Volcano landslides bury large areas with rock debris

Hummocks of landslide deposit, Mount St. Helens, Washington
North Fork Toutle River valley, Mount St. Helens, Washington
Photograph by L. Topinka in 1981

A scientist stands on one of the many small hills called hummocks that form the chaotic surface of a massive landslide deposit in the upper North Fork Toutle River valley below Mount St. Helens volcano (10 km in distance). Before the landslide and eruption on May 18, 1980, a forest grew on this part of the valley floor, and a highway followed the meandering river to Spirit Lake, a popular recreation area.

The landslide deposit extends about 22 km from the volcano and buries the river valley to an average depth of about 45 m. In places, the deposit is nearly 200 m thick! The landslide covers an area of about 60 km2.

An exceptionally large landslide deposit was discovered at Mount Shasta shortly after the eruption of Mount St. Helens. This landslide has a volume of about 45 km3--nearly 20 times larger than the one that buries the North Fork Toutle River valley (above)--and it covers an area of 675 km2.

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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
URL http://volcanoes.usgs.gov/Imgs/Jpg/MSH/30212265-050_caption.html
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Last modification: 10 December 1999 (SRB)