Sedimentological analysis of the tephra from the 12–15 August 1991 eruption of Hudson volcano
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Pumice
Lapilli
Dense-rock equivalent
Volcanic ash
Phreatomagmatic eruption
Volcanic glass
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Silicic
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Kutcharo caldera is situated in the eastern part of Hokkaido, Japan. It is subcircular with a diameter of 20-26 km and was formed by repeated violent rhyolitic explosive eruptions from 340 to 35 ka. The caldera has three post-caldera volcanoes: Atosanupuri, Nakajima and Mashu. Our new tephrostratigraphical survey suggests that a pyroclastic fall deposit (Nakajima pumice), which was extruded from Nakajima, is exposed at the western slope of Atosanupuri. The deposit comprises dacitic pumice clasts (< 9 cm across) in a fine-grained matrix. The pumice clasts are coated with fine-grained ash, suggesting the deposit was produced by a phreatomagmatic eruption. The deposit directly overlies a scoria-fall deposit, which was ejected from Mashu volcano at 17-12 ka, and is overlain by Ma-k tephra, which was extruded from Mashu volcano at 10 ka. The stratigraphy of the deposit suggests that a phreatomagmatic eruption occurred at Nakajima volcano between 17 and 10 ka.
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Caldera
Lapilli
Scoria
Tephrochronology
Volcanic ash
Dense-rock equivalent
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Pumice
Caldera
Silicic
Peléan eruption
Lapilli
Dense-rock equivalent
Volcanic ash
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In this thesis, the internal stratigraphy, facies, and facies architecture of the rhyodacitic 184 ka Lower Pumice 1 (LP1) and 172 ka Lower Pumice 2 (LP2) eruption sequences, erupted from Santorini Caldera, Greece, are reassessed as the basis for understanding the initiation and development of caldera-forming Plinian eruptions, including the changing conditions of the magmatic system, the conduit and eruption processes, the transition between eruption phases and ultimately the evolution of the vent system. Key findings from the study of these eruption sequences include the identification of complex precursory eruption phases, evidence for diverse histories of degassing and magma ascent, and for rapid transitions between eruption phases (e.g., precursor to Plinian, plume to flow and catastrophic caldera collapse).
184 ka Lower Pumice 1 eruption
The stratigraphy of the LP1 eruption defines two main eruption phases, including an initial precursor phase and a major Plinian phase. The precursor deposit sequence of the LP1 eruption (LP1-Pc) consists of 13 internal stratigraphic subdivisions that incorporate four major lithofacies associations. These include a series of pumice lapillistone deposits, interpreted as fallout of pumice from a buoyant eruption column (LP1-Pc-a1, 2, 3, 4), a finely-laminated tuff, interpreted as a pyroclastic surge deposit (LP1-Pc-b), a sequence of interbedded tuffs and pumice lapillistone horizons, interpreted as phreatomagmatic ash fallout/surge and pumice fallout deposits (LP1-Pc-c1, 2, 3, 4), respectively, and a series of incipiently bioturbated, matrix-supported, lapilli-tuffs, interpreted as phreatomagmatic ignimbrites (LP1-Pc-d1, 2, 3, 4). Incipient bioturbation, variability in unit thickness and lithology in LP1-Pc, attest to multiple time breaks, sufficient for insects and other burrowing organisms to colonise and plant roots to grow. The vesicularity characteristics (collapsed vesicle textures) of dacitic pumice pyroclasts from LP1-Pc-d support these conclusions, and suggests stalling or slow magma rise rates prior to magma evacuation. An interpreted repose period of up to several months preceded the Plinian phase, which commenced with the development of a buoyant convective eruption column (LP1-A1, 2, 3). Vent widening and the increased incorporation of lithics into the eruption column resulted in eruption column collapse and the development of valley-confined pyroclastic flows (LP1-B1, 2, 3-1, 3-2, 4). Lithic-rich lag breccias, which cap the sequence, are indicative of a late-stage catastrophic caldera collapse event (LP1-C1, 2, 3, 4, 5, 6). The vesicularity characteristics of pumice and occurrence of basement-derived lithic assemblages in LP1-C1 (when compared to LP1-B4), indicate a rapid increase in decompression (doubling of decompression rate from 7.4-10.5 MPasˉ¹ during LP1-B to 15.3-28.4 MPasˉ¹ at the onset of LP1-C) and a deepening of the fragmentation surface at the onset of caldera collapse. This data supports previously unsubstantiated numerical models which predict rapid increases in decompression and mass discharge rates at the onset of caldera collapse. We suggest that lithic-lag breccias are indicative of vertical or steeply inwards dipping faults that initially hinder roof block collapse. However, once fractures propagate to the Earth’s surface, sudden decompression of magma system, and deepening of the fragmentation surface because of this, triggers explosive widening of the fractures as vents, so allowing roof block collapse.
