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Science 30 September 1983:
Vol. 221. no. 4618, pp. 1385 - 1387
DOI: 10.1126/science.221.4618.1385
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Articles

Petrologic Monitoring of 1981 and 1982 Eruptive Products from Mount St. Helens

KATHARINE V. CASHMAN 1 and JOSEPH E. TAGGART 2

1 U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98661
2 U.S. Geological Survey, Denver, Colorado 80225

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.

Submitted on January 26, 1983
Revised on June 22, 1983


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Petrologic Reconstruction of Magmatic System Variables and Processes.
J. Blundy and K. Cashman (2008)
Reviews in Mineralogy and Geochemistry 69, 179-239
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Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longavi Volcano (36{middle dot}2{degrees}S; Andean Southern Volcanic Zone, Central Chile).
C. RodrIguez, D. Selles, M. Dungan, C. Langmuir, and W. Leeman (2007)
J. Petrology 48, 2033-2061
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Unraveling the 1974 eruption of Fuego volcano (Guatemala) with small crystals and their young melt inclusions.
K. Roggensack (2001)
Geology 29, 911-914
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Gas Emissions and the Eruptions of Mount St. Helens Through 1982.
T. J. Casadevall, T. CASADEVALL, W. ROSE, T. GERLACH, L. P. GREENLAND, J. EWERT, R. WUNDERMAN, and R. SYMONDS (1983)
Science 221, 1383-1385
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Monitoring the 1980-1982 Eruptions of Mount St. Helens: Compositions and Abundances of Glass.
W. G. Melson and W. G. MELSON (1983)
Science 221, 1387-1391
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Deep Earthquakes Beneath Mount St. Helens: Evidence for Magmatic Gas Transport?.
C. S. Weaver, C. S. WEAVER, J. E. ZOLLWEG, and S. D. MALONE (1983)
Science 221, 1391-1394
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