Volcano landslides generate lahars

House partially buried by a lahar deposit, Mount St. Helens, Washington
Photograph by L. Topinka in 1981

This house is partially buried in a lahar deposit that was formed by the dewatering of a large volcano landslide from Mount St. Helens, Washington. Early on the morning of May 18, 1980, the landslide swept into the upper North Fork Toutle River valley and came to rest within about 22 km of the volcano. The landslide deposit, however, was saturated with water, and contained snow and ice blocks from the volcano's former glaciers. As soon as the landslide stopped moving, water percolated to the top of the deposit and poured across its irregular surface, forming many lahars that merged as they rushed down the valley. The peak flow swept from the deposit about 5 hours after the landslide was emplaced!

The lahar flowed down the Toutle River throughout the afternoon and evening, reaching its peak at midnight about 60 km downstream from the volcano. The lahar destroyed roads, bridges, and homes.

Many volcano landslides do not stop so close to their source, but instead keep moving by transforming directly into a lahar. These lahars can be extremely hazardous because of their size and mobility (they may travel more than 100 km).

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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/30410914-057_caption.html
Contact: VHP WWW Team
Last modification: 16 December 1999 (SRB)