144 research outputs found

    VOLCANIC HAZARD ASSESSMENT AT SANTORINI VOLCANO: A REVIEW AND A SYNTHESIS IN THE LIGHT OF THE 2011-2012 SANTORINI UNREST

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    Το 2011 και το πρώτο εξάμηνο του 2012 η Σαντορίνη γνώρισε την πρώτη σεισμο-ηφαιστειακή κρίση από το 1950, όπως αυτή ανιχνεύθηκε από τα μόνιμα εγκατεστημένα δίκτυα παρακολούθησης και ένα μεγάλο αριθμό παροδικών μετρήσεων. Η διέγερση αυτή χαρακτηρίστηκε από μικρού μεγέθους, αλλά έντονη σεισμική δραστηριότητα, σημαντική ανύψωση και διόγκωση του νησιού, αλλαγές της θερμοκρασίας του νερού και των γεωχημικών αερίων. Ενώ η διέγερση έληξε την άνοιξη του 2012, το παγκόσμιο ενδιαφέρον οδήγησε στην εκπόνηση αρκετών μελετών, σε μια προσπάθεια να αξιολογηθούν τα πιθανά σενάρια για την εξέλιξη της διέγερσης. Στο πλαίσιο αυτό, συνοψίζουμε τα σημαντικότερα ευρήματα σχετικά με την επικινδυνότητα του ευρύτερου ηφαιστειακού συγκροτήματος της Σαντορίνης, καθώς και τις πληροφορίες που πρέπει να ληφθούν υπόψη για τη διαχείριση μιας πιθανής μελλοντικής ηφαιστειακής κρίσης.In 2011 and the first half of 2012 Santorini experienced its first seismo-volcanic unrest since 1950, as detected by the permanently installed monitoring networks and a large number of campaign measurements. The unrest was characterized by small magnitude but intense seismic activity, significant uplift and inflation deformation rates, and changes of water temperature as well as of fluid and soil gases. While the unrest ended in the spring of 2012, the world-wide interest led to the performance of several studies, in an attempt to assess the possible scenarios for the unrest evolution. Within this framework, we summarize the most important findings regarding the volcanic hazard assessment of the broader Santorini volcanic complex, as well as the constraints that need to be taken into account for a possible future volcanic crisis management

    Crystal-mush reactivation by magma recharge: Evidence from the Campanian Ignimbrite activity, Campi Flegrei volcanic field, Italy

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    International audienceProcesses of crystal-mush remobilization by mafic magma recharges are often related to the outpouring of largevolumes of silicic melt during caldera-forming eruptions. This occurred for the Campanian Ignimbrite (CI) erup-tion (Campi Flegrei, Italy), which produced a voluminous trachy-phonolitic ignimbrite in southern-central Italyabout 40 ka ago. We focussed on the proximal-CI deposits at San Martino that are composed of a main sequenceof early-erupted, crystal-poor units and a late-erupted (post-caldera collapse) crystal-rich Upper Pumice FlowUnit (UPFU). Detailed micro-analytical geochemical data were performed on glasses and crystals of pyroclastsfrom these deposits and coupled with Sr-Nd isotopic measurements on glasses. Results show that the CI eruptionwas fed by two distinct melts for the early-erupted units and the late UPFU, respectively. The glasses of the early-erupted units have negative Eu anomalies and show more evolved compositions and higher Nd isotope ratiosthan those of the UPFU, which have positive Eu/Eu*. The magmas of the early units formed the main volume oferuptible meltof the CI reservoir, and are interpretedashaving beenextractedfrom cumulate crystal-mushwith-out a vertical geochemical gradient within the magma reservoir. The data indicate that the generation of thedistinctive UPFU melts involved the injection of a new batch of mafic magma into the base of the CI reservoir.The mafic magma allowed heating and reactivation of the CI crystal-mush by melting of low-Or sanidines(+/−low-An plagioclases), leaving high-An plagioclases and high-Mg# clinopyroxenes as residual phases anda crystal-mush melt, made of 20% of the initial mush interstitial melt (with a composition similar to the early-erupted units) and 80% of sanidine melt. When the mush crystallinity was sufficiently reduced, the maficmagma was able to penetrate into the reactivated crystal-mush, mixing with variable proportions of crystal-mush melt and generating cooler hybrid melts, which underwent further crystallization of high-Or sanidine atvariable degrees (10–25%). Finally, possibly a short time before the eruption, the UPFU magmas were able tomix and mingle with the crystal-poor eruptible melts still persisting in the CI reservoir at the time of UPFU emis-sion. We suggest that the complex mechanisms described for the magma evolution feeding the CI eruption mayoccur whenever a crystal-mush is reactivated by new mafic magma input

