that might have developed over a rifted arc. The bimodal volcanic rock-BIF-ultramafic succession of the SIOG with evidence of a convergent margin geodynamic setting is an important example for Palaeoarchaean plate tectonic processes operating on Earth. Copyright © 2011 John Wiley & Sons, Ltd.

How Small-volume Basaltic Magmatic Systems Develop: a Case Study from the Jeju Island Volcanic Field, Korea

Abstract: Jeju is a volcanic field that has erupted from around 1·8 Myr to c. 1 kyr ago. Activity began with dispersed, basaltic, monogenetic, phreatomagmatic eruptions. Continuing monogenetic volcanism was later joined by more voluminous lava effusion events building a central composite shield. Samples from older (40·7 Ma) and younger (50·2 Ma) monogenetic centres were analysed for their whole-rock major element, trace element and Sr^Nd^Pb isotopic compositions. Pyroclastic products from the monogenetic centres are dominantly alkali basalt to trachybasalt, whereas the more voluminous lava flows and domes of the central edifice consist of subalkali basalt and alkali basalt to trachyte. Lavas from the Early Pleistocene monogenetic centres are depleted in MgO, Cr and Ni, reflecting considerable olivine fractionation. By contrast, Late Pleistocene^Holocene monogenetic centre magmas fractionated clinopyroxeneþ olivine at deeper levels. Isotopic compositions show little variation across the suite; however, the Late Pleistocene^Holocene monogenetic centres have generally lower Sr/Sr and Pb/Pb and higher Nd/Nd than the older centres and subalkali lavas. Major and trace element and isotope data suggest a common, shallower source for the high-Al alkali and subalkali lavas, in contrast to a deeper source for the low-Al alkali magmas. We propose that mantle melting was initiated under partially hydrous conditions at a pressure of near 2·5 GPa, followed by drier conditions and extension of the melting zone to 3^3·5 GPa, with a concomitant increase in the volume of melt derived from the shallower part of the system to produce subalkaline magmas. Increasing melt production at shallow depths may be related to accelerated heat transfer resulting from deepening of the melting zone, or increased mantle upwelling. Mantle lenses were uplifted, probably lubricated by shear zones created during the opening of the Sea of Japan c. 15 Myr ago, and reactivated during rotation of the Philippine Sea plate direction of subduction at around 2 Ma. This is the first hypothesized link between subduction processes and intraplate volcanism atJeju.

Volcanic activity on differentiated asteroids: A review and analysis

Abstract: Many early-forming asteroids differentiated, as a result of incorporating the heat-generating isotope 26Al, and experienced various kinds of volcanic activity. The Dawn spacecraft's investigation of the best-preserved of these asteroids, 4 Vesta, warrants a reappraisal of the factors controlling asteroid volcanism. We conclude that silicate melts were removed efficiently from the mantles of all asteroids by complex networks of small and large veins and dikes. This implies that only a few percent of the interior consisted of melt at any one time. Thus although large amounts of mantle melting may ultimately have occurred, “magma oceans”, in the sense of mantles containing many tens of percent of melt, can not have formed. The total rate of silicate melt production in an asteroid of a given size can be predicted as a function of time after formation. If magma were erupted directly to the surface it must have done so through a small number of major dikes, brittle fractures that penetrated the outer thermal boundary crustal layer ∼10 km deep within which heat was conducted to the surface fast enough that temperatures stayed below the silicate solidus. By modeling the link between magma flow rate and the stresses needed to keep fractures open and allow magma to rise through them without excessive cooling, we show that continuous eruptions direct from mantle to surface were impossible on asteroids with radii less than ∼190–250 km. Instead, magma must have accumulated, in sills at the base of the thermal boundary layer or in magma reservoirs in its lower part. Magma could then erupt intermittently to the surface from these steadily replenished reservoirs. The eruption rates from the reservoirs are not linked directly to the melt production rate in the mantle and could be very large, hundreds to thousands of m3 s−1, comparable to rates in historic basaltic eruptions on Earth. Many asteroid magmas are expected to have contained at least a few hundred ppm of volatiles. On asteroids with radii less than ∼100 km, the gases and small (sub-mm) pyroclastic melt droplets ejected in explosive eruptions as volatiles expanded into the surrounding vacuum will have had velocities exceeding escape speed. Only pyroclasts of at least cm size will have had small enough speeds to be retained on the surface. Asteroids significantly larger than ∼100 km in radius will have retained all pyroclasts, and most clasts will have reached the surface after passing through optically dense fire fountains. These clasts suffered negligible cooling and coalesced into lava ponds feeding lava flows. Only if eruption rates were low and volatile contents high will enough clasts have suffered sufficient cooling that spatter or cinder deposits formed. Thicknesses of lava flows are controlled by the acceleration due to gravity, surface slope, and the effective yield strength that lava develops due to cooling. Low gravity on asteroids caused flows to be thick and, coupled with high eruption rates, induced initially turbulent flow. Cooling caused a change to laminar flow and eventually brought flows to a halt, but comparison of expected cooling rates and flow thicknesses suggests that many flows attained lengths of tens of km and stopped as a result of cessation of magma supply from the reservoir rather than cooling. If more than ∼30% melting of the mantle of an asteroid occurred and the resulting erupted volcanic products were retained on the surface, as is expected for asteroids with radii >∼100 km, the volcanic deposits will have buried the original chondritic surface layers of the asteroid to such great depths that they were melted, or at least heavily thermally metamorphosed, leaving no meteoritical evidence of their existence today. Tidal stresses caused by close encounters between asteroids and proto-planets may have briefly and temporarily increased melting and melt migration speeds in asteroid interiors but will not greatly have changed volcanic histories unless gross structural disruption took place.

