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. Away from the vent gases pose no more than an irritant or nuisance.
Many volcanic cones are steep sided and unstable due to rapid growth of the cone.
Rising magma, earthquakes, weakening due to hydrothermal alteration and heavy rain can trigger a debris avalanche of this unstable material. Avalanched material follows valleys as it moves down the side of the volcano under the force of gravity. Debris avalanches can be wet, dry or both, and if wet, an avalanche may evolve and continue to flow further down slope as a lahar. They can be produced by a submarine earthquakes, by debris avalanches and by underwater volcanic eruptions. A number of waves may be produced and they may travel long distances to far-off shores.