Relation of the Pumice Fall to the Pumice Flows

Moore thought that the pumice fall (granular pumice) was deposited after the pumice flows (lump pumice) because in the region south of Chemult granular pumice lies on top of the more rudely sorted deposits of the glowing clouds. He seems to have been mistaken, for the relations at Chemult are, as we shall see, abnormal. Nowhere in the area covered by the pumice flows is there a cover of pumice fall comparable with that just described. Had the main pumice fall followed the flows, the latter would be buried by a heavy sheet of granular pumice, for where the two deposits adjoin at Chemult the fall is more than 15 feet thick. Near Crater Lake, therefore, the flows would have been buried under an even thicker sheet of granular pumice. The truth is, however, that except along the margins of the pumice flow and along the axial parts of the flows in the canyons, there is no cover of pumice fall of any kind. The possibility that an overlying layer of pumice fall has been removed by erosion is out of the question.

Where the pumice flows are actually blanketed with fine pumice, the younger deposits are radically different from the main pumice fall just discussed. Notably the pumice on top of the flows in the canyons is much richer in crystals, lithic debris, and basaltic scoria; elsewhere, it is almost entirely composed of fine glass dust. Its origin is discussed on a later page, but it may be stated here that part of the ejecta represents fine dust which settled slowly from the air after the flows had come to rest, and the remainder is a product of the weak dying explosions of Mazama. The succession of events is precisely what might be expected on theoretical grounds; eruptions of pumice increase in violence until they give way to more voluminous explosions of the glowing-avalanche type, and finally the last explosions remove the debris which crumbles and slides into the conduits enlarged by the principal activity. At Komagatake in 199 this was the sequence: first the pumice fall, then the pumice flows, and lastly the fall of dark ash.

Since the main pumice fall from Mount Mazama preceded the flows, it may be asked why deposits of the fall cannot be seen beneath those of the pumice flows. The answer is that as the glowing avalanches rushed down the flanks of the volcano they plowed up and incorporated the incoherent deposits of the pumice fall. The latter, being mingled with the onrushing flows, can no longer be recognized. It must be remembered, also, that the base of the pumice flows is visible only in the canyons on the south and west sides of the volcano, precisely where the earlier fall of pumice was thinnest. The ability of glowing avalanches to scour the surfaces over which they race has often been noted by volcanologists.

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