Air-fall Deposits Research Articles

Gamma absorbed radiation dose in Marrah mountain series, western Sudan

Measurements of ambient gamma radiation dose and radioactivity of soil and rocks samples have been carried out in 103 sites located in the region of Marrah mountain (western Sudan) using survey meters and gamma spectrometry techniques, respectively. The ambient dose varied from 0.05 to 0.25 µSv/h with mean value of 0.18 ± 0.05 (SD) µSv/h. At high altitudes (> 1500 m), relatively high values were observed (0.18–0.22 µSv/h). The levels of 238U, 232Th and 40K in soil–rocks were in the ranges 12–80, 12–66 and 120–937 Bq/kg with mean ± SD of (41 ± 6.9), (24 ± 12) and (540 ± 99) Bq/kg, respectively, equivalent to total gamma absorbed dose of 23–142 (66 ± 20) nGy/h. The ambient dose data were presented in cartography format, and correlation to geological maps was attempted. Surface map of the absorbed dose created using geostatistical kriging tools showed the spatial distribution of dose. Analyses showed that trends of high values were located in and around areas characterized by local flow and basaltic flow lithology, while the low values were observed at locations of basement rocks (granitic) in general. That was attributed to the differences in environmental conditions in various altitude and weathering processes. The highest readings (0.25 µSv/h and 142 nGy/h) were found in Deriba Complex Pumice air-fall deposits (younger explosive phase). Health risk was assessed by calculating annual effective dose for the inhabitants resulted in values range between 0.14 and 1.4 mSv/y (mean ± SD of 0.40 ± 0.12 mSv/y). The observed values of dose rates are comparable with some countries but higher than the worldwide average for outdoor annual affective dose of 0.07 mSv/y.

Controls on the nature of loess particles and the formation of loess deposits

The story (history) of a single loess particle can be told from the formation of the planet to the making of bricks. The modal loess particle chosen for study is a 30 μm quartz particle. Many factors bear on its complex history, starting with the formation of quartz units in the original granitic crustal rocks. Quartz units appear in the old granites as the product of a eutectic-like reaction. This controls particle size and ensures that sand is much the same wherever (and whenever) it is encountered. The eutectic reaction controls sand size; the silt size is controlled by crystalline defects which are introduced into the quartz by the high-low displacive transformation which the quartz undergoes as the system cools. Thus all quartz silt is much the same, as is the case for sand. The quartz clastic particles are surprisingly bi-modal, a more even size distribution might have been expected. The silt particles are largely formed by glacial grinding, by continental and mountain glaciers. These silt particles are distributed across the landscape by the action of large rivers, and deposited on floodplains. A critical stage in the story is the aeolian transportation of the particles which then form an airfall deposit. The glacially provided silt particles are quite close in size to the optimum for aeolian pick-up (∼80 μm). At this stage, on wind-fall deposition, the characteristic metastability is introduced into the loess deposit. The chief post-depositional process is the development of collapsibility, which is largely due to the movement of clay minerals to the main particle contacts. This low clay content of most loess deposits makes loess an ideal material for brick production.

Quaternary basaltic volcanism in the Golden Trout Volcanic Field, southern Sierra Nevada, California

