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.

Approximately 6 miles south of Chemult, at Diamond Lake Station (formerly Lonroth) on the Southern Pacific Railroad, the pumice is 65 feet thick and rests on at least 96 feet of stratified sand, gravel, and clay. Near by, in the hamlet of Beaver Marsh, a well penetrated 73 feet of pumice without reaching bottom. At Yamsay Station, approximately 5 miles farther south on the Southern Pacific tracks, the pumice is 53 feet thick. A mile or so northwest of this station, at the junction of the Dalles-California highway with the Diamond Lake highway, the pumice is reported to be 114 feet thick and to rest on "black sand."

At Lenz Station, 9 miles south of Yamsay, the pumice is 65 feet thick and rests on sand and gravel. At Fuego, approximately another 9 miles to the south, two wells were drilled, one 500 feet and the other 136 feet west of the station. The former penetrated pumice to a depth of 70 feet without reaching the underlying bedrock; the other passed out of pