This photograph shows a large dacitic block from Mount St. Helens within a matrix of smaller sized and more mafic composition fragments. Erosion has carved into the debris avalanche to provide exposures of individual blocks. Although the blocks are coherent masses, they have been extensively fractured and can be excavated with a shovel. An avalanche typically starts movement on a slide plane, but then begins to disintegrate into separate particles that move against and over one another, with the small ones serving as the matrix that carries the larger particles, although this is not necessary for the movement of avalanches. There is uusually abundant moisture around the fragments, derived from ground water initially retained in the rocks before they dissociated into large and small particles.
Gigantic avalanches can originate by the structural collapse of a composite volcanoes.
Since then, avalanche deposits have been inventoried world-wide. Although collapse is infrequent on a human life-span scale, it is a common volcanologic phenomenon of composite volcanoes. Composite volcanoes may grow to great heights if volcanic activity persists long enough, but the pull of gravity limits growth. Slopes steepen, they become unstable and then may collapse. A rising internal dome within the Mount St. Helens volcano contributed to the oversteepening and instability of its north slope.