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Date Permissions Signed

5-15-2018

Date of Award

1983

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Babcock, R. Scott (Randall Scott)

Second Advisor

Brown, Edwin H.

Third Advisor

Pevear, David R.

Abstract

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.

Type

Text

Keywords

Pyroclastic flow deposits, Pumice

Publisher

Western Washington University

OCLC Number

1080953187

Subject – LCSH

Geochemistry--Washington (State)--Saint Helens, Mount; Petrology--Washington (State)--Saint Helens, Mount; Volcanic ash, tuff, etc.--Washington (State)--Saint Helens, Mount

Geographic Coverage

Saint Helens, Mount (Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this thesis in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.

Included in

Geology Commons

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