Landslides

Start of a landslide	Illustrations by B. Meyers	Landslide enters valley

Landslides are large masses of rock and soil that fall, slide, or flow very quickly under the force of gravity. Landslides can be wet, dry, or both. They usually begin as rockslides or avalanches. As they move, they carry fragments of soil and rock as small as dirt particles or as large as city blocks. If there is enough water in the landslide, it can become a lahar.

Landslides are common on volcanoes because the cone of a volcano usually causes it to rise thousands of feet above the land that surrounds the volcano. The heat and acids inside a volcano can weaken rocks and turn them to clay. The molten rock that creates a volcano in layer after layer can weaken the land because of faults or weak spots in the rock. All of these things can trigger a landslide when magma, eruptions, earthquakes, or heavy rains cause them to give in to the force of gravity.

A landslide can destroy everything in its path. . In some cases, such as Mount St. Helens 1980 eruption, landslides actually cause volcanic eruptions because the movement of rock decreases the pressure in the volcano so that steam and magma can be released Some landslides form a dam in a river system that blocks the river’s flow and creates a new lake. In other cases, landslides trigger some of the largest and most deadly lahars. The most deadly volcano landslide killed nearly 15,000 people when it slammed into the sea and caused a tidal wave. Landslides usually leave behind a deep gash or a large horse-shoe shaped craters in the side or top of the volcano.

Volcanic Landslides can:

trigger volcanic explosions.	create lahars that travel far.	cause waves or tsunamis in a lake or ocean.	bury river valleys with rock debris.
Original illustrations by T.R. Alpha	Photograph by L. Topinka in 1981	Photograph by T. Casadevall in 1991	Photograph by L. Topinka in 1981
	dam streams to form lakes.	create a crater or scar on a volcano.
	Photograph by L. Topinka on January 13, 1984	Photograph by C.D. Miller in 1980

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