everything in their path and may cause additional hazards

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 landslides often bury 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.