The Golden Trout Volcanic Field (GTVF) produced the only Quaternary eruptions of mafic magma within the southern Sierra Nevada block. Approximately 38×106m3 of basalt, trachy-basalt, basaltic trachy-andesite, and basaltic andesite (50.1–56.1% SiO2, 1.1–1.9% K2O, and 5.4–9.1% MgO) was erupted from four vents within a ~10km2 portion of the GTVF, which also includes rhyolite domes that are not considered in this study. The vents include, from oldest to youngest: Little Whitney Cone, South Fork Cone, Tunnel Cone, and unglaciated Groundhog Cone. Little Whitney Cone is a 120m-high pile of olivine-CPX-phyric scoria produced during a Strombolian-style eruption overlying two columnar jointed lava flows. Tunnel Cone formed through a Hawaiian-style eruption along a 400m-long north-south trending fissure that excavated at least three 25–65m-wide craters. Crater walls up to 12m high are composed of plagioclase-olivine-phyric spatter-fed flows that dip radially away from the crater center and crumble to form Tunnel Cone's steep unconsolidated flanks. South Fork Cone is a 170m-high pile of plagioclase-olivine-phyric scoria that formed during Strombolian to violent Strombolian eruptions. South Fork Cone overlies the South Fork Cone lava, a 9.5km-long flow (~12×106km3) that reached the Kern River Canyon to the west. Scoria and airfall deposits originating from South Fork Cone are located up to 2km from the vent. Groundhog Cone is a 140m-tall cinder and spatter cone breached on the north flank by a 13×106m3 lava flow that partially buried the South Fork Cone lava and extends 7.5km west to Kern River Canyon. Incompatible trace element concentrations and ratios show vent-specific trends but are unsystematic when plotted in terms of all mafic GTVF vents, implying that GTVF basalts were derived from a lithospheric mantle source and ascended through thick granitic Sierra Nevada crust as discrete batches that underwent different degrees of crustal contamination, differentiation, and magma mixing. Clinopyroxene-liquid thermobarometry calculations of the oldest and most primitive GTVF sample indicate that most clinopyroxene crystals record nucleation conditions of 1172–1196°C and 896–1115MPa. These pressures correspond to depths equivalent to, or up to 10km shallower than, the Moho in this region. Deposits from the oldest GTVF vents are more primitive and homogeneous in terms of major and trace element concentrations, phenocryst textures and compositions, and their general absence of crustal xenoliths and xenocrysts compared to younger eruptive products. The eruption rate of the GTVF (~0.05km3/Myr) is two orders of magnitude less than neighboring and contemporaneous Big Pine and Coso volcanic fields to the north and east, respectively. We interpret these differences to result from a relative lack of strain in the GTVF region of the southern Sierra Nevada block, which limits magma accumulation and ascent.

A pedological case study of volcanoclastically impacted landscapes: The Vesuvian Avellino air-fall deposits, Southern Italy

Volcanic sediments are involved in both stratigraphic architecture and pedological environment of the impacted surfaces. Volcanic deposits are widespread in the southern Italian region of Campania, generating extensive tephrostratigraphic and pedological studies. Nevertheless, the pedogenetic effects of the distribution of volcanic sediments have not been fully investigated. The implications of tephra burial and composition on pedogenetic processes have been studied by comparing soil profiles in two different locations influenced by the pumice fall-deposit from the Vesuvian Avellino eruption (about 3360years BP): Mt. Somma volcano and Mt. Arciano. The latter forms part of the carbonate Campanian Apennines, where the carbonate rock has been excluded from the main pedogenetic processes, due to emplacement of pyroclastic products. Among the described soils on Mt. Somma, the analyzed soil profile was extracted from an earlier work (see the text for the pertinent citation). For the investigation, soil morphological, mineralogical, chemical and lithological properties were determined. On Mt. Somma, the pedogenesis from the Avellino pumices was prevented by their rapid and massive burial by other volcanic sediments. In the absence of overlying materials on Mt. Arciano, the Avellino pumices provided a homogeneously developed soil with well-expressed andic soil properties. In this environment, the pumice substratum was also enriched in the glassy fraction as an effect of selection caused by the pyroclastics during the air-fall distribution. The results indicated the distance directly and indirectly influencing the pedogenesis, due to its implications on both the potential burial processes and the lateral variation in the pedogenetic substratum.

INTERPRETATION OF BOUGUER ANOMALY TO DETERMINE FAULT AND SUBSURFACE STRUCTURE AT BLAWAN-IJEN GEOTHERMAL AREA

Gravity survey has been acquired by Gravimeter Lacoste & Romberg G-1035 at Blawan-Ijen geothermal area. It was a focusing study from previous research. The residual Bouguer anomaly data was obtain after applying gravity data reduction, reduction to horizontal plane, and upward continuation. Result of Bouguer anomaly interpretation shows occurrence of new faults and their relative movement. Blawan fault (F1), F2, F3, and F6 are normal fault. Blawan fault is main fault controlling hot springs at Blawan-Ijen geothermal area. F4 and F5 are oblique fault and forming a graben at Banyupahit River. F7 is reverse fault. Subsurface model shows that Blawan-Ijen geothermal area was dominated by the Ijen caldera forming ignimbrite (ρ1=2.670 g/cm3), embedded shale and sand (ρ2=2.644 g/cm3) as Blawan lake sediments, magma intrusion (ρ3=2.814 g/cm3 & ρ7=2.821 g/cm3), andesite rock (ρ4=2.448 g/cm3) as geothermal reservoir, pyroclastic air fall deposits (ρ5=2.613 g/cm3) from Mt. Blau, and lava flow (ρ6=2.890 g/cm3).