172 ka Lower Pumice 2 eruption
The stratigraphy of LP2 eruption records two major eruption phases, including an initial precursor event followed by a major Plinian phase. The precursor deposit (LP2-A1) is characterised by a clast-supported, diffusely-stratified framework of rhyodacitic white pumice, reflecting the development of a short-lived volcanic plume. These deposits pass conformably upwards into a massive, moderately to well-sorted, clast-supported, fallout deposit (LP2-A2, 3), reflecting the development of a 36 km high Plinian eruption column. The vesicularity characteristics of rhyodacitic pumice, in addition to the progressive increase in volcanic and basement lithic components vertically within the stratigraphy of LP2-A2, A3, attest to a progressive increase in vent diameter, a deepening of the fragmentation surface and an increase in decompression rate from 13.7 to 18.0 MPasˉ¹ throughout the Plinian phase. Continued vent widening and the inability to effectively transfer air into the eruption column resulted in column collapse and the development of pyroclastic flows. The transition in the stratigraphy from diffusely-stratified to massive pyroclastic flow deposits reflects the progressive increase in depositional and mass discharge rate throughout the eruption phase (LP2-B). The incorporation of water into the magmatic system, possibly associated with incipient caldera collapse or a major vent excavation event, facilitated phreatomagmatism and the development of widespread pyroclastic flows (LP2-C). Late-stage caldera collapse resulted in the explosive breakup of the reservoir roof block, a deepening of the fragmentation surface and an enlargement of the pre-existing LP1 caldera, producing lithic-rich pyroclasts flows and depositing lithic lag breccias (LP2-D).
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Caldera
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At Kawah Ijen (Indonesia), vigorous SO2 and HCl degassing sustains a hyperacid lake (pH ~0) and intensely alters the subsurface, producing widespread residual silica and advanced argillic alteration products. In 1817, a VEI 2 phreatomagmatic eruption evacuated the lake, depositing a widespread layer of muddy ash fall, and sending lahars down river drainages. We discovered multiple types of opaline silica in juvenile low-silica dacite pumice and in particles within co-erupted laharic sediments. Most spectacular are opal-replaced phenocrysts of plagioclase and pyroxene adjacent to pristine matrix glass and melt inclusions. Opal-bearing pumice has been found at numerous sites, including where post-eruption infiltration of acid water is unlikely. Through detailed analyses of an initial sampling of 1817 eruption products, we find evidence for multiple origins of opaline materials in pumice and laharic sediments. Evidently, magma encountered acid-altered materials in the subsurface and triggered phreatomagmatic eruptions. Syn-eruptive incorporation of opal-alunite clasts, layered opal, and fragment-filled vesicles of opal and glass, all suggest magma-rock interactions in concert with vesiculation, followed by cooling within minutes. Our experiments at magmatic temperature confirm that the opaline materials would show noticeable degradation in time periods longer than a few tens of minutes. Some glassy laharic sedimentary grains are more andesitic than the main pumice type and may represent older volcanic materials that were altered beneath the lake bottom and were forcefully ejected during the 1817 eruption. A post-eruptive origin remains likely for most of the opal-replaced phenocrysts in pumice. Experiments at 25°C and 100°C reveal that when fresh pumice is bathed in Kawah Ijen hyperacid fluid for six weeks, plagioclase is replaced without altering either matrix glass or melt inclusions. Moreover, lack of evidence for high-temperature annealing of the opal suggests that post-eruption alteration of pumice is more likely than pre-eruption envelopment of euhedral opal-replaced phenocrysts in dacitic melt. At Ijen and elsewhere, the ascent of magma into hydrous acid-altered mineral assemblages (e.g., opal, kaolinite, alunite) could induce rapid dehydration of hydrous minerals and amorphous materials, generating considerable steam and contributing to magmatic-hydrothermal and phreatomagmatic explosions.
