If all the pumice fragments were equally vesicular, the products of a single eruption would tend to diminish in size away from the vent. The fact is, however, that the porosity of the pumice varies considerably; some pieces are extremely cellular, whereas others are scarcely “frothed” at all. Large, light lumps may therefore be found among much smaller and more compact ejecta. Local variations in the content of large lumps must also have resulted from changes in the intensity and direction of explosions.
Generally, at any given locality the pumice becomes coarser from the bottom upward. This reflects the growing violence of the eruptions. When the explosions began to hurl out lumps between 2 and 3 inches across, the winds from the west began to veer toward the southwest. Hence in the region east and southeast of Klamath Marsh, where the deposits are more than 10 feet thick, the proportion of lumps greater than 3 inches is extremely small; farther north the proportion is high even where the deposits are only half as thick. At no time did large lumps fall south of Crater Lake. Even 2 miles south of the caldera, on the Rim Road, where the deposit is 8 feet thick, few lumps measure more than 2 inches across.
Compared with the deposits of the pumice flows, the material of the pumice fall is much better sorted (see histograms, figure 24). In view of the mode of deposition, this is not surprising, for the material of the pumice fall was subjected to long transport in the air and fell largely according to the dictates of gravity.
Fig. 24. Size distribution of samples of pumice flow (lump pumice). Fraction less than 1/8 mm. omitted. (After B. N. Moore, Journal of Geology, vol. 42, p. 365, 1934. Numbers in parentheses refer to Moore’s samples.) |
The deposits of the pumice flows, on the other hand, ere laid down rapidly in the manner of avalanches and therefore suffered no sorting by wind.
Particularly striking is the paucity of fine dust in the pumice fall and its abundance in the pumice flows. Samples taken from the snout of the pumice flow poured down the Rogue River contain no less 70 per cent by volume of material finer than 0.5 mm. Elsewhere, more than half the flows are made up of particles less than 1 mm. in diameter. On the contrary, the proportion of fine dust is insignificant in the deposits of the pumice fall, particularly close to Crater Lake (see figure 19). Less than I per cent of samples taken from Timber Crater, Mount Thielsen, Miller Lake, and Windigo Pass consist of ejecta smaller than 1 mm. in size. Farther away, the content of fine particles increases rapidly (figure 21).
