9.5

Volcanic landslides are larger and more devastating than non-volcanic collapses

This could be due to the presence of shallow magma bodies, location of the volcano’s conduits (a conduit is a pipe that carries this magma from the magma chamber), or dike placement that acts as a plane of weakness. Non-volcanic environments lack these factors.

Cross sections showing amount of material that failed in volcanic collapses (left) versus non-volcanic (right) (Siebert, 1984)

The large size of volcanic collapses can perhaps explain why the run-out distance of debris is so great. When an ‘ordinary’ block of rock slides down an inclined plane, it travels 166 m horizontally for every 100 m it descends vertically. In contrast, volcanic avalanches can travel as much as 1000 m horizontally for every 100 m descended. The large amount of kinetic energy from rock falling from high distances can carry material for a long way. More importantly, the boulders and blocks involved in the avalanche have very short collisional contact with their neighbors, hence there is very little friction to counteract the large amount of kinetic energy.

Mount Shasta ancestral debris avalanche deposit that traveled 43 km from the volcano (USGS)

Other hypotheses have been suggested for run-out distance, but the physics is far from being fully understood. The style of volcanic avalanche movement has been compared to plug flow. Plug flow occurs when the body of the flow goes through very little deformation and only the layer in contact with the ground is sheared. In other words, the movement of material is equal throughout the flow in a laminar arrangement– there is very little turbulence, or mixing of material. The results of a flow like this that the original relationships of layers are preserved. Avalanches also have high yield strengths, such that heavy boulders can rest on the surface and not sink into a weak medium.

Taking these ideas into account, early workers explained the long run-out with a ‘magic-carpet’ hypothesis. This suggested that the avalanche rode on a ‘cushion’ of compressed, trapped air. Another hypothesis is that the finer-grained particles in the avalanche can be fluidized by trapped air.

These hypotheses became debunked when scientists discovered large avalanches on the Moon and Mars, which have no atmosphere (for example, see McGovern, P. publications). Therefore magic carpets, air cushions, and partial fluidization are discarded as being the only explanations of transport.

(Francis, P. and Oppenheimer, C., 2003)

 

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