New estimates of the 1815 Tambora eruption volume
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Peléan eruption
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Caldera
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Many timed observations make it possible to subdivide the 9‐hour Plinian eruption of Mount St. Helens on May 18, 1980, into six phases, defined by eruption style. The phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits. The suite of pyroclastic deposits indicates that the eruption became more pumice‐rich and compositionally diverse with time, perhaps owing to concurrent eruption of less evolved, gas‐poor parts of the magma body with the more evolved, gas‐rich parts. The paroxysmal phase I (0832–0900) consisted of landslides, lithic pyroclastic flows of a lateral blast and other explosions, and a weak pre‐Plinian column. Phase I pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer. The early Plinian phase II (0900–1215) consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks (1010–1035), a brief period of lithic ash ejection (1035–1100), and a pumice‐rich pulse that accompanied growth in height of the eruption column (1100–1215). Deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer, the lower lithic ash layer, and the middle pumice lapilli layer, all of which consist of evolved white dacitic pumice (63–64% SiO 2 ). During the early ash flow phase III (1215–1500) the height of the eruption column decreased, vertical ejection of tephra ceased, and pyroclastic flows were fed from intermittent fountains. Phase III deposits consist of a poorly exposed sequence (.≤12 m) of ash flow tuff that consist of many thin flow units (≤2 m each) containing pumiceous white dacite (63–64% SiO 2 ) and denser, gray silicic andésite (61–62% SiO 2 ), and fine‐grained ash cloud deposits interbedded with a nongraded middle pumice ash layer. The climactic phase IV (1500–1715) developed in two stages: fountain‐fed pyroclastic flows, followed by a short pulse (1625–1715) of vigorous vertical ejection of tephra. These stages were accompanied by the peak seismic energy release and peak eruption column height, respectively. Climactic deposits consist of a thick (≤35 m) sequence of thick, lapilli‐rich ash flow sheets (4–12 m each) with white and gray pumice, and streaky scoria bands (60% SiO 2 ) in pumice breccia clasts, and the reversely‐graded, upper pumice lapilli layer that is interbedded with fine‐grained ash cloud deposits. During the late ash flow phase V (1715–1815) eruption intensity waned but included a brief episode of small pyroclastic flows (1745–1815). Phase V deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice, and minor fine‐ash deposits. Phase VI activity (1815 to May 19, 1980) consisted of a low‐energy ash plume, with transient increases in intensity, while seismicity continued at depth. Sustained vertical discharge of phase II prodeced evolved dacitewith high S/Cl ratios. Ash flow activity of phase III is attributed to decreases in gas content, indicated by reduced S/Cl ratios and increased clast density of the less evolved, gray pumice. Climactic events are attributed to vent clearing and exhaustion of the evolved dacite.
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The mechanisms controlling the transport of pyroclasts in a plinian-type eruption column are discussed. An estimate is made of the density of the magmatic gas in the vent, and initial (‘muzzle’) velocities are deduced for 18 eruptions using the a real distribution of pyroclasts in the resulting air-fall deposits. A model of the physical properties of the lower part of an eruption column is presented, and used to deduce the heights to which plinian eruption columns should commonly extend. It is demonstrated that column collapse to form ignimbrites may be a common result of the change, with time, of the volatile content of the erupted material as progressively deeper levels in a magma chamber are tapped. Vent radii are estimated for those eruptions for which the duration of the eruption is known.
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Peléan eruption
Pyroclastic fall
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On 13 to 14 February 2014 a ~4 h long, VEI 4 eruption occurred at Kelut volcano (Java, Indonesia). Pyroclastic density currents (PDCs) and extensive ash fall led to 7 fatalities and disruption to flights across the Asia-Pacific region. New sedimentological descriptions of the pyroclastic deposits from the 2014 eruption were compared with eyewitness and satellite reports to elucidate temporal variations in eruptive dynamics. The stratigraphy of the deposits is presented in 3 stages, associated with two eruptions that occurred approximately ~15–30 min apart. Stage 1 PDC deposits originate from the smaller onset eruption. The PDC deposits from Stage 2, and tephra fall deposits from Stage 3 originate from the second, plinian eruption. During the onset eruption (Stage 1), low energy PDCs were produced that ran out to <2.6 km. Basal layers show characteristics of deposits similar to ash-cloud surges that carried dominantly fine ash and crystal fragments. These are capped by deposits typical of high-particle concentrated pyroclastic flows. All Stage 1 deposits have high contents of dense lithic fragments (up to 44% by vol.), sourced from the 2007–2008 lava dome and conduit walls, indicating that the eruption onset was driven by an explosive release of gas-overpressure below the vent-capping dome. Increases in the magma flux and transition to a more constant eruption led to a growing eruption column during the ~2-hour long Stage 2 plinian eruption. Pumice rich (>70% by vol.) PDC deposits ran out to 4.7 km from the vent. The deposits reflect an increased output of fresh fragmented magma, and some conduit widening evidenced by dense lithic fragments. Vent instabilities and incorporation of dense material into basal margins of the plume led to the marginal collapse and formation of these PDCs. Stage 3 occurred in the final hour at the peak of the plinian eruption, around 01:00 to 02:00, and produced reversely graded lapilli fall deposits with ≤90 vol% pumice from a 26 km-high plume. This indicates that there was a sustained flux of juvenile magma to the now open vent system, and expansion and fragmentation of the gas-rich magma was at its most efficient. Our study of the eruptive sequence of Kelut provides some constraints on predicted patterns for future explosive activity, critical for further hazard assessment of the volcano. Since 1901 Kelut has erupted on intervals of 1 to 23 years, and the 2014 event characterises a typical “explosive” style of eruption that alternates regularly with effusive dome-formation and collapse events. This pattern depends on the dynamics of magma renewal to the system, and degassing conditions in the shallow magma reservoir and upper conduit. If this pattern holds, and the next eruption occurs within the next two decades, a return of prolonged dome growth could be anticipated.
