Volcanic Landslides

Volcanoes have unstable surfaces, so landslides are likely even when the volcano is dormant. In addition to the fact that the surface is composed mostly of loose rock, the volcanic gases create acidic groundwater. This contributes to the rocks' breakdown, making the them more likely to be carried away.

Landslides can also occur during volcanic activity. There are two types of volcanic landslides.

Pyroclastic flows occur during an eruption or after the collapse of a volcano's dome. These high-speed flows, composed of ash, lava, rocks and gas, can reach temperatures of 1,500 degrees Fahrenheit (815.5 degrees Celsius) and have been clocked at up to 450 mph (724 kph) [source: Landslide Hazards, USGS]. Pyroclastic flows that surge with hurricane-force haste are capable of tearing up and burning anything in their path.

pyroclastic flow
J.D. Griggs/Hawaiian Volcano Observatory/U.S. Geological Survey
Lava flow near the shore line at Pupapau Point, Kalapana, on Nov. 27, 1989 during the Kilauea east rift eruption.


Lahars, which don't necessarily occur during an eruption, are set off by water. The water could come from a rainstorm, melted snow and ice or a glacier melted by lava flow. Along with the contributing water, lahars comprise mud and rock. Sixty to 90 percent of a lahar's weight may come from rock debris, lending it grave force. Generally considered the most deadly type of volcanic hazard, lahars can reach speeds of 20 mph to 40 mph (32 kph to 64 kph) and travel over 50 miles (80.4 kilometers) [source: Landslide Hazards, USGS], ripping up any trees and homes in their path.

Lahar after 1982 eruption of Mount St. Helens
U.S. Geological Survey
Melting snow and ice on the north flank of Mount St. Helens triggered this lahar, which traveled rapidly down the valley of the North Fork of the Toutle River.


Not all causes of landslides are as obvious as the ones listed so far. Humans certainly play a part in causing equally devastating landslides. Read on to find out what mistakes we make and how we can prevent and prepare for landslides.

Caution: Hazardous Area
­­B­e aware: Certain landscapes are more prone to landslides than others. As you might expect, any hilly or mountainous area that receives frequent rain or snow is a prime location for a landslide. Canyon bottoms, valleys, gullies, stream channels, culverts and other avenues for runoff are dangerous as well. In the United States, landslides occur in all 50 states but are most prevalent in Southern California and the northernmost regions of Hawaii, California, Idaho, Oklahoma and Washington. The Rocky and Sierra Nevada mountain ranges in the West and the Appalachians in the East are also susceptible due to their slope and amount of weathered rock.