The fluidization behaviour of ignimbrite at high temperature and with mechanical agitation

Experiments were carried out to study the fluidization behaviour of ignimbrite at high temperature and while being mechanically agitated. Geldart group C behaviour occurs up to 565 degreesC when the material is subjected to increasing gas flow ( without agitation) from the loosely packed state. In contrast, even gentle mechanical agitation inhibits channelling and results in group-A type behaviour with homogeneous (non-bubbling) expansions of up to 30 - 40%. Bed collapse tests exhibit group-C behaviour at room temperature, group-A behaviour at 200 - 565 degreesC, and transitional behaviour at 55 degreesC. Both elevated temperature and mechanical agitation greatly increase the fluidizability of ignimbrite. It is inferred that a combination of high temperature and shear during transport will promote Geldart group A behaviour in pyroclastic flows

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

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

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

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

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

Pre-eruptive magmatic processes re-timed using a non-isothermal approach to magma chamber dynamics

Open Source PaperThis work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. The attached file is the published version of the article

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

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

Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor-sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre-existing low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro-faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor-sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved

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

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