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Volcanic glass
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Phreatic eruption
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Phreatomagmatic eruption
Lapilli
Caldera
Dense-rock equivalent
Magma chamber
Strombolian eruption
Scoria
Silicic
Peléan eruption
Phreatic eruption
Maar
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Kolumbo submarine volcano, located 7 km northeast of Santorini in the Aegean Sea, last erupted in 1650 AD resulting in significant coastal destruction from tsunamis and about 70 fatalities on nearby Thera from gas discharge. Pyroclastic materials were reported as far south as Crete and as far northeast as Turkey. Tephra from the 1650 AD submarine eruption has been correlated in sediment box cores using a combination of mineralogy and major element composition of glass shards. The biotite-bearing rhyolite of Kolumbo can be readily discriminated from other silicic pyroclastics derived from the main Santorini complex. In general, the tephra deposits are very fine-grained (silt to fine sand), medium gray in color, and covered by ~10 cm of brown hemipelagic sediment. This corresponds to an average background sedimentation rate of 29 cm/kyr in the area. The distribution of the 1650 AD Kolumbo tephra covers at least 446 km2 around the crater, nearly 5 times the approximated 97 km2 previously inferred from seismic profiles on the volcano’s slopes and in adjacent basins. Despite the expansion of the inferred deposition area, the estimated eruption volume is not enlarged significantly, and therefore remains a minimum estimate, because the box cores did not penetrate the bases of the tephra units. SEM images reveal particle morphologies attributed to multiple fragmentation mechanisms, including primary volatile degassing and phreatomagmatic activity. It is likely that phreatomagmatic activity became more important in the latter stages of the eruptive sequence when eruptions column broke the surface and a small ephemeral island was formed.
Phreatomagmatic eruption
Pyroclastic fall
Pumice
Lapilli
Submarine volcano
Silicic
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Volcanic glass
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Volcano represent channel of systems fluid (lava), which has a depth up to 10 km from the earth surface. One ofthe active volcanoes is Mount Rinjani which recorded the eruption was 9 times from 1846 to 1994. The result ofthe eruption of Mount Rinjani is pyroclastic rocks dominated by pumice, which accumulated at lot of areas ofresearch that will be determined by the density value calculation method. The resulting rock density values canbe used to see the spread of the volcanic eruption material with simulation software based on data Hazmaperuption in 1994. The result of this research is the density of pumice and simulated the spread of the eruption ofMount Rinjani 1994. The density of pumice is about 693 kg/m3 and deployment simulation shows the distributionof the eruption of Rinjani to the diameter size of the fine dust (<1/16 mm) spread towards the Northwest (NorthLombok) with total mass about 6,38x109 kg and diameter size of lapilli (2-10) mm spread around the center ofthe eruption (Mount Rinjani) with total mass about 5, 16x109 kg.
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Dense-rock equivalent
Lateral eruption
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Volcanology
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The explosive phase of the 1979 Soufriere eruption produced 37.5 x 10(6) cubic meters (dense-rock equivalent) of tephra, consisting of about 40 percent juvenile basaltic andesite and 60 percent of a nonjuvenile component derived from the fragmentation of the 1971-1972 lava island during phreatomagmatic explosions. The unusually fine grain size, poor sorting, and bimodality of the land deposit are attributed to particle aggregation and the formation of accretionary lapilli in a wet eruption column.
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Strombolian eruption
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The 2005–6 eruption of Augustine Volcano produced tephra-fall deposits during each of four eruptive phases. Late in the precursory phase (December 2005), small phreatic explosions produced small-volume, localized, mostly nonjuvenile tephra. The greatest volume of tephra was produced during the explosive phase (January 11–28, 2006) when 13 discrete Vulcanian explosions generated ash plumes between 4 and 14 km above mean sea level (asl). A succession of juvenile tephra with compositions from low-silica to high-silica andesite is consistent with the eruption of two distinct magmas, represented also by a low-silica andesite lava dome (January 13–16) followed by a high-silica andesite lave dome (January 17–27). On-island deposits of lapilli to coarse ash originated from discrete vent explosions, whereas fine-grained, massive deposits were elutriated from pyroclastic flows and rock falls. During the continuous phase (January 28–February 10, 2006), steady growth and subsequent collapses of a high-silica andesite lava dome caused continuous low-level ash emissions and resulting fine elutriate ash deposits. The emplacement of a summit lava dome and lava flows of low-silica andesite during the effusive phase (March 3–16, 2006) resulted in localized, fine-grained elutriated ash deposits from small block-and-ash flows off the steep-sided lava flows. Mixing of two end-member magmas (low-silica and highsilica andesite) is evidenced by the overall similarities between tephra-fall and contemporaneous lava-dome and flow lithologies and by the chemical heterogeneity of matrix glass compositions of coarse lapilli and glass shards in the ash-size fraction throughout the 2005–6 eruption. A total mass of 2.2×1010 kg of tephra fell (bulk volume of 2.2×107 m3 and DRE volume of 8.5×106 m3) during the explosive phase, as calculated by extrapolation of mass data from a single Vulcanian blast on January 17. Total tephra-fall volume for the 2005–6 eruption is about an order of magnitude smaller than other historical eruptions from Augustine Volcano. Ash plumes of short duration and small volume caused no more than minor amounts (≤1 mm) of ash to fall on villages and towns in the lower Cook Inlet region, and thus little hazard was posed to local communities. The bulk of the ash fell into Cook Inlet. Monitoring by the Alaska Volcano Observatory during the eruption helped to prevent hazardous encounters of ash and aircraft.
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Effusive eruption
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