(Editor's Note: The following is an abstract of a paper presented at the Mount Rainier National Park Science Symposium on January 21, 1984.)

HYDROTHERMAL ALTERATION AT MOUNT RAINIER

Field studies and thermal-infrared mapping indicate that present excess surface heat flux from thermal activity at Mount Rainier may be estimated at about 13 megawatts, a value between that of Mount Baker (82 megawatts for 1975) and Mount Hood (4-10 megawatts for 1977). Three representative settings of hydrothermal activity at the cone include:

a. An extensive area (>10,000m²) of boiling-point fumaroles at 76-82°C and heated ground along and marginal to the overlapping rims of East and West Craters at the volcano's summit;

b. A small area (<500m²) of sub-boiling-point fumaroles at 55-60°C and heated ground on the upper flank at Disappointment Cleaver, and other probably similar but inaccessible areas on the upper flank at Willis Wall and Sunset Amphitheater;

c. Thermal springs at 9-24°C on the lower flank of the volcano in valley walls of the Winthrop and Paradise Glaciers.

Thermal springs also issue from thin sediments that overlie Tertiary rocks at, or somewhat beyond, the base of the volcanic edifice in valley bottoms of the Nisqually and Ohanapecosh Rivers where maximum spring temperatures are 19-25°C and 38-50°C respectively.

During the period of study, fumaroles at the summit had a mildly acidic condensate (pH 4-5), and contained no noticeable H₂S or SO₂. Summit rocks are mainly augite hypersthene andesite flows with minor amounts of lithic explosion rubble. Hydrothermal alteration at the summit is localized only around active fumarole clusters, and is more extensive in the somewhat older West Crater than in the East Crater. In contrast, vapor temperatures are typically 4-6°C higher in the East Crater. Alteration products include an abundance of dioctahedral smectite, poorly crystallized kaolinite, cristobalite, tridymite, opal, and calcite. Alunite is common in the West Crater but not the East, attesting to the recently past presence of a higher sulfate activity westward.

Thermal springs on the lower flank are low-temperature NA-SO₄-HCO₃ waters that are greatly mixed with shallow, cold ground water. Thermal springs at Longmire and Ohanapecosh are Na-HCO₃-Cl waters that may have little or no relationship to the present Mount Rainier hydrothermal system. Presently forming alteration products around all springs typically include calcite, with aragonite in some areas; additionally, opal and gypsum occur at the lower-flank springs.

David Frank
Department of Geological Science
University of Washington