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Temporal Trends In The Geochemistry And Petrology Of The 1980 Mount St. Helens Pyroclastic Flow Deposits, Robert L. Logan
Temporal Trends In The Geochemistry And Petrology Of The 1980 Mount St. Helens Pyroclastic Flow Deposits, Robert L. Logan
WWU Graduate School Collection
Petrographic and geochemical analyses were performed on pumice from the May 18, June 12, July 22, August 7, and October 16-18 pyroclastic flow deposits. The pumice is dacitic and contains, in order of decreasing abundance, the minerals plagioclase An30-57, hypersthene, hornblende, magnetite-illmenite, ± augite, ± apatite, in a groundmass of highly vesiculated glass and plagioclase microlites. Vesiculation occurred over a period of about one second, but at times during the eruption probably within a zone in the vent rather than at the atmosphere-magma interface.
An increase with time in the crystal to glass ratio indicates continued cooling of …