The NSS Bulletin - ISSN 1090-6924
Volume 37 Number 3: 43 - July 1975

A publication of the National Speleological Society

Firn Caves in the Valcanic Craters of Mount Rainier, Washington
Eugene P. Kiver and William K. Steele


Sub-ice fumaroles and warm air currents form and maintain over 2 km of cave pasage beneath ice filling the summit craters of Mount Rainier. Passage size increases from 1970 to 1973 indicate recent, minor, heat flow increases. Large heat flow decreases would allow plastic flowage to close passages and large increases would produce enlargement, collapse, and large crater lakes. Complete melting of summit ice would produce about 1.1 billion liters of water in the west crater and 7.4 billion liters in the east crater, creating a serious potential geological hazard.

The caves are called firn caves because ice density ranges from 0.55 to 0.81 gm/cm3. Ice tempteratures as low as -10°C partly account for low densities, high viscosity ice, and open passages as deep as 100 m below the east crater snow surface.

Subsising ice replaces walls and ceilings that are melted back 2.0 to 3.5 m/year. Subsidence, melting, and snowfall are in dynamic equilibrium. Thus, cave dimensions and the snow surface within the crater remain relatively constant. The discovery of a climber's glove and the debris from a 1959 expedition shows that subsiding ice reaches deeper cave areas in a few decades.

Entrance passages lead down the crater slope and perimeter passages parallel elevations contours. An east crater perimeter passage 50 m below the surface in its central part is 915 m long and winds three-fourths of the way around the crater. Narrow passages lead downward from the main perimeter passage in each crater to large grottos. A 40 m long lake in the west crater grotto is the highest crater lake in North America.

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