Reconstructing the architectural evolution of volcanic islands from combined K/Ar, morphologic, tectonic, and magnetic data: the Faial Island example (Azores)

Abstract: The morpho-structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass-wasting processes. We present here a multi-disciplinary study of the successive stages of development of Faial (Azores) during the last 1 Myr. Using high-resolution digital elevation model (DEM), and new K/Ar, tectonic, and magnetic data, we reconstruct the rapidly evolving topography at successive stages, in response to complex interactions between volcanic construction and mass wasting, including the development of a graben. We show that: (1) sub-aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca. 0.85 Ma, followed by its partial destruction over half a million years; (2) beginning about 360 ka a new small edifice grew on the NE of the island, and was subsequently cut by normal faults responsible for initiation of the graben; (3) after an apparent pause of ca. 250 kyr, the large Central Volcano (CV) developed on the western side of the island at ca 120 ka, accumulating a thick pile of lava flows in less than 20 kyr, which were partly channelized within the graben; (4) the period between 120 ka and 40 ka is marked by widespread deformation at the island scale, including westward propagation of faulting and associated erosion of the graben walls, which produced sedimentary deposits; subsequent growth of the CV at 40 ka was then constrained within the graben, with lava flowing onto the sediments up to the eastern shore; (5) the island evolution during the Holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano-tectonic depression. We conclude that the whole evolution of Faial Island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation. Each pulse has been separated by considerable periods of volcanic inactivity during which the Faial graben gradually developed. We propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement, as observed historically, when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the Capelinhos eruptive crises (1957-58).

Paleogeographic evolution of the Southern Pannonian Basin: 40Ar/39Ar age constraints on the Miocene continental series of Northern Croatia

Abstract: The Pannonian Basin, originating during the Early Miocene, is a large extensional basin incorporated between Alpine, Carpathian and Dinaride fold-thrust belts. Back-arc extensional tectonics triggered deposition of up to 500-m-thick continental fluvio-lacustrine deposits distributed in numerous sub-basins of the Southern Pannonian Basin. Extensive andesitic and dacitic volcanism accompanied the syn-rift deposition and caused a number of pyroclastic intercalations. Here, we analyze two volcanic ash layers located at the base and top of the continental series. The lowermost ash from Mt. Kalnik yielded an 40Ar/39Ar age of 18.07 ± 0.07 Ma. This indicates that the marine-continental transition in the Slovenia-Zagorje Basin, coinciding with the onset of rifting tectonics in the Southern Pannonian Basin, occurs roughly at the Eggenburgian/Ottnangian boundary of the regional Paratethys time scale. This age proves the synchronicity of initial rifting in the Southern Pannonian Basin with the beginning of sedimentation in the Dinaride Lake System. Beside geodynamic evolution, the two regions also share a biotic evolutionary history: both belong to the same ecoregion, which we designate here as the Illyrian Bioprovince. The youngest volcanic ash level is sampled at the Glina and Karlovac sub-depressions, and both sites yield the same 40Ar/39Ar age of 15.91 ± 0.06 and 16.03 ± 0.06 Ma, respectively. This indicates that lacustrine sedimentation in the Southern Pannonian Basin continued at least until the earliest Badenian. The present results provide not only important bench marks on duration of initial synrift in the Pannonian Basin System, but also deliver substantial backbone data for paleogeographic reconstructions in Central and Southeastern Europe around the Early–Middle Miocene transition.

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

Abstract: 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.

Characterization of previously unidentified lunar pyroclastic deposits using Lunar Reconnaissance Orbiter Camera data

Abstract: [1] We used a Lunar Reconnaissance Orbiter Camera (LROC) global monochrome Wide-angle Camera (WAC) mosaic to conduct a survey of the Moon to search for previously unidentified pyroclastic deposits. Promising locations were examined in detail using LROC multispectral WAC mosaics, high-resolution LROC Narrow Angle Camera (NAC) images, and Clementine multispectral (ultraviolet-visible or UVVIS) data. Out of 47 potential deposits chosen for closer examination, 12 were selected as probable newly identified pyroclastic deposits. Potential pyroclastic deposits were generally found in settings similar to previously identified deposits, including areas within or near mare deposits adjacent to highlands, within floor-fractured craters, and along fissures in mare deposits. However, a significant new finding is the discovery of localized pyroclastic deposits within floor-fractured craters Anderson E and F on the lunar farside, isolated from other known similar deposits. Our search confirms that most major regional and localized low-albedo pyroclastic deposits have been identified on the Moon down to ∼100 m/pix resolution, and that additional newly identified deposits are likely to be either isolated small deposits or additional portions of discontinuous, patchy deposits.

Syn-eruptive morphometric variability of monogenetic scoria cones

Abstract: According to Wood's model, morphometric parameters such as slope angle can provide valuable information about the age of conical volcanic edifices such as scoria cones assuming that their initial slopes range from 30° to 33°, measured manually on topographic maps, and assuming that their inner architectures are homogenous. This study examines the morphometric variability of nine young (a few thousand years old) small-volume scoria cones from Tenerife, Canary Islands, using high-resolution digital elevation models in order to assess their slope angle variability. Because of the young age and minimal development of gullies on the flanks, their morphometric variability can be interpreted as the result of syn-eruptive processes including: (1) pre-eruptive surface inclination, (2) vent migration and lava outflow with associated crater breaching and (3) diversity of pyroclastic rocks accumulated in the flanks of these volcanic edifices. Results show that slope angles for flank sectors differ by up to 12° among the studied volcanoes, which formed over the same period of time; this range greatly exceeds the 2–3° indicated by Wood. The greater than expected original slope range suggests that use of morphometric data in terms of morphometry-based relative dating and detection of erosional processes and settings must be done with great care (or detailed knowledge about absolute ages and eruption history), especially in field-scale morphometric investigation.