Evidence for a large-magnitude Holocene eruption of Mount Rittmann (Antarctica): A volcanological reconstruction using the marine tephra record

The Kikai caldera volcano located under water in East China Sea is one of the most gigantic calderas in southern Kyushu. At the caldera, a violent eruption occurred from the submarine vent, at ca. 70-80 ka. The eruption is interpreted to have been phreatomagmatic throughout. Each eruptive phase of the eruption sequence generated its own characteristic deposits. The sequence of the events can be summarized as fallows ; (1) a small phreatomagmatic eruption, which generated the fine grained ash including accretionary lapilli, (2) the catastrophic pyroclastic-flow eruption, which formed a large-scale pyroclastic flow (the Nagase pyroclastic flow), two pyroclastic surges (Nishinoomote-1 member : Ns-1, Nishinoomote-3 member : Ns-3), and a wide-spread co-ignimbrite ash fall (Nishinoomote-2 member : Ns-2).The Nagase pyroclastic flow came down from the rim of the caldera, and entered the sea. Then, the flow body, which included a large amount of large pumice blocks and heavy lithic fragments, was disintegrated as gas-particle flow by violent phreatomagmatic explosions, or continued subaqueously as water-supported mass flow. Dilute and fine-particle-rich pyroclastic surges, probably with a density much less than that of water, 1.0 g/cm3, generated off the top or head of subaerial Nagase pyroclastic flow. They could cross on the smooth surface of the sea, becoming water-cooled, vaporish and depleted in large clasts which dropped into the sea. Eventually, the cool and wet pyroclastic surges attacked the islands around the caldera, and deposited as Ns-1 and Ns-3.Ns-2 co-ignimbrite ash fall, composing of glass shards were generated from the upper convective part of the eruption column of the Nagase pyroclastic flow. Included accretionary lapilli indicate that the eruption column was very moisture because of much sea water flash-out subaerially for very violent explosions from the submarine vent. Ns-2 is probably correlated with the Kikai-Tozurahara ash which was found in central Japan more than 500 km off the source.

Phreatomagmatic eruption

Pyroclastic fall

Peléan eruption

Caldera

10.5026/jgeography.97.3_156

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Citations (2)

Stratigraphy, chronology, and character of the 1976 pyroclastic eruption of Augustine volcano, Alaska

Bulletin of Volcanology (1991)

Hiroki Kamata David A. Johnston Richard B. Waitt

Peléan eruption

Pyroclastic fall

Dense-rock equivalent

Pumice

Effusive eruption

Volcanic ash

Phreatic eruption

Lava dome

Volcanic hazards

Phreatomagmatic eruption

Lateral eruption

10.1007/bf00258182

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Citations (10)

Monitoring of Eruptive Products: Deposits Associated with Pyroclastic Fallout

Lucia Gurioli Alessandro Tadini Simon Thivet Étienne Médard Carole Berthod

"Pyroclastic fallout" is the process of fallout of the particles, which is one of the most common processes in volcanology and is generally associated with all types of explosive eruptions. This chapter shows how the study and monitoring of pyroclastic fallout products play a key role in volcanic risk assessment. The pyroclastic fallout process is, in its simplest formulation, the sedimentation of pyroclasts through the atmosphere and their deposition on the Earth's surface. For fallout deposits, the subdivision into proximal, medial or distal deposits depends on the size of the eruption considered. During eruptive crises a sampling of the eruptive products is generally carried out in the hours following the beginning of each eruption. Geochemical and petrographic analysis of pyroclasts can constrain the initial conditions from the magma chamber to the surface via the conduit. Total grain size distribution represents the theoretical eruptive mixture injected into the atmosphere during volcanic explosive eruptions.

Pyroclastic fall

Peléan eruption

Volcanology

Deposition

Phreatomagmatic eruption

10.1002/9781394173730.ch2

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Volcanic Eruption and Nature of the Pyroclastic Deposits

The Quaternary Research (Daiyonki-Kenkyu) (1963)

Kazuaki Nakamura Shigeo Aramaki Isamu Murai

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.

Peléan eruption

Pyroclastic fall

Strombolian eruption

Caldera

Lava dome

Dense-rock equivalent

Tephrochronology

Volcanic plateau

Phreatomagmatic eruption

Effusive eruption

10.4116/jaqua.3.13

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