火星衝突クレーターの特異なエジェクタ地形と劣化過程

Mars has fascinated us in terms of discovering vital activities and examining planetary surface evolutions. Recent explorations detected subsurface ice and methane, which reveal the importance of investigating ancient environments and their evolution. Impact craters and their ejecta morphologies are natural probes for obtaining knowledge on ancient surface and subsurface environments, because detailed morphologies of impact craters and their ejecta are sensitive to impact conditions and degradation processes. Martian enigmatic ejecta morphologies and their currently-proposed origins are summarized, and some degradation processes effective on Martian craters are introduced. Ejecta of Martian impact craters are different from those of lunar or Mercurian impact craters. They possess clear distal ridges and cliffs, and are diverted by preexisting obstacles, while edges of lunar or Mercurian ejecta are ambiguous. Their unique morphologies suggest that the ejecta are emplaced by radial ground-hugging flows generated at the time of the impact. Processes and agents of the fluidization draw attention, because the atmosphere and/or subsurface volatiles are suggested for its formation. Degradation processes on Mars vary, and include superimposed impacts, viscous relaxation, mass movements, lava infilling, fluvial incision, gully formation, lacustrine erosion and deposition, ice-related processes, and airfall deposition. Due to high ancient erosion rates, degraded crater morphologies also provide useful information on the ancient environment.

Photogeologic mapping and the geologic history of the Hellas basin floor, Mars

The Hellas basin on Mars is the second-largest topographically well-defined impact structure in the Solar System and has repeatedly been interpreted as a major sink of volcanic, glacio-fluvial and eolian materials. Based on established guidelines for planetary mapping, we compiled a comprehensive photogeological map of Hellas Planitia, i.e., the Hellas basin floor (1:2,000,000; ∼1.8×106km2; see Supplementary online material), using the Thermal Emission Imaging System (THEMIS-IR) day-time mosaic as basemap (supplemented by several other datasets). We identified 33 units, which were categorized into a “Rim Assemblage”, containing “Dissected units”, a “Layered rim sequence”, and “Other basin rim units”, as well as a “Floor Assemblage”, containing the “Honeycomb formation”, an “Interior formation”, and a “Plains sequence”. Relative dating of units was performed wherever contacts revealed stratigraphic relationships and was complemented by absolute model ages (AMAs) of all units that lend themselves to reliable crater-size frequency distribution (CSFDs) measurements. On the basis of our results, as well as AMAs of circum-Hellas volcanic provinces by previous authors, we compiled a chronostratigraphic model of the Hellas basin floor. The northern basin rim shows evidence (vast layered, hydrous mineral-bearing deposits containing meandering, channel-like valleys), that the early history of the basin until ∼3.8Ga ago experienced extended periods of low-energy fluvial, and possibly lacustrine, activity. Superposing the layered rim sequence, the majority of the Hellas basin infill (∼1.5–1.7×106km3) consists of volcanic material (a lower and an upper wrinkle-ridged plains unit), which was shortened by a compressive stress field relatively soon after its emplacement. Based on their ages and stratigraphic considerations, we identified Malea Patera (and possibly also Tyrrhena Patera) as a suitable source for the older, lower plains (∼3.8Ga), and Hadriaca and/or Amphitrites Paterae for the younger, upper plains (∼3.7Ga). Shortly after these volcanic episodes, the entire basin floor was covered by large amounts of deposits (>106km3) containing aqueously altered mafic materials. We interpret the deposits to have originated from Hesperia Planum and the Hesperia-Hellas trough, where they might have been removed by glacio-fluvial processes, which also sculpted the Hellas rim sections along northern Promethei Terra and Malea Planum around the same time. Such an apparent correlation between nearby volcanic activity and increased erosion/deposition is in agreement with previous models, which suggest outgassing by a maximum in global volcanism at that time caused warmer episodes enabling repeated subaerial water run-off. After the emplacement of these deposits, intense erosion, likely deflation along a rim-parallel annulus, began to exhume the outer parts of the wrinkle-ridged plains again, forming an interior formation of more elevated erosional remnants including the Alpheus Colles. Since ∼3.7Ga ago, fluvial activity of Dao/Niger and Harmakhis Valles dissected the eastern plains and emplaced pancake-shaped deposits in the basin center. Later, deflation and abrasion by katabatic winds moving through the Hellas basin in a clock-wise direction parallel to the basin floor outline carved out the northwestern Hellas Planitia trough (called Peneus Palus), possibly within a time span of few hundreds of Ma or less within the Amazonian. Being consistent with erosional patterns we observed within the basin, circulation models predict such winds to be ongoing today and might, thus, have recently exhumed the so called “honeycomb” formation on parts of Peneus Palus. This enigmatic, unique terrain was possibly formed by ductile deformation of pre-3.8Ga material and appears to be partially covered by another unique landform, the “banded terrain”. Although we cannot rule out that it is the surface expression of a thicker unit, the banded terrain might represent flows of volatile-rich airfall deposits emplaced along the edges of the Hellas wind alley by turbulent, marginal wind currents, which currently/recently prevail on both sides of the Hellas Planitia trough. In summary, our investigations enabled us to draw an updated, comprehensive and self-consistent picture of the geologic evolution of the Hellas basin floor, including volcanic, (peri-)glacial, fluvial and eolian processes, their possible interactions, and the implications on the climatic and geologic development of the circum-Hellas region and the entire planet.

