Lahars are "mudflows", mixtures of volcanic ash, blocks and water, formed on volcanoes.

The source of a lahar maybe a crater lake, a dam collapse or heavy rainfall washing ash from the slope of a volcano. People caught in the path of a lahar have a high risk of death from severe crush injuries, drowning or asphyxiation. Lahars are often highly erosive to river banks and eyewitnesses should remain at a safe distance. Lahar events will cause destruction of buildings, installations and vegetation caught in their path.