69 The Glowing Avalanches: Pumice and Scoria Flows – The Lack of Sorting

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

     The Lack of Sorting

From what has been said it may be anticipated that the deposits of the glowing avalanches show a chaotic mixture of ill assorted fragments. Such is strikingly the case. Essentially, the deposits consist of large, sub-rounded pumice lumps in a dusty matrix. Nor is there any regular variation in coarseness as the flows are followed away from their source. In fact, the size of the pumice lumps bears no relation to distance from the vent, and the largest, up to 14 feet in diameter, occur 20 miles away. Lumps more than I foot across are plentiful throughout, and in general they are somewhat more rounded at the distal ends of the flows, owing, no doubt, to longer abrasion.

In the description of the earlier pumice fall, stress was laid on the scarcity of material less than I mm. in diameter, and the still more pronounced scarcity of material less than 0.5 mm. across. But in the pumice flows, the proportion of fine dust is extremely high. For emphasis it may be repeated that in the fraction of the flows less than 64 mm. in diameter approximately two-thirds consists of particles less than I mm., and even the portion less than 0.25 mm. in diameter averages 36 per cent. Whereas the weight percentage of material between 0.125 and 0.25 mm. in the pumice fall averages only 0.9 per cent, that in the flows averages 11.7 per cent.

Considering the manner in which the material of the flows was transported, this extraordinary abundance of fine dust is not to be wondered at, for not only were the larger lumps of pumice rapidly triturated and abraded by crashing against one another, but they must have been repeatedly shattered by internal explosions.

Typical histograms of the pumice flows are shown in figure 24, reproduced from Moore’s paper. From these it may be seen that the deposits show far less sorting by size than do those of the earlier pumice fall. Chaotic mixture of fragments of all sizes is typical of glowing-avalanche debris; so is the absence of any regular, lateral variation. For these characters, rapid deposition and the absence of wind action during transit are largely responsible.


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