Robustness of the Vesuvian Roofs Under the Combined Overload and High Temperatures Due to Air Falls

During an explosive eruption, a construction is hit by several actions, always associated to elevated temperatures, causing fires, possible explosions and reduction of the mechanical properties of the structural materials. In this paper the attention is focused on the analysis of a specific volcanic event, the so-called air fall deposits, generally falling from the eruptive column due to gravity. The study, starting within the European COST Action C26 project “Urban habitat construction under catastrophic events” (2006-2010), is concerned with the Vesuvius area at Naples in Italy. It faces two important aspects. The first items the proposal of a model for the air fall deposits action on the basis of a similitude with the snow load. The second item is the robustness evaluation against the air fall deposits of the most common roof types in the Vesuvian area, which are made of timber, steel and reinforced concrete. Therefore, some protection systems for mitigating the effects of the combination of overloading and high temperatures on the examined roofing structures are identified.

Eruptive history of a low-frequency and low-output rate Pleistocene volcano, Ciomadul, South Harghita Mts., Romania

Based on a new set of K–Ar age data and detailed field observations, the eruptive history of the youngest volcano in the whole Carpathian-Pannonian region was reconstructed. Ciomadul volcano is a dacitic dome complex located at the southeastern end of the Călimani-Gurghiu-Harghita Neogene volcanic range in the East Carpathians. It consists of a central group of extrusive domes (the Ciomadul Mare and Haramul Mare dome clusters and the Koves Ponk dome) surrounded by a number of isolated peripheral domes, some of them strongly eroded (Balvanyos, Puturosul), and others topographically well preserved (Haramul Mic, Dealul Mare). One of the domes (Dealul Cetăţii) still preserves part of its original breccia envelope. A large number of bread-crust bombs found mostly along the southern slopes of the volcano suggest that the dome-building activity at Ciomadul was punctuated by short Vulcanian-type explosive events. Two late-stage explosive events that ended the volcanic activity of Ciomadul left behind two topographically well-preserved craters disrupting the central group of domes: the larger-diameter, shallower, and older Mohos phreatomagmatic crater and the smaller, deeper and younger Sf. Ana (sub)Plinian crater. Phreatomagmatic products of the Mohos center, including accretionary lapilli-bearing base-surge deposits and poorly sorted airfall deposits with impact sags, are known close to the eastern crater rim. A key section studied in detail south of Băile Tusnad shows the temporal succession of eruptive episodes related to the Sf. Ana (sub)Plinian event, as well as relationships with the older dome-building stages. The age of this last eruptive event is loosely constrained by radiocarbon dating of charcoal pieces and paleosoil organic matter at ca. 27–35 ka. The age of the Mohos eruption is not constrained, but we suggest that it is closely related to the Sf. Ana eruption. The whole volcanic history of Ciomadul spans over ca. 1 Myr, starting with the building up of peripheral domes and then concentrating in its central part. Ciomadul appears as a small-volume (ca. 8.74 km3) and very low-frequency and low-output rate volcano (ca. 9 km3/Myr) at the terminus of a gradually diminishing and extinguishing volcanic range. A number of geodynamically active features strongly suggest that the magma plumbing system beneath Ciomadul is not completely frozen, so future activity cannot be ruled out.

