The 7500 BP Socompa sector collapse emplaced 25 km 3 of fragmented rock as a thin, but widespread (500 km 2) avalanche deposit, followed by late-stage sliding of 11 km 3 as Toreva blocks. Most of the ...avalanche mass was emplaced dry, although saturation of a basal shear layer cannot be excluded. Modelling was carried out using the depth-averaged granular flow equations in order to provide information on the flow behaviour of this well-preserved, long-runout avalanche. Results were constrained using structures preserved on the surface of the deposit, as well as by deposit outline and run-up (a proxy for velocity). Models assuming constant dynamic friction fail to produce realistic results because the low basal friction angles (1 to 3.5°) necessary to generate observed runout permit neither adequate deposition on slopes nor preservation of significant morphology on the deposit surface. A reasonable fit is obtained, however, if the avalanche is assumed simply to experience a constant retarding stress of 50-100 kPa during flow. This permits long runout as well as deposition on slopes and preservation of realistic depositional morphology. In particular the model explains a prominent topographic escarpment on the deposit surface as the frozen front of a huge wave of debris reflected off surrounding hills. The result that Socompa avalanche experienced a small, approximately constant retarding stress during emplacement is consistent with a previously published analysis of avalanche data.
The 18,500 yr. b.p. Cape Riva (CR) eruption of Santorini vented several km3 or more of magma, generating four eruption units: a basal Plinian fall deposit (CR-A) and three pyroclastic flow deposits ...(CR-B to CR-D upwards). CR-B and CR-D are welded ignimbrites; CR-C consists predominantly of up to 25 m thick coarse, lithic-rich co-ignimbrite lag breccias resulting from a climactic phase of the eruption. The initial Plinian phase occurred from a localized vent in N Santorini, and subsequent column collapse resulted in emplacement of CR-B. Towards the end of CR-B, new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias (CR-C). CR-D probably records a return to a localized vent as the eruption waned. The eruption sampled a zoned magma chamber containing rhyodacite overlying andesite, and leaks of these magmas were manifested as the Skaros-Therasia lavas preceding the CR eruption. Plinian and initial ignimbrite stages occurred while the magma chamber was overpressured; subsequent underpressuring, due to magma discharge, caused fracturing of the chamber roof, caldera collapse, and eruption of pyroclastic flows from multiple vents. Activation and widening of new conduits during collapse resulted in the rapid escalation of discharge rate favoring the formation of lag breccias by: (i) promoting erosion of lithic debris at the surface vent; and (ii) raising surface exit pressures, thereby resulting in a dramatic increase in the grain size of the ejecta.
The intensive variables of dacitic-rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were ...determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850-900 degree C, fluid (H sub(2)O + CO sub(2))-saturation, XH sub(2)O = molar H sub(2)O/(H sub(2)O + CO sub(2)) between 0.6 and 1 (melt H sub(2)O contents of 2-10 wt %), and redox conditions of FMQ (fayalite-magnetite-quartz buffer) or NNO + 1 (where NNO is Ni-NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850-900 degree C and P greater than or equal to 2 kbar, under moderately reduced conditions ( Delta NNO = -0.9 to -0.1), and were poor in fluorine (500-800 ppm) and sulphur ( less than or equal to 100 ppm), but rich in water and chlorine (5-6 wt % and 2500-3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H sub(2)O, but most probably saturated with respect to H sub(2)O + Cl plus or minus CO sub(2) and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived ( greater than or equal to 200 kyr) predominantly silicic magma storage region situated at greater than or equal to 8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system.
The role of volcanogenic halogen-bearing (i.e. chlorine and bromine) compounds in stratospheric ozone chemistry and climate forcing is poorly constrained. While the 1991 eruption of Pinatubo resulted ...in stratospheric ozone loss, it was due to heterogeneous chemistry on volcanic sulfate aerosols involving chlorine of anthropogenic rather than volcanogenic origin, since co-erupted chlorine was scavenged within the plume. Therefore, it is not known what effect volcanism had on ozone in pre-industrial times, nor what will be its role on future atmospheres with reduced anthropogenic halogens present. By combining petrologic constraints on eruption volatile yields with a global atmospheric chemistry-transport model, we show here that the Bronze-Age 'Minoan' eruption of Santorini Volcano released far more halogens than sulfur and that, even if only 2% of these halogens reached the stratosphere, it would have resulted in strong global ozone depletion. The model predicts reductions in ozone columns of 20 to >90% at Northern high latitudes and an ozone recovery taking up to a decade. Our findings emphasise the significance of volcanic halogens for stratosphere chemistry and suggest that modelling of past and future volcanic impacts on Earth's ozone, climate and ecosystems should systematically consider volcanic halogen emissions in addition to sulfur emissions.
Santorini volcano in the Aegean region (Greece) is characterized by andesitic- to silicic-dominated explosive activity and caldera-forming eruptions, sourced from magmatic reservoirs located at ...various structural levels beneath the volcano. There is a good understanding of the silica-rich magmatism of the island whereas the andesite-dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood. To fill this gap we have performed crystallization experiments on a representative basalt from Santorini with the aim of determining the conditions of differentiation (pressure, temperature, volatile fugacities) and the parental magma relationship with the andesitic eruptive rocks. Experiments were carried out between 975 and 1040 degree C, in the pressure range 100-400 MPa, fO sub(2) from QFM to NNO + 3.5 (where QFM is quartz-fayalite-magnetite and NNO is nickel-nickel oxide), with H sub(2)O sub(melt) contents varying from saturation to nominally dry conditions. The results show that basalt phenocrysts within the basalt crystallized at around 1040 degree C in a magma storage reservoir located at a depth equivalent to 200-400 MPa pressure, with 3-5 wt % dissolved H sub(2)O, and fO sub(2) around QFM. Comparison with the xenocryst and phenocryst assemblages of the Upper Scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 degree C and 400 MPa, following 60-80 wt % crystallization of an ol + cpx + plag + Ti-mag + opx plus or minus pig-ilm assemblage, with melt water contents around 4-6 wt %. At Santorini, the andesitic low-viscosity and water-rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs, eventually erupting upon mixing with resident magma. Changes in prevailing oxygen fugacity may control the tholeiitic-calc-alkaline character of Santorini magmas, explaining the compositional and mineralogical differences observed between the recent Thyra and old eruptive products from Akrotiri.
Despite their global societal importance, the volumes of large-scale volcanic eruptions remain poorly constrained. Here, we integrate seismic reflection and P-wave tomography datasets with computed ...tomography-derived sedimentological analyses to estimate the volume of the iconic Minoan eruption. Our results reveal a total dense-rock equivalent eruption volume of 34.5 ± 6.8 km³, which encompasses 21.4 ± 3.6 km³ of tephra fall deposits, 6.9 ± 2 km³ of ignimbrites, and 6.1 ± 1.2 km³ of intra-caldera deposits. 2.8 ± 1.5 km³ of the total material consists of lithics. These volume estimates are in agreement with an independent caldera collapse reconstruction (33.1 ± 1.2 km³). Our results show that the Plinian phase contributed most to the distal tephra fall, and that the pyroclastic flow volume is significantly smaller than previously assumed. This benchmark reconstruction demonstrates that complementary geophysical and sedimentological datasets are required for reliable eruption volume estimates, which are necessary for regional and global volcanic hazard assessments.
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