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This article deals with the mechanism of eruption and transportation of the pyroclastic material and the nature of the resultant deposits from the geological standpoint.In Japan, the method of tephrochronology is best applied to pyroclastic deposits of the Quaternary central volcanoes and those related to the Krakatoan calderas. Most of the rocks are andesitic in composition with subordinate amount of basalt and dacite.Three modes of volcanic eruption may be distinguished: 1) projection of pyroclastic materials which form pyroclastic fall deposits, 2) eruption of pyroclastic flows, and 3) outflow of lava flows or extrusion of dome and spine. Table 1 shows characteristic features of the deposits formed by the three modes of volcanic eruption.Tephra, as originally defined by Thorarinsson, signifies only the air-fall pyroclastic materials and its relation to pyroclastic flow is not clear. In this article, all the pyroclastic materials directly connected with volcanic eruptions, irrespective of their origin (i. e. essential, accessory, or accidental) and of their mode of emplacement, are included in the term tephra. The chronology using the deposits of pyroclastic flows are included in the tephrochronology.The small-scale vesiculation occurring at or close to the top of the magma column results in the so-called Strombolian and Vulcanian eruptions. Larger scale vesiculation with longer time duration leads to the Plinian eruption. The greatest vesiculation takes place within the magma reservoir resulting in the formation of a depression caldera. The larger the size of eruption column, the more effective the sorting of the erupted pyroclastic fragments. The larger and denser particles fall first and closer to the vent while the smaller and more vesicular fragments fall farther away. Consequently the deposits of pyroclastic falls are well sorted and exhibit pronounced lateral regular grading in texture and composition. This is in strong contrast with the poorly sorted character of pyroclastic flow deposits, in which all particles travel en masse in a state of turbulent flow.Welding of the deposit is not uncommon in the pyroclastic flow deposits while it is rare in pyroclastic fall deposits except those deposited near the vents of basaltic eruptions.To reconstruct past eruptions from volcanic deposits, it may be necessary to establish definite correlation between stratigraphic units by which volcanic deposits are grouped and time duration by which specific eruptive activity is grouped. A single eruptive cycle, the deposits of which represent such a time unit, is defined as a series of eruptive events limited by fairly long intervals of quiescence. Historic examples indicate that the duration of a single eruptive cycle ranges from a day to several years in most cases. The intervening periods are generally far longer than the duration of single eruptive cycle.From many examples of single eruptive cycles, a rule has been established: the degree of vesiculation of magma gradually decreases toward the end of the cycle. This is expressed in successive eruption of pyroclastic fall, pyroclastic flow, and lava flow from the same vent in case of felsic magma, and of pyroclastic fall and lava flow in case of mafic magma, which fact may indicate that the original magma column responsible for the eruptive cycle was more enriched in volatiles in the upper part than the lower.The close correlation between the recorded sequence of single eruptive cycles and the reultant beds of volcanic materials is described for a few examples. The beds produced by a single cycle of witnessed eruption conformably superpose each other and do not include a layer representing weathering break. It is stressed that such a group of beds of volcanic ejecta, volcanic deposits of a single eruptive cycle, should be taken into account as a stratigraphic unit when precise tephrochronology is undertaken.
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Shikotsu volcano is a caldera volcano in southwestern Hokkaido. It is considered that a large-scale eruption started at ca. 60 ka (named as 60 ka Shadai eruption), mainly ejecting scoria fall and scoria flow deposits. The volcano repeated explosive eruptions every several thousand years, and after 10 ky of dormancy, a caldera-forming eruption took place at ca. 46 ka. Trench and boring surveys were carried out in the eastern part of Shikotsu caldera, and stratigraphy and changes in the components of the 60 ka Shadai eruption were reexamined. There are three types of juvenile materials in 60 ka eruption deposits: dacitic pumice, olivine-bearing andesitic scoria, and intermediate banded/gray pumice. As a result, tephra layers of the eruption are mainly classified into three units: Units A to C in ascending order. Soils and volcanic ash soils were not discovered among these layers, so these must derive from a single eruption sequence. Unit A consists of pumice fall deposits and ash fall deposits; Unit B of scoria fall deposits; and Unit C comprises a pyroclastic flow deposit and a following pyroclastic fall deposit. Each eruptive unit is subdivided into A1-A3, B1-B5 and C1-C2. Based on these eruptive units, the sequence of the 60 ka Shadai eruption is constructed as follows: Phase 1 was a pumiceous plinian eruption (A1, A2), and eruption rate abruptly decreased in A3. Phase 2 was a scoriaceous plinian eruption (B1-B5). Eruption rate was unstable in early Phase 2 (B1-B4); however, it gradually increased in late Phase 2 (intermittent eruption had repeated in B5). Consequently, a pyroclastic flow eruption occurred in Phase 3 (C1), followed by a plinian eruption (C2). Andesitic and mixed magmas were found to begin to erupt from Phase 2, while dacitic magma survived through the 60 ka Shadai eruption. These characteristics are difficult to explain based on the eruption from a zoned magma chamber or from a common eruptive vent. Dacitic and andesitic magmas are suggested to have existed in separated magma chambers, and dacitic magma at least was supplied through an independent vent system.
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