Marine Volcanosedimentary Basins Hosting Porphyry Au-Cu Deposits, Cadia Valley, New South Wales, Australia

The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithofacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account.

Stratigraphy, sedimentology and eruptive mechanisms in the tuff cone of El Golfo (Lanzarote, Canary Islands)

The tuff cone of El Golfo on the western coast of Lanzarote (Canary Islands) is a typical hydrovolcanic edifice. Along with other edifices of the same age, it was constructed along a fracture oriented NEE–SWW that coincides with the main structural trend of recent volcanism in this part of the island. We conducted a detailed stratigraphic study of the succession of deposits present in this tuff cone and here interpret them in light of the depositional processes and eruptive dynamics that we were able to infer. The eruptive sequence is represented by a succession of pyroclastic deposits, most of which were emplaced by flow, plus a number of air-fall deposits and ballistic blocks and bombs. We distinguished five different eruptive/depositional stages on the basis of differences in inferred current flow regimes and fragmentation efficiencies represented by the resulting deposits; the different stages may be related to variations in the explosive energy. Eight lithofacies were identified based on sedimentary discontinuities, grain size, components, variations in primary laminations and bedforms. The volcanic edifice was constructed very rapidly around the vent, and this is inferred to have controlled the amount of water that was able to enter the eruption conduit. The sedimentological characteristics of the deposits and the nature and distribution of palagonitic alteration suggest that most of the pyroclastic succession in El Golfo was deposited in a subaerial environment. This type of hydrovolcanic explosive activity is common in the coastal zones of Lanzarote and the other Canary Islands and is one of the main potential hazards that could threaten the human population of this archipelago. Detailed studies of these hydrovolcanic eruptions such as the one we present here can help volcanologists understand the hazards that this type of eruption can generate and provide essential information for undertaking risk assessment in similar volcanic environments.

Re-deposited cryptotephra layers in Holocene peats linked to anthropogenic activity

Tephra layers can form useful age-equivalent stratigraphic markers for correlating palaeoenvironmental sequences and they provide information about the spatio-temporal nature of past volcanic ash fall events. The use of microscopic ‘cryptotephra’ layers has both increased the stratigraphic resolution of tephra sequences in proximal areas and extended the distal application of tephrochronology to regions of the world situated far from volcanoes. Effective tephrochronology requires the discrimination between in situ tephra deposited directly from volcanic plumes and tephras that have been remobilised since their initial deposition. We present tephrostratigraphic and glass chemistry data from two proximal peat profiles (one lowland, one upland) from the Shetland Islands, UK. Both profiles contain the Hekla-Selsund tephra (deposited c. 1800–1750 cal. bc), whilst the Hekla 4 ash (c. 2395–2279 cal. bc) is present in the upland record. Overlying the Hekla-Selsund tephra are a number of distinct peaks in tephra shard abundance. The geochemistry of these layers shows that they represent re-working of the Hekla 4 and Hekla-Selsund layers rather than primary air-fall deposits. Pollen analysis of the peat sequences illustrates that these re-deposited tephra layers are coincident with a rise in heather-dominated vegetation communities (heath and/or moorland) and a subsequent intensification of burning in the landscape. We suggest that burning caused increased erosion of peats resulting in the remobilisation of tephra shards. The study demonstrates both the need for caution and the opportunities created when applying tephrochronologies in regions heavily affected by past human activity that contain both reworked tephra layers and in situ fallout.

