Double Character of the Deposits - Geology of Crater Lake NP

Probably more than seven-eighths of the flows consists of pale-gray and buff dacite pumice similar mineralogically to the material of the preceding pumice fall. The pumice itself is highly vesicular and often fibrous, and lightly charged with crystals of plagioclase, pyroxene, and hornblende. In the fraction less than I mm. in diameter, the content of crystals ranges from a quarter to two-thirds and averages about a third, feldspar predominating over the heavier minerals. The ratios of the minerals and their amounts in typical samples are shown in figures 22, 25, and 26. After the magma chamber had been exhausted of its dacitic part, the pumice flows were immediately followed by avalanches of smoke-gray basic scoria. These dark scoria flows are well seen in the canyons of Annie, Sun, Sand, and Castle creeks, where they make a striking contrast with the underlying pumice (plate 16). The transition between the two deposits takes place within a few feet. There are no intermediate andesitic ejecta. Elsewhere it has been shown that the building of the main andesite cones of Mounts Mazama and Shasta was followed by extrusion of thick masses of dacite and formation of basaltic cinder cones. The culminating pumice and scoria eruptions of Mazama likewise involved the expulsion of the two extreme magma types in rapid succession.

Fig. 22. Samples of pumice fall and flow. Sun Pass samples from fall; the other two from flows.

Fig. 25. Histograms of pumice deposits. All but the first from pumice flows.

Plate 16. The Pinnacles, Sand Creek canyon, showing pale pumice flow beneath smoke-gray scoria flow, above which lie 10 feet of fine ash. Near the contact of the ash and scoria layers is the red zone caused by oxidation of iron-bearing fumarole gases. The pinnacles result from erosion controlled partly by vertical joints and partly by local compaction of scoria by hot gases. (Photograph by George Grant, National Park Service.)

The basic scoria flows did not spread so far as the earlier pumice flows; many of them extended no farther than the limits of the park. Where they followed the canyons they were restricted to the central parts, as if they poured down median depressions in the pumice deposits and to some extent carved their own channels. On the north and northeast sides of the volcano they spread more widely, covering much of the so-called Pumice Desert and flooding the wide valley of Desert Creek. Some of the basic ejecta even traveled as far as the eastern foot of Mount Mazama and well out toward the Klamath Marsh.

In the canyons, the dark scoria commonly shows a well developed columnar structure (plate 17, figure I), caused by slow cooling of the compacted ejecta. So well is this structure developed that the deposits have often been mistaken at a distance for columnar flows of basaltic lava.

Plate 17. Fig. 1. Columnar scoria flow overlain by crystal- and lithic-rich ash, Annie Creek canyon. (Photograph by William Schoeb.)

Most of the basic scoria consists of highly vesicular brown glass, heavily charged with slender prisms of hornblende and fewer crystals of pyroxene and plagioclase. Some bombs, particularly in the Pumice Desert, are almost wholly composed of hornblende prisms.

Compared with the pumice flows immediately below, the scoria flows are generally much more crystalline. A few typical samples will illustrate the point. Considering only the fraction in the size range 0.25 to 10 mm, the pumice flow in Godfrey's Glen carries only 3 per cent by volume of crystals, whereas the overlying zone, transitional between the pumice and scoria flows, carries 9.6 per cent and the dark scoria, 14 per cent. Farther down Annie Creek, at the main falls, the pumice flow contains 8.7 per cent of discrete crystals and the scoria above carries 14 per cent. There are, however, other pumice flows which are richer than any of the scoria flows in crystals, as the histograms, figures 25 and 26, show.

Another feature distinguishes the pumice flows from the scoria flows, namely, the higher proportion of heavy minerals in the latter. Though the ratio of feldspar to heavy minerals in the pumice deposits ranges between 5:3 and 7:1, averaging approximately 3:1, the ratio in the scoria flows rarely departs much from 1:1. Presumably this increase in heavy minerals among the scoria flows was caused by gravitative settling in the magma chamber prior to eruption. For the same reason, the final ash fall which followed the scoria flows is abnormally rich in crystals.

Significant in connection with the mode of deposition of both the pumice and the scoria flows is their high porosity. Coupled with the high content of fine dust, it signifies continuous frothing of the ejecta during transport. It was this auto-explosive property that enabled the flows to move for such long distances even down slight gradients.

Neither the pumice flows nor the scoria flows contain more than a very few breadcrust bombs. The reason is not far to seek: in order to form breadcrust surfaces, bombs must be suddenly chilled on the outside while the gases within continue to expand and produce tension cracks in the glassy skins. When bombs are blown high in the air the tendency is, of course, for the crust to chill to glass. If, on the other hand, the bombs remain hot until most of their gas is exhausted, as they must do in glowing avalanches, they fail to acquire glassy "breadcrusts." Accordingly the conclusion seems warranted that when the pumice and scoria flows were erupted, few of the ejecta rose high above the crater. The bulk of the fragments fell to earth almost at once and, enclosed in the hot gases of the avalanches, lost their heat slowly.