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Petrologic monitoring of 1981 and 1982 eruptive products from Mount St. Helens

Science

By:
and

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Abstract

New material from the dacite lava dome of Mount St. Helens, collected soon after the start of each successive extrusion, is subjected to rapid chemical and petrologic analysis. The crystallinity of the dacite lava produced in 1981 and 1982 is 38 to 42 percent, about 10 percent higher than for products of the explosive 1980 eruptions. This increase in crystallinity accompanies a decrease in the ratio of hornblende to hornblende plus orthopyroxene, which suggests that the volatile-rich, crystal-poor material explosively erupted in 1980 came from the top of a zoned magma chamber and that a lower, volatile-poor and crystal-rich region is now being tapped. The major-element chemistry of the dacite lava has remained essentially constant (62 to 63 percent silica) since August 1980, ending a trend of decreasing silica seen in the products of the explosive eruptions of May through August 1980.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Petrologic monitoring of 1981 and 1982 eruptive products from Mount St. Helens
Series title:
Science
Volume
221
Issue:
4618
Year Published:
1983
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Science
First page:
1385
Last page:
1387