Pyroclastic density current from the 1888 phreatic eruption of Bandai volcano, NE Japan
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Lapilli
Pyroclastic fall
Phreatomagmatic eruption
Phreatic eruption
Peléan eruption
Phreatic
Volcanic ash
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Introduction: Construed as the result of a collapsing ejecta plume since 1977 [1], the formation and emplacement of the Ries suevite was recently reinterpreted as a result of (a) ground-hugging impact melt flows [2], or (b) ‘phreatomagmatic eruptions‘ that were caused by the interaction of surficial water with an impact melt pool [3,4]. Furthermore, [5] pointed out a striking similarity between structural features in suvites and ignimbrites (compare Figs. 1 and 2). Ignimbrites are deposits of pyroclastic density currents (pyroclastic flows), a hot suspension of particles and gas driven by the collaps of an eruptive column. The deposits are composed of a poorly-sorted mixture of volcanic ash and pumice, commonly with scattered lithic fragments; various stages of welding and reomorphic flow structures can be observed [6]. They usually exhibit a fine-grained, often nonerosive, basis (surge), followed by ash layers that contain inversely graded rock fragments. Bottom-up, ignimbrites are dominated by pumice-rich ash layers, overlain by very fine-grained fall-back ashes [7]; elutriation (degassing) pipes are frequently developed at the top.
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After a precursory phreatic stage (2008 to 2010), the 2011 Shinmoe-dake eruption entered a phreatomagmatic stage on January 19, a sub-Plinian and lava accumulation stage at the end of January, a vulcanian stage in February–April, and a second phreatomagmatic stage in June–August. Component ratio, bulk composition, and particle size of the samples helped us define the eruptive stages. The juvenile particles were first found in the January 19 sample as pumice (8 vol%) and were consistently present as scoria and pumice particles thereafter (generally ~50 vol%, decreasing in weaker events). The January 19 pumice has water-quench texture. After the lava accumulation, particles of that lava origin came to account for 30~70 vol% of the ash. The second phreatomagmatic stage is proposed because of fine ash and long eruption period. The SiO2 contents of bulk ash are lower in post-January 19, 2011 eruptions, reflecting lower average SiO2 contents in 2011 ejecta than in past ejecta. The free-crystal assemblages were two pyroxenes + plagioclase + Fe-Ti oxides until 2010; olivine joined the assemblage in 2011, when juvenile ash was erupted. This change is consistent with the absence or smaller sizes of olivine phenocrysts in past ejecta.
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Abstract The 21.9 ka Okareka and 17.6 ka Rerewhakaaitu rhyolite eruption episodes began the construction of Tarawera Volcano in the Okataina Volcanic Centre, Taupo Volcanic Zone. Examination of the proximal and medial stratigraphy of these moderate‐size (c. 5 km3 magma) but poorly exposed pyroclastic deposits has increased understanding of their eruption and dispersal processes. The Okareka Tephra consists of at least nine units (A‐I), with unit A basaltic scoria at the base, overlain by the rhyolitic units B‐I. Unit C is the largest individual plinian fall deposit (c. 0.4 km3), dispersed from an eruption column that reached c. 19 km height in the presence of strong crosswinds. The other pyroclastic units record a variety of phreatomagmatic, sub‐plinian, and small ignimbrite eruptions, which were followed by extrusion of voluminous lava flows. The Rerewhakaaitu Tephra consists of 15 rhyolitic fall units A‐N. An initial short plinian phase dispersed lapillifall unit A, mostly to ENE, from columns c. 15 km in height. Units B‐D have high ash contents, indicating phreatomagmatic eruptions with varying magma/water ratios, and were widely dispersed, with lobes to the northeast and southeast. Units E‐J were deposited from 20–25 km high plinian eruption columns into strong crosswinds that dispersed tephra to the southeast. The E‐J package contains the largest tephra volume of the episode (1.18 km3) and is thought to dominate the deposits widely dispersed in Pacific Ocean sediments to the east of New Zealand. Rerewhakaaitu pyroclastic deposits are interbedded with, and underlie, voluminous lavas.
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