Lahars Triggered by Sudden Landslides at Volcanoes
During the day of peak rainfall from Hurricane Mitch on October 30, 1998, part of Casita Volcano collapsed to form a landslide (note scar at top of volcano). The sliding debris quickly transformed into a lahar that eroded rocks and vegetation from along the stream channel, destroyed two towns, and killed more than 2,000 people. In this photograph, the erosive power of landslides and lahars is clearly visible (note areas devoid of vegetation) (Full story).
Large muddy lahars commonly begin as volcanic landslides. Part of a volcano may suddenly break loose under the constant pull of gravity, especially (1) during a large earthquake that strongly shakes the ground; (2) following a period of heavy rains that saturate the ground; (3) when magma rises into the core of a volcano and wedges the cone apart; or (4) during a sideways-directed explosion that rips through one or more sides of a volcano.
Volcanoes are susceptible to landslides for several reasons. Their cones are steep and often rise several thousand meters above the surrounding land. They are built layer upon layer of weakly consolidated and brittle rock debris. Because of the frequent intrusion of magma into their cones and the shear mass of lava that form their cones, internal faults or shear zones commonly develop. Finally, hot, acid-rich water commonly circulates throughout the cone to form an extensive hydrothermal system. Over time, this hot water changes hard volcanic rocks to weak, muddy and clay-rich material. A volcano with a hydrothermal system is like a house infested with termites--the house gradually weakens to the point that it may collapse. The collapse of a volcano, and the resulting lahar, are natural, expectable events during its life history and long after it stops erupting.
The collapse or "flank failure" of a volcano will generate a fast-moving landslide that usually transforms into a lahar after traveling a few kilometers. Depending on the size of the landslide, its water content, and extent to which the volcano's rocks have been weakened and turned into clay by a hydrothermal system, the resulting lahar may travel more than 100 km downstream. Such large muddy lahars are extremely dangerous.
Case histories of lahars triggered by landslides
More About Landslides and Lahars
At least three large landslides have swept down from the west flank of Mount Rainier volcano, Washington, (hazard assessment report, online) in the past 3,000 years. Each generated large lahars that swept into the Puyallup River valley and traveled to the Puget Sound more than 50 km downstream. The most recent occurred only 500 years ago, and scientists have not yet found evidence to indicate it was associated with eruptive activity. The U.S. Geological Survey and Pierce County, Washington, Department of Emergency Management are developing a lahar-detection system for the Puyallup and Carbon river valleys downstream from Mount Rainier.