Knob fields in the Terra Cimmeria/Terra Sirenum region of Mars: Stratigraphy, mineralogy and morphology

We investigate the stratigraphy, morphology and mineralogy of five major knob fields in the region between Terra Cimmeria and Terra Sirenum on Mars based on HRSC, CTX, MOC and HiRISE imagery together with hyperspectral data from CRISM. The knob fields comprise Ariadnes Colles, Atlantis Chaos and Gorgonum Chaos and further, unnamed fields of mounds. They have been mapped in previous studies as Hesperian or Amazonian units and are located within the shoreline of the proposed “Eridania lake”, the putative source of Ma’adim Vallis. The mounds contain Mg/Fe-bearing phyllosilicates and locally Al-rich phyllosilicates. Our geological mapping shows that the knob fields have a late Noachian age, which indicates later phyllosilicate formation than typically observed on Mars. The knob fields formed by alteration of the “Electris deposit”, an airfall deposit possibly rich in basaltic glass (Grant, J.A., Schultz, P.H. [1990]. Icarus 84, 166–195), in local depressions, possibly in the Eridania lake. The spectroscopic detection of phyllosilicates here may indicate that liquid water persisted longer in this region than elsewhere on Mars. The knob fields are embayed by the Hesperian ridged plains. Numerous valleys carve into the ridged plains and document that the aqueous history of this region continued into the Hesperian and Amazonian. The study area is traversed by the Sirenum Fossae. These graben appear to post-date the aqueous activity in the study area except in the Gorgonum basin, where a lake developed after their formation.

A significant middle Pleistocene tephra deposit preserved in the caves of Mulu, Borneo

A distinctive white sediment in the caves of Mulu, Sarawak, Borneo is a well-preserved tephra, representing a fluvially transported surface air-fall deposit, re-deposited inside the caves. We show that the tephra is not the Younger Toba Tephra, formerly considered as most likely. The shards are rod-shaped with elongate tubular vesicles; the largest grains ~170μm in length; of rhyolitic composition; and 87Sr/86Sr ratio of 0.70426±0.00001. U–Th dating of associated calcites suggest that the tephra was deposited before 125±4ka, and probably before 156±2ka. Grain size and distance from closest potential source suggests an eruption of VEI 7. Prevailing winds, grain size, thickness of deposit, location of potential sources, and Sr isotopic ratio limit the source to the Philippines. Comparisons with the literature give the best match geochemically with layer 1822 from Ku et al. (2009a), dated by ocean core stratigraphy to 189ka. This tephra represents a rare terrestrial repository indicating a very substantial Plinian/Ultra-Plinian eruption that covered the Mulu region of Borneo with ash, a region that rarely receives tephra from even the largest known eruptions in the vicinity. It likely will be a valuable chronostratigraphic marker for sedimentary, palaeontological and archaeological studies.

Variations in welding characteristics within the Plinian air-fall deposit of the Middle Pumice eruption, Santorini, Greece

The welded Plinian air-fall deposit of the Middle Pumice A (MP-A) eruption, Santorini, Greece (144.6ka), was analysed in order to document welding characteristics and determine the factors that control welding due to the sintering of hot ash- to block-sized pyroclasts. There are vertical and lateral variations in welding intensity, with welding increasing upwards within deposit sections and decreasing laterally away from the source. Four welding grades are distinguished: a = densely-welded, b = slightly-welded, c = tack-welded, and d = non-welded. Lateral welding zones are defined by the highest welding grade observed: zone A = densely-welded (<0.25km from the source); zone B = slightly-welded (0.25–1.26km); zone C = tack-welded (1.26–3.7km), and zone D = non-welded (>3.7km). Pumice density increases (and porosity decreases) with welding, with the most densely-welded part having a density of 2290kgm−3 and a porosity of 5%. Distal, non-welded pumices have a density of 370kgm−3 and porosities of >75%. Clast oblateness varies from 0.79 in the densely-welded proximal deposit sections to 0.54 in the distal, non-welded deposits in southern Thera. Strain, determined using the Rf/ϕ method, indicates that the deposit is moderately flattened but relatively undeformed. The MP-A deposit is dacitic to andesitic in composition, becoming more mafic with stratigraphic height, where the degree of welding is highest. Welding is controlled by geochemical variations, compactional load and local variations in accumulation rate and clast sizes.

