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.
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.
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.