238 research outputs found
Peralkaline felsic magmatism at the Nemrut volcano, Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV
Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO2 close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas
Towards a dendrochronologically refined date of the Laacher See Eruption
The precise date of the Laacher See eruption (LSE), central Europe’s largest Late Pleistocene volcanic event that occurred around 12,900 years ago, is still unknown. Here, we outline the potential of combined high-resolution dendrochronological, wood anatomical and radiocarbon (14C) measurements, to refine the age of this major Plinian eruption. Based on excavated trees that were killed during the explosive LSE and buried under its pyroclastic deposits, we describe how a firm date of the eruption might be achieved, and how the resulting temporal precision would further advance our understanding of the environmental and societal impacts of this event. Moreover, we discuss the relevance of an accurate LSE date for improving the synchronization of European terrestrial and lacustrine Late Glacial to Holocene archives, and outline how the proposed, interdisciplinary dating approach can be applied to other large, yet undated, volcanic eruptions
Preserving the impossible: conservation of soft-sediment hominin footprint sites and strategies for three-dimensional digital data capture.
Human footprints provide some of the most publically emotive and tangible evidence of our ancestors. To the scientific community they provide evidence of stature, presence, behaviour and in the case of early hominins potential evidence with respect to the evolution of gait. While rare in the geological record the number of footprint sites has increased in recent years along with the analytical tools available for their study. Many of these sites are at risk from rapid erosion, including the Ileret footprints in northern Kenya which are second only in age to those at Laetoli (Tanzania). Unlithified, soft-sediment footprint sites such these pose a significant geoconservation challenge. In the first part of this paper conservation and preservation options are explored leading to the conclusion that to 'record and digitally rescue' provides the only viable approach. Key to such strategies is the increasing availability of three-dimensional data capture either via optical laser scanning and/or digital photogrammetry. Within the discipline there is a developing schism between those that favour one approach over the other and a requirement from geoconservationists and the scientific community for some form of objective appraisal of these alternatives is necessary. Consequently in the second part of this paper we evaluate these alternative approaches and the role they can play in a 'record and digitally rescue' conservation strategy. Using modern footprint data, digital models created via optical laser scanning are compared to those generated by state-of-the-art photogrammetry. Both methods give comparable although subtly different results. This data is evaluated alongside a review of field deployment issues to provide guidance to the community with respect to the factors which need to be considered in digital conservation of human/hominin footprints
Walking through volcanic mud: the 2000 year-old Acahualinca footprints (Nicaragua) I. Stratigraphy, lithology, volcanology and age of the Acahualinca section
500,000 Years of Environmental History in Eastern Anatolia: The PALEOVAN Drilling Project
International Continental Scientific Drilling Program
(ICDP) drilled a complete succession of the lacustrine sediment
sequence deposited during the last ~500,000 years in
Lake Van, Eastern Anatolia (Turkey). Based on a detailed
seismic site survey, two sites at a water depth of up to 360 m
were drilled in summer 2010, and cores were retrieved from
sub-lake-floor depths of 140 m (Northern Basin) and 220 m
(Ahlat Ridge). To obtain a complete sedimentary section, the
two sites were multiple-cored in order to investigate the paleoclimate
history of a sensitive semi-arid region between the
Black, Caspian, and Mediterranean seas. Further scientific
goals of the PALEOVAN project are the reconstruction of
earthquake activity, as well as the temporal, spatial, and
compositional evolution of volcanism as reflected in the deposition
of tephra layers. The sediments host organic matter
from different sources and hence composition, which will be
unravelled using biomarkers. Pathways for migration of continental
and mantle-derived noble gases will be analyzed in
pore waters. Preliminary 40Ar/39Ar single crystal dating of
tephra layers and pollen analyses suggest that the Ahlat
Ridge record encompasses more than half a million years of
paleoclimate and volcanic/geodynamic history, providing
the longest continental record in the entire Near East to
date
Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions
This work is a contribution to the Natural Environment Research Council (NERC) funded RiftVolc project (NE/L013932/1, Rift volcanism: past, present and future) through which several of the authors are supported. In addition, Clarke was funded by a NERC doctoral training partnership grant (NE/L002558/1).Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. The dynamics of peralkaline rhyolite eruptions remain elusive with no direct observations recorded, significantly hindering the assessment of hazard and risk. Here we describe uniquely-preserved, fluidal-shaped pyroclasts found within pumice cone deposits at Aluto, a peralkaline rhyolite caldera in the Main Ethiopian Rift. We use a combination of field-observations, geochemistry, X-ray computed microtomography (XCT) and thermal-modelling to investigate how these pyroclasts are formed. We find that they deform during flight and, depending on size, quench prior to deposition or continue to inflate then quench in-situ. These findings reveal important characteristics of the eruptions that gave rise to them: that despite the relatively low viscosity of these magmas, and similarities to basaltic scoria-cone deposits, moderate to intense, unstable, eruption columns are developed; meaning that such eruptions can generate extensive tephra-fall and pyroclastic density currents.Publisher PDFPeer reviewe
Post-depositional fracturing and subsidence of pumice flow deposits: Lascar Volcano, Chile
Unconsolidated pyroclastic flow deposits of the
1993 eruption of Lascar Volcano, Chile, have, with time,
become increasingly dissected by a network of deeply
penetrating fractures. The fracture network comprises
orthogonal sets of decimeter-wide linear voids that form a
pseudo-polygonal grid visible on the deposit surface. In this
work, we combine shallow surface geophysical imaging
tools with remote sensing observations and direct field
measurements of the deposit to investigate these fractures
and their underlying causal mechanisms. Based on ground
penetrating radar images, the fractures are observed to have
propagated to depths of up to 10 m. In addition, orbiting radar interferometry shows that deposit subsidence of up to
1 cm/year occurred between 1993 and 1996 with continued
subsidence occurring at a slower rate thereafter. In situ
measurements show that 1 m below the surface, the 1993
deposits remain 5°C to 15°C hotter, 18 years after
emplacement, than adjacent deposits. Based on the observed
subsidence as well as estimated cooling rates, the fractures are
inferred to be the combined result of deaeration, thermal
contraction, and sedimentary compaction in the months to
years following deposition. Significant environmental factors,
including regional earthquakes in 1995 and 2007, accelerated
settling at punctuated moments in time. The spatially variable
fracture pattern relates to surface slope and lithofacies
variations as well as substrate lithology. Similar fractures
have been reported in other ignimbrites but are generally
exposed only in cross section and are often attributed to
formation by external forces. Here we suggest that such
interpretations should be invoked with caution, and deformation
including post-emplacement subsidence and fracturing of
loosely packed ash-rich deposits in the months to years postemplacement
is a process inherent in the settling of pyroclastic
material
Correlating the Holocene records: Icelandic tephra found in Schleswig-Holstein (Northern Germany)
Stratigraphy and textural characteristics of the 1982–83 tephra of Galunggung volcano (Indonesia): implications for volcanic hazards
Morphology of the Faial Island shelf (Azores): the interplay between volcanic, erosional, depositional, tectonic and mass-wasting processes
[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.publishe
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