63 The Main Pumice Fall – Distribution and Thickness

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 Main Pumice Fall

     Distribution and Thickness

The culminating activity of Mount Mazama began with the expulsion of finely divided dacite pumice, projected high above the summit of the volcano and then drifted afar and winnowed by the winds. From the distribution of this pumice (map, figure 16), it follows that during the eruptions the prevailing winds were from the same quarters as at present. At first the winds blew toward the east, but as the explosions increased in violence the winds veered toward the northeast. Two features are thus explained: first, the pronounced eastward bulge of the isopachytes toward Summer Lake and the absence in that direction of much coarse pumice, particularly in the lower part of the deposit; second, the prevalence of coarse pumice on top of the fine in the region northeast of Crater Lake.

Except near the crest of the Cascade Range, the pumice fall mantles the surface with amazing fidelity. On steep, rocky slopes much pumice has been washed to lower levels, and elsewhere it has drifted into depressions to a depth of 50 feet, but in general erosion has done little to modify the thickness of the sheet. The meager rains which fall east of the Cascade crest percolate readily through the pumice and there is little transport of material save on the banks of the principal streams. So faithfully does the pumice sheet reflect the underlying surface that test pits excavated at the tops and bottoms of large hills show the thickness to vary only within a few inches over areas of several square miles. It is as if the ejecta had been “ducoed” onto the preexisting slopes. Where the thickness of the mantle exceeds 2 feet, hardly a single exposure of “bedrock” may be seen for miles.

The simplified isopachytes shown on the map (figure 16) are based on measurements made along highway and railroad cuts and on a large number of test pits.3 No attempt was made to plot the outermost limit of the pumice, for where the deposit diminishes to less than 6 inches in thickness, it is generally so mixed with soil that accurate estimates are no longer possible. Nor was it feasible to plot the isopachytes within the park boundaries, for close to the source, where erosion has been more marked and where the onrush of the later pumice flows obliterated much of the pumice fall, the thickness is extremely variable. Near the northern margin of the sheet, the ejecta are buried beneath younger pumice deposits from the Newberry volcano and beneath basaltic cinders erupted from scores of cones in that vicinity. Beyond the northern limits of these younger ejecta, the pumice from Crater Lake occurs only as a light sprinkle in the soil.

A few of the outstanding features of the pumice distribution deserve to be emphasized. The margins of the deposit do not diverge from the source as might be expected, nor does the pumice thin uniformly in radial directions. On the contrary, the isopachytes run more or less parallel, and along the south and southwest edges of the deposit the, pumice thins much more rapidly than elsewhere. Generally, the front of an ash or pumice fall widens with increasing distance from the vent; that it did not in this case suggests that strong winds were confined to a narrow belt. A similar lack of spreading was observed in the pumice blown across Argentina during the 1932 eruptions of the Chilean volcano Quizapu.4