74 The Glowing Avalanches: Pumice and Scoria Flows – Erosive Action of the Flows

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Climax: Culminating Explosions of Pumice and Scoria


The Glowing Avalanches: Pumice and Scoria Flows

     Erosive Action of the Flows

Near the top of the Javanese volcano Merapi, there are places where the rock surfaces have been so severely abraded and striated by the passage of glowing avalanches that they might easily be mistaken for glaciated pavements. On the Japanese volcano Komagatake, the upper parts of the canyons were likewise swept bare by the onrush of pumice flows. Although there may have been vigorous erosion on the higher, now vanished parts of Mount Mazama, there are no signs that the pumice and scoria flows scratched or polished the visible slopes of the cone. There is proof, however, that they were able to sweep much incoherent material from the valley floors and to incorporate it.

The flows on the west and southwest flanks of the volcano, as noted already, did not begin to deposit their load until they reached a distance of 2 or 3 miles beyond the present rim of the caldera. About the headwaters of National, Bybee, Copeland, and Castle creeks, and to a lesser extent in the upper parts of Munson, Kerr, and Sun valleys, there is generally a transition zone between 1/4 and mile wide where the pumice and scoria are intimately mingled with reworked glacial debris. Where the streams have cut down to the base of the flows a similar transitional zone is exposed. Admittedly, some of this mingling may have been brought about by floods following in the wake of the first flows as they melted snow banks in their path, but most of the admixture seems to have been caused by the powerful plowing action of the heavy avalanches as they swept over incoherent moraines and fluvioglacial outwash.


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