Surface age of the ice–dust mantle deposit in Malea Planum, Mars

The mid- and high-latitudes of Mars are covered by a smooth young mantle that is interpreted as an atmospherically derived air-fall deposit of ice and dust related to recent climate changes. In order to determine relative and absolute ages of this surface unit within the southern hemisphere, a systematic survey of all available HiRISE and CTX images in the Malea Planum region from 55–60°S latitude and 50–70°E longitude was performed and the distribution and the morphology of small impact craters on the mantle deposit were investigated. Using crater size-frequency measurements, we derived absolute model ages of ∼3–5Ma for the surface of the mantle, immediately south of the Hellas basin rim. Morphologic observations of the mantle, its fresh appearance, very low number of craters, and superposition on older units support this very young Amazonian age. Nearly all observed craters on the smooth mantle in Malea Planum are small and show signs of erosion, evidence for the ongoing modification of the ice–dust mantle. However, this modification has not been strong enough to reset the surface age. Compared to the ice–dust mantle at higher latitudes in the northern and southern hemisphere, the surface of the mantle in Malea Planum is older and thus has been relatively stable during obliquity changes in the last ∼3–5Ma. This is consistent with the hypothesis that the ice–dust mantle is a complex surface deposit of different layers, that shows a strong latitude dependence in morphology and has been deposited and degraded at different times in martian history.

The Toba volcanic super-eruption, environmental change, and hominin occupation history in India over the last 140,000 years

A prolonged, interdisciplinary fieldwork program was initiated in India to investigate the impact of the Toba super-eruption on terrestrial ecosystems and hominins. Fieldwork was centered on the Jurreru River Valley and the Middle Son River Valley, in southern and northern India. Archaeological sites span from 140,000 years ago through to the Holocene, providing a long-term view about hominin occupation history. Primary air-fall deposits of the Young Toba Tuff (YTT) were identified and changes in terrestrial environments prior to and after the super-eruption are indicated. Prior to the Toba eruption, drying of local ecological settings was underway. The emplacement of ash on landscapes and its subsequent erosion had local ecological and hydrological effects. Although drier conditions are indicated after the deposition of the ash deposits, a mosaic of environments was present in India, indicating that animals and hominins could have survived the volcanic event. Evidence indicates that Middle Paleolithic industries in stratified contexts are present in India prior to and after the Toba super-eruption. Middle Paleolithic hominins appear to have survived the negative effects of the volcanic eruption and climatic fluctuations in the Late Pleistocene. Middle Paleolithic industries, produced from ca. 77–38,000 years ago in India, were probably manufactured by Homo sapiens.

