| Abstract: | Secondary hydroeruptions occur in pyroclastic-flow deposits when water or ice is trapped beneath hot pyroclastic debris and rapidly heated to steam. These eruptions display various styles of activity including fumarolic degassing, tephra fountaining, and explosive cratering. The deposits, which occupy the layer 3 stratigraphic position on the top of pyroclastic-flow units, can be distinguished from ash-cloud material by lateral thickness variation, clast composition, and other sedimentary features. The ejecta of secondary hydroeruptions comprise a subset of hydrovolcanic pyroclastic deposits. A small secondary hydroeruption observed on the Mount St. Helens pumice plain in 1981 produced tephra that was emplaced ballistically, by deposition from base surges, and by fallout from an eruption column. Stratigraphic descriptions and grain-size analysis of the ejecta from several secondary craters on the pumice plain demonstrate that the bedforms produced by a hydroeruption change with crater diameter. In particular, craters of small diameter are surrounded by interbedded ripple-laminated ash horizons and nonstratified, fines-depleted units; large craters have ejecta ramparts comprised of coarse dunes and antidunes. These bedform changes are related to a progressive increase in eruptive energy, which produces base surges of greater power and eruptive columns of greater height. We suggest that the style of activity displayed during a secondary hydroeruption is controlled by both the total thermal energy of the system and the permeability of the pyroclastic overburden. ?? 1987. |
| Genre: | Article |
| ProdID: | 70015322 |
| Citation Author: | Moyer, T. C.; Swanson, D. A. |
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| Citation End Page: | 319 |
| Citation Issue: | 4 |
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| Citation Language: | English |
| Citation Larger Work Title: | Journal of Volcanology and Geothermal Research |
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| Citation Number Of Pages: | 21 |
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| Citation Search Results Text: | Secondary hydroeruptions in pyroclastic-flow deposits: Examples from Mount St. Helens; 1987; Article; Journal; Journal of Volcanology and Geothermal Research; Moyer, T. C.; Swanson, D. A. |
| Citation Start Page: | 299 |
| Citation Volume: | 32 |
| Citation Year: | 1987 |
| Type: | citation/reference |
| Text: | Secondary hydroeruptions in pyroclastic-flow deposits: Examples from Mount St. Helens; 1987; Article; Journal; Journal of Volcanology and Geothermal Research; Moyer, T. C.; Swanson, D. A. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Thu, 1 Jan 1987 00:00 -0600 |
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