84 Volume of the Pumice-Scoria Flows

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

Volume of the Pumice-Scoria Flows

Little difficulty was experienced in calculating the volume of the pumice fall, for the base of the deposit is exposed in many places and the thickness is easy to determine elsewhere by excavation. The pumice and scoria flows, on the other hand, were voluminous enough to obliterate much of the pre-existing topography and convert deep glacial valleys into broad plains. Locally, rivers have cut through the deposits and exposed the moraines and lavas below. From these known thicknesses and by making reasonable assumptions as to the form of the buried parts of the glacial valleys, it is possible to calculate the volume of pumice and scoria in the principal valleys.

On the plateau east of Mount Mazama, the topography offers little evidence concerning the thickness of the ejecta. Fortunately, several wells were drilled by the Southern Pacific Company along their tracks in this region, a few were drilled in Chemult and elsewhere for local residents, and three were drilled for the purposes of the present study, so that we now have a fairly accurate picture of the form and character of the pre-pumice surface. Between the base of Mount Mazama and the Klamath Marsh, this pre-pumice surface was a broad piedmont plain sloping eastward. Here and there low knolls of basalt rose above the general level, but for the most part the surface consisted of volcanic sands and gravels laid down by streams draining the eastern slopes of Mount Mazama. The following data concerning the pumice and underlying deposits are relevant in the present connection.

Close to the snout of the pumice flow at Chemult several wells have yielded records. One, drilled for the Great Northern Railroad in 1932, revealed these data: 0-41 feet, pink pumice; 41-62 feet, clay; 62-68 feet, ash; 68-180 feet, interbedded hard and soft clay; 180-223 feet, basaltic lava.

In the village of Chemult itself, six wells provided logs. The thickness of the pumice ranges between 34 and 45 feet. In all, the topmost pumice is extremely fine, to a depth varying from 20 to 25 feet. Beneath this, the pumice is heavily charged with large lumps. Apparently the coarse lump pumice represents the pumice flow proper, and the overlying fine ejecta rep resent material that settled from the air after the flow had come to rest. In some wells the pumice was found to rest on red clay; in others it rests on basaltic sand and gravel; in one at least it is underlain immediately by vesicular basalt.