The formation and evolution of Hule and Río Cuarto maars, Costa Rica

The Hule and Río Cuarto maars are respectively located 11 and 18 km northward of the active crater of Poás volcano, on the Caribbean side of the Central Volcanic Range of Costa Rica. They lie on the northern part of Poás volcano massif, along a N–S trending, ~ 27 km-long volcanic fracture crossing the Poás volcano. The volcanic products from Hule maar (2.3 km × 1.8 km, area ~ 3.5 km 2) are mainly pyroclastic surges (poorly vesiculated andesites with very small plagioclases), silica-rich andesitic pumice flows, air-fall deposits, ballistic blocks, and reworked deposits that overlie the regional Pleistocene volcanic basement. They were produced during three main explosive phases. Two overlapping pyroclastic cones have developed within the Hule maar, and at least three lava fields are related to them (high-Al basalt to basaltic andesite). Another maar, Pata de Gallo (400 m across), is located less than 1 km off the SE rim of Hule. Río Cuarto is a nearly circular maar (700–850 m across) with a surface area of 0.33 km 2. Río Cuarto products include surges, ballistics and air-fall tephra, produced during three main explosive phases. These deposits show a narrow fan oriented westward, according to westerly wind direction. They indicate a westerly-directed surge (first 2 km), followed by air-fall deposits (up to 5 km away). Radiocarbon dating has shown that Hule was formed ~ 6.2 ka ago and Pata de Gallo probably formed ~ 2.8 ka ago, while the intra-maar products could have ages of ~ 1.7 ka or ~ 0.7 ka, indicating that Hule is a polygenetic maar. There are no radiocarbon ages yet for dating the formation of Río Cuarto maar, but archaeological data suggest that it erupted between 3–4 ka ago. The volume of pyroclastic deposits associated to Hule maar is estimated to be 0.51–0.53 km 3, from which ~ 20% is juvenile material, therefore 0.07–0.08 km 3 of new dense rock equivalent (DRE) magma, after subtracting 20–30% of porosity. The tephra from Río Cuarto is estimated to be 4.4 × 10 7 m 3, of which 0.008 m 3 correspond to DRE magma. The Hule and Río Cuarto maars are occupied by lakes and, in the last decades, several lake-overturn events have taken place, with a repeat cycle of six to seven years. The main outcome of these events has been the mass death of fish accompanied by changes in the lake color. In these systems, the hazard related to the possible occurrence of Nyos-type gas eruptions can be considered negligible or very local, but significant for tourists who camp by the lakes.

Reconnaissance assessment of the prospects for development of high-enthalpy geothermal energy resources, Montserrat

Abstract The Soufriere Hills Volcano on Montserrat has been in eruption since 1995. Before then, a number of hot springs and fumaroles high on the volcanic edifice attested to the presence of hydrothermal circulation systems. All of these features were buried beneath thick deposits of volcanic ash during the early years of the eruption. However, examination of low-lying sites in the vicinity of the abandoned town of Plymouth has revealed the persistence of a zone of hot (≤64 °C) groundwater discharge in a narrow valley that opens onto the northern beaches of the town. This small valley has not been significantly affected by ash deposition, with any airfall deposits having been subsequently washed downstream during periods of rain. The seepages and small springs in this zone all emit Na–Cl groundwaters, which appear from the concentrations of major ions and B and Li to be hydrothermally altered seawaters that have undergone extensive rock–water interaction at depth. Taken together with previously published volcanological observations, the persistence of this hydrothermal system throughout the sustained eruption period suggests that a robust hydrothermal circulation system exists at depths of several hundred metres in an area that closely adjoins the still-inhabited half of the island. Comparison with similar volcanogenic hydrothermal systems in the Caribbean and elsewhere suggests that there is significant potential for development of high-enthalpy geothermal energy resources in this vicinity. Using a portable well-head generating unit, a single successful borehole in this area could reasonably be expected to yield up to 5 MW of electrical power. This output significantly exceeds the current peak electricity demand on the island ( c . 3.5 MW). In concept, it would be possible to exploit any excess capacity to operate an electrolysis plant to produce hydrogen for use in vehicles on the island. Establishing energy self-sufficiency would be a major step towards the economic and social regeneration of this devastated island, with the further possibility of increasing prosperity in the future through export of surplus energy to nearby non-volcanic islands.

Characterization of volcanic geomorphology and geology by slope and topographic openness

We used a digital elevation model (DEM) of Iwate Volcano, Japan to calculate slope and topographic openness for every geological unit in that area. Further, we conducted principal component analysis (PCA) and stepwise discriminant analysis (SDA) to evaluate statistical differences in the morphometric parameters of the geological units. We categorized the geological units according to their ages and their formation processes. The categorization based on formation processes was as follows: lava-flow/lava, central cone/stratocone, air fall deposit, pyroclastic flow deposit, volcanic fan deposit, and debris avalanche deposit. PCA allowed us to select geological units suitable for analysis, while SDA was used to select some morphometric parameters, which could explain the differences in the formation processes and ages of the geological units. We found that the formation processes correspond well to mean slope, and that the geological ages that control the degree of surface dissection correspond well to the standard deviations of slope and negative topographic openness.