    Forecasting magma-chamber rupture at Santorini volcano, Greece

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    How much magma needs to be added to a shallow magma chamber to cause rupture, dyke injection, and a potential eruption? Models that yield reliable answers to this question are needed in order to facilitate eruption forecasting. Development of a long-lived shallow magma chamber requires periodic influx of magmas from a parental body at depth. This redistribution process does not necessarily cause an eruption but produces a net volume change that can be measured geodetically by inversion techniques. Using continuum-mechanics and fracture-mechanics principles, we calculate the amount of magma contained at shallow depth beneath Santorini volcano, Greece. We demonstrate through structural analysis of dykes exposed within the Santorini caldera, previously published data on the volume of recent eruptions, and geodetic measurements of the 2011–2012 unrest period, that the measured 0.02% increase in volume of Santorini’s shallow magma chamber was associated with magmatic excess pressure increase of around 1.1 MPa. This excess pressure was high enough to bring the chamber roof close to rupture and dyke injection. For volcanoes with known typical extrusion and intrusion (dyke) volumes, the new methodology presented here makes it possible to forecast the conditions for magma-chamber failure and dyke injection at any geodetically well-monitored volcano

    Experimental study of dense pyroclastic density currents using sustained, gas-fluidized granular flows

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    © 2014, Springer-Verlag Berlin Heidelberg. We present the results of laboratory experiments on the behaviour of sustained, dense granular flows in a horizontal flume, in which high-gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base. The flows were fed by a sustained (0.5–30 s) supply of fine (75 ± 15 μm) particles from a hopper; the falling particles impacted an impingement surface at concentrations of ~3 to 45 %, where they densified rapidly to generate horizontally moving, dense granular flows. When the gas supplied through the flume base was below the minimum fluidization velocity of the particles (i.e. aerated flow conditions), three flow phases were identified: (i) an initial dilute spray of particles travelling at 1–2 m s−1, followed by (ii) a dense granular flow travelling at 0.5–1 m s−1, then by (iii) sustained aggradation of the deposit by a prolonged succession of thin flow pulses. The maximum runout of the phase 2 flow was linearly dependent on the initial mass flux, and the frontal velocity had a square-root dependence on mass flux. The frontal propagation speed during phase 3 had a linear relationship with mass flux. The total mass of particles released had no significant control on either flow velocity or runout in any of the phases. High-frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts. When the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles (i.e. fluidized flow conditions), the flows remained within phase 2 for their entire runout, no deposit formed and the particles ran off the end of the flume. Sustained granular flows differ significantly from instantaneous flows generated by lock-exchange mechanisms, in that the sustained flows generate (by prolonged progressive aggradation) deposits that are much thicker than the flowing layer of particles at any given moment. The experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick, voluminous ignimbrites

    Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera

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    Caldera-forming eruptions of silicic volcanic systems are among the most devastating events on Earth. By contrast, post-collapse volcanic activity initiating new caldera cycles is generally considered less hazardous. Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the Kameni Volcano in the southern Aegean Sea enables the eruptive evolution of a recharging multi-cyclic caldera to be reconstructed. Santorini’s eruptive record has been documented by onshore products and historical descriptions of mainly effusive eruptions dating back to 197 bce. Here we combine high-resolution seismic reflection data with cored lithologies from International Ocean Discovery Program Expedition 398 at four sites to determine the submarine architecture and volcanic history of intra-caldera deposits from Kameni. Our shore-crossing analysis reveals the deposits of a submarine explosive eruption that produced up to 3.1 km3 of pumice and ash, which we relate to a historical eruption in 726 ce. The estimated volcanic explosivity index of magnitude 5 exceeds previously considered worst-case eruptive scenarios for Santorini. Our finding that the Santorini caldera is capable of producing large explosive eruptions at an early stage in the caldera cycle implies an elevated hazard potential for the eastern Mediterranean region, and potentially for other recharging silicic calderas

    Post-depositional fracturing and subsidence of pumice flow deposits: Lascar Volcano, Chile

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    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

    Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

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    International audienceCaldera-forming volcanic eruptions are low-frequency, highimpact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales1. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained2,3. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-formingeruption of Santorini volcano,Greece4, which occurred in the late 1600s BC. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption5,6. Despite the large volume of erupted magma4 (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicicmagmabatches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems

    Readout technologies for directional WIMP Dark Matter detection

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    The measurement of the direction of WIMP-induced nuclear recoils is a compelling but technologically challenging strategy to provide an unambiguous signature of the detection of Galactic dark matter. Most directional detectors aim to reconstruct the dark-matter-induced nuclear recoil tracks, either in gas or solid targets. The main challenge with directional detection is the need for high spatial resolution over large volumes, which puts strong requirements on the readout technologies. In this paper we review the various detector readout technologies used by directional detectors. In particular, we summarize the challenges, advantages and drawbacks of each approach, and discuss future prospects for these technologies
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