Landslides are large masses of wet or dry rock and soil that fall, slide, or flow very rapidly under the force of gravity. Landslides commonly originate as massive rock falls or avalanches, which disintegrate during movement into fragments ranging from small particles to blocks hundreds of meters across. If the landside is large enough and contains a high-enough percentage of water and fine material (typically, >3-5 percent of clay-sized particles), it may transform into a lahar that can travel as much as 200 km (120 mi) downstream.
Landslides are common on volcanic cones because they are tall, steep, and weakened by the rise and eruption of molten rock. Magma releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering minerals to clay. Furthermore, the mass of thousands of layers lava and loose fragmented rock debris can lead to fault zones that move frequently.
Volcano landslides (debris avalanches) range in size from less than 1 km3 to more than 100 km3. The high velocity and great momentum of landslides allows them to cross valley divides and run up slopes several hundred meters high. For example, the landslide at Mount St. Helens on May 18, 1980, had a volume of 2.5 km3, reached speeds of 50-80 m/s (100-180 mi/hr), and surged up and over a 400-m-tall (1300 ft) ridge located about 5 km from the volcano.Several conditions can trigger landslides:
By removing a large part of a volcano's cone, a landslide may abruptly decrease pressure on the shallow magmatic and hydrothermal systems, which can generate explosions ranging from a small steam explosion to large steam- and magma-driven directed blasts. These result in tephra and ashfall hazards for surrounding areas. A recent example of this occurred at Mount St. Helens in 1980.
Large landslide often buries valleys with tens to hundreds of meters of rock debris, forming a chaotic landscape marked by dozens of small hills (hummocks) and closed depressions. If the deposit is thick enough, it may dam tributary streams to form lakes; the lakes may eventually drain catastrophically forming lahars and floods downstream. Landslides also generate some of the largest and most deadly lahars, either by transforming directly into a lahar or, after it stops moving from settling out of the deposit.
Historically, the most deadly volcano landslide occurred in 1792 when sliding debris from Mt. Mayuyama near Unzen Volcano in Japan slammed into the Ariaka Sea and generated a tsunami that reached the opposite shore and killed nearly 15,000 people.
Large horseshoe-shaped craters, open at one end, have long been noted in many volcanic regions around the world. The origin of these breached craters has been controversial, but since the debris avalanche and eruption of Mount St. Helens in 1980, scientists believe that many of them formed as the result of a landslide. If the primary eruptive vent is located within these deep craters, they will likely direct subsequent volcanic activity (lava flows, pyroclastic flows, or lahars) toward its breached opening.