Structural collapse of part of a vo

Structural collapse of part of a volcano can result in catastrophic avalanches of material downslope as a debris-flow avalanche. They may be triggered by explosive eruptions, volcanic earthquakes or by the oversteepening of the side of a volcanic edifice due to addition of material during eruption, such that part of it fails under gravity. Large amounts of unstable volcanic material move downslope under gravity including blocks that may be tens to hundreds of metres across in a matrix of finer-grained volcanic ash (Urgeles et al. 1997). The deposits of these vents are extremely poorly sorted, chaotic masses of detritus that may be tens to hundreds of metres thick and cover hundreds of square kilometres. Where water is involved in the debris-flow avalanche it may pass into a lahar (see below).