Correlation of submarine deposits and witness accounts of the 1952 Myojinsho submarine eruption, Izu-Bonin arc, by bathymetric survey
Abstract
The relationship between eruptive phenomena during the 1952 phreatomagmatic eruption and consequent deposits under seawater is discussed, on the basis of bathymetric survey of Myojinsho volcano, Izu-Bonin island arc (32°55’N, 140°00’E). We carried out some research cruises by the ship Natsushima (JAMSTEC) in 2006-2008. We used unmanned bathymetric vehicle, Hyperdolphin, for observation and sampling of the submarine deposit. Myojinsho volcano is an active post-caldera volcano that grew on the northeastern rim of Myojinsho caldera (8 x 6 km in diameter). There have been many reports of colored seawater, and we also recognized a bubbly column above the summit of Myojinsho (Myojin reef; ca. 50 m below sea level) during acoustic survey. The 1952 eruption was the latest eruption that formed and destroyed new island above sea level, and would have been the first submarine eruption to be recorded by good scientific standard. This eruption was firstly recognized at the middle of September, 1952, and explosions and dome growth have been documented for about 1 year till the middle of September, 1953. There are many time series color photographs of explosions so that we can evaluate these explosions quantitatively (Ossaka, 1991). These records indicate that most of the cock’s tail jets are limited within the proximal area (ca. 500m) from the center of explosions, whereas the base surges and lateral steam clouds reach farther than ca. 500 m. During the bathymetric survey, we found several small lobes of pumice-rich deposits (< 1 m thick, several meter wide, several tens of meter long) on the sandy flat slope father than ca. 500 m from the summit of Myojinsho (deeper than ca. 300 m below sea level). Large pumices are concentrated at the front of each lobe, and the lobes become thinner toward the summit. On the other hand, the proximal deposit (<500 m from the summit) is characterized by scattered large angular blocky rocks or pumices. The largest blocks are as large as several meters in diameter. The compositional variations and textural features of these pumices and blocks are similar to those reported for samples obtained during the eruption. The correspondence of the lateral changes in witnessed phenomena (cock’s tail jets to base surges) and in depositional features (scattered blocks to pumice lobes) would indicate that (1) the lobes of pumices are derived either by base surges or by lateral steam clouds, and that (2) large angular blocks are derived by cock’s tail jets during explosions or dome collapse occurred at the end of the eruption.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51D1744S
- Keywords:
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- 8400 VOLCANOLOGY;
- 8404 VOLCANOLOGY / Volcanoclastic deposits;
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8427 VOLCANOLOGY / Subaqueous volcanism