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

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.

Science at the policy interface: volcano-monitoring technologies and volcanic hazard management

“…Rymer 1994) and petrological laboratory tools (e.g. Corsaro and Miraglia 2005;Cashman and Taggart 1983). These are currently in various stages of development, and are mostly employed at observatories with healthier funding, or by university scientists in collaboration with observatories.…”

Tiltmeter analysis of Mount St. Helens, Skamania County, Washington

“…The changes in bulk chemistry (Table 7) Crystallinity of the eruptive products is also increasing, indicative of material being derived from a more crystal-rich volatile-poor region of the magma chamber (Cashman and Taggert, 1983). Decreases in co 2 and S02 gas emmission since May 1980 also indicate that additional new magma has not entered the system (Casadevall and others, 1983).…”

“…Since that time major studies at Kilauea, Hawaii (Kinoshita and others, 1969, 1974, Swanson and others, 1976, and Krafla, Iceland (Tryggvason, 1978, Bodvarsson and others, 1980 Helens between 1980and 1984(Cashman and Taggert, 1983, Lipman and others, 1981, Melson, 1983, Sarna-Wojcicki and others, 1981 have indicated that there has been only one source for eruptive outputs. Tiltmeter studies at Mount St. Helens (Dvorak andothers, 1981, Dzurisin andothers, 1983) have indicated that discernible changes in tilt occur in conjunction with both explosive volcanic activity and exogenous dome growth.…”

Road guide to volcanic deposits of Mount St. Helens and vicinity, Washington

“…[Dome growth occurred both by endogenous and exogenous processes. Tephra and pyroclastic-flow pumice erupted during 1980-83 contain Fe-Mg phenocryst-assemblage hypersthene-hornblende-augite (Kuntz and others, 1981;Cashman and Taggart, 1983) -11, I-12, I-13, I-14, I-15, I-17, I-20. Blast deposit 1 ----------II-1 2, II-13.…”