The Glowing Avalanches: Pumice and Scoria Flows

     Content of Lithic Fragments

Moore expressed the opinion that there is a lack of "accidental" or foreign material in the pumice and scoria flows; on the contrary, we have found that they may be readily distinguished from the preceding pumice fall by their richness in such debris. In the main pumice fall, as we have seen, it is rare to find old lava fragments more than 5 cm. in diameter, and the total content of lithic debris averages only between 3 and 4 per cent. In the flows, on the other hand, the percentage of lithic material in the size range between 0.125 and 10 mm. varies between 9 and 31.7 and averages approximately 25 (figures 25, 26). The amount of lithic material less than I mm. in diameter is laborious to estimate, requiring careful bromoform separation. No attempt was therefore made in the field to calculate this fraction. The coarser ejecta were readily screened and separated by flotation in water. It was then found that, considering all material larger than I mm. in diameter, the percentage of lithic detritus in the combined pumice and scoria flows averages approximately 15 to 20, being higher in the scoria than in the pumice. These figures, it will be seen, are smaller than those given for the lithic content in the size range 0.125 to 10 mm. The reason is that there are few old lava fragments larger than 3 inches in size, whereas bombs of pumice occasionally reach a diameter of more than 6 feet, and bombs of scoria 3 feet across are not uncommon.

It might be supposed that the rapid eruption of many cubic miles of magma would be attended by the expulsion of large blocks of lava torn from the sides of the conduit and the summit of the volcano. Yet such blocks are rare. The debris torn from the sides and top of Mount Mazama during the eruption of the glowing avalanches is mostly of the size of coarse sand and gravel. This paucity of large blocks is in marked contrast with their abundance among the nuée ardente deposits of most volcanoes.

We cannot properly attribute this scarcity of large rock fragments to shattering during transport, for dense lava blocks would hardly be comminuted when immersed in gas-charged flows of the type in question. More likely, the magma which produced the flows rose high in the conduits and only began to froth violently when it had almost reached the surface. Instead of shattering the sides of the conduit and crater, the frothing pumice and scoria merely cored them by abrasion. Perret's masterly account of the recent eruptions of Mount Pelée gives a vivid picture of what the conditions at Mount Mazama must have been like.⁸

Finally, it should be noted that although the scoria flows usually carry more lithic material than the underlying pumice flows, the distribution of the foreign debris is otherwise quite haphazard, the fragments being thoroughly scattered throughout.

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