Journal of Cave and Karst Studies - ISSN 0146-9517
Volume 60 Number 1: 44-50 - April 1998

A publication of the National Speleological Society

Crater Firn Caves of Mount St. Helens, Washington
Charles H. Anderson, Jr., Christopher J. Behrens, Gary A. Floyd and Mark R. Vining


Systematic observation, photo-reconnaissance, mapping, and sampling were performed in the crater firn caves of Mount St. Helens, Washington, from 1981 through 1996 by members of the International Glaciospeleological Survey in cooperation with the United States Forest Service and Mount St. Helens National Monument.

Mount St. Helens is an active dacitic volcano, which is currently in a semi-dormant state after a catastrophic explosive eruption in May 1980. A dacite dome occupies the crater and plugs the volcanic vent. The crater area has been progressively covered by a layer of snow, firn, and glacier ice since as early as 1986. Heat, steam, and volcanic gases from the crater fumaroles melted over 2415 meters of cave passage in the crater ice mass. The caves are in approximate balance with the present geothermal heat release. Future changes in the thermal activity will influence the dimensions, location, ceiling, wall, and wall ablation features of these caves. Cave passages are located above fumaroles and fractures in and adjacent to the crater lava dome. Cave passages gradually enlarge by ablation, caused by outside air circulation and by geothermal sources beneath the ice. The passages form a circumferential pattern around the dome, with entrance passages on the dome flanks. Passages grow laterally and vertically toward the surface, spawning ceiling collapse.

The crater ice body has been expanding since 1986 and its mean density increases each year. It possesses at least two active crevasses. Trends and changes in geothermal activity in the crater of Mount St. Helens have been noticeable through cave passage observation and re-mapping.

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