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Date of Award
Master of Science (MS)
Babcock, R. Scott (Randall Scott)
Brown, Edwin H.
Pevear, David R.
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 the magma. Resorption or recrystallization of hornblende in younger flows indicates degassing by loss of H2O. Barring new intrusion of magma, these textural trends suggest that less explosive eruptions might be expected in the future.
Using inductively coupled plasma spectroscopy, elemental oxide abundances were determined for Si, Al, Ti, Fe, Mn, Ca, Mg, K, Na, and P; and trace element abundances were determined for Ba, Cr, Cu, La, Nb, Sc, Sr, V, Y, Zn, and Zr. Temporal trends in major and minor element abundances show that Si02 decreases, while FeO (total), CaO, MgO, Ti02, and MnO increase. Temporal trends in trace element abundances show an increase in Cr, Cu, Sc, and V with a decrease in Ba. These trends are toward a more andesitic composition. These data are consistent with the hypothesis that orogenic calk-alkaline rocks are formed by a subcrustal two stage melting process.
Pyroclastic flow deposits, Pumice
Western Washington University
Subject – LCSH
Geochemistry--Washington (State)--Saint Helens, Mount; Petrology--Washington (State)--Saint Helens, Mount; Volcanic ash, tuff, etc.--Washington (State)--Saint Helens, Mount
Saint Helens, Mount (Wash.)
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Logan, Robert L., "Temporal Trends in the Geochemistry and Petrology of the 1980 Mount St. Helens Pyroclastic Flow Deposits" (1983). WWU Graduate School Collection. 819.