No abstract

Radiant flux. The flow of radiated heat per unit time. Stratigraphic. Pertaining to layered deposits of rock. Tephra. General term for airfall debris produced by a volcanic eruption. Includes but is not limited to ash, pumice, and bombs. Thermal area. Spatial area of thermal ground. Thermal ground. In this report, ground heated by a volcanic source. Water equivalent. The amount of water that would result from the complete melting of a body of deposited snow, firn, or ice, expressed as thickness (depth) or volume.

Abstraet--Phyllosilicates are major components of the <2-/xm fraction (1-3 wt. % of most bulk specimens) in more than 50 samples of air-fall tephra from several 1980 eruptions of Mount St. Helens. In all samples, trioctahedral smectite is the major clay mineral. The integral series of 00~ reflections in ethylene glycoltreated samples indicates a lack of interstratification; absence of a peak near 5/~ after heat treatment, the 060 peak at 1.54 A, and energy dispersive chemical analyses indicate that the smectite is a Mg-and Ferich, trioctahedral saponite. Minor mica and chlorite are present in the <2-/xm fraction of most samples, and some samples show a peak near 12/~ after heating to 550~ which is probably due to the presence of an interstratifled chlorite/collapsed smectite or chlorite/collapsed vermiculite. The tephra contains glass and crystals originating from new magma and lithic fragments incorporated from the pre-existing cone. The clay minerals in the tephra are lithic components stripped from older, hydrothermally altered rocks during explosive ejection. Cleaned pumice fragments, which are new magmatic components, lack smectite, but contain rare biotite in xenoliths. Old, hydrothermally altered rocks from the volcano's summit and from the debris-avalanche (former north flank) contain saponite together with chlorite and chlorite/smectite which may have formed from it. Saponite and zeolites that precipitated from neutral to alkaline hydrothermal solutions line cavities in some of these rocks. The saponite was probably not subjected to magmatic temperatures because heating this material for 5 min at 750~ collapses it irreversibly to 10 A. Kaolinite, alunite, and opal, indicative of acid-sulfate alteration, were found only in the pre-1980 summit crater and the southwest thermal area, but were not evident in the lithic components of the 1980 deposits.

Clay minerals are locally abundant in two hydrothermal areas at Mount Baker-Sherman Crater and the Dorr Fumarole Field. The silt-and clay-size fractions of volcanic debris that is undergoing alteration at and near the ground surface around areas of current fumarolic activity in Sherman Crater are largely dominated by alunite and a silica phase, either opal or cristobalite, but contain some kaolinite and smectite. Correspondingly, the chemistry of solutions at the surface of the crater, as represented by the crater lake, favors the formation of alunite over kaolinite. In contrast, vent-filling debris that was ejected to the surface from fumaroles in 1975 contains more than 20 percent clay-size material in which kaolinite and smectite are dominant. The youngest eruptive• deposit (probably 19th century) on the crater rim was also altered prior to ejection and contains as much as 27 percent clay-size material in which kaolinite, smectite, pyrophyllite, and mixed-layer illitesmectite are abundant. The hydrothermal products, kaolinite and alunite, are present in significant amounts in five large Holocene mudflows that originated at the upper cone of Mount Baker. The distribution of kaolinite in crater and valley deposits indicates that, with the passage of time, increasingly greater amounts of this clay mineral have been incorporated into large mass movements from the upper cone. Either erosion has cut into more kaolinitic parts of the core of Sherman Crater, or the amount of kaolinite has increased through time in Sherman Crater.

In late afternoon on March 10, 1975, Roy Ashe, operator of the Upper Baker Dam at the Baker Lake hydroelectric reservoir, reported than an unusually large dark gray plume of vapor was rising above the rim of the Sherman Crater near the top of Mount Baker, 21 km upstream. A flurry of aerial observations the following day noted distinct changes from previously known fumarolic activity in the ice‐covered crater. New fumaroles, ice pits, semicircular crevasses, and ponded water in the crater had appeared, as well as a thin dusting of gray debris 100 to 300 m outside the crater. The apparently sudden change in the thermal regime of Mount Baker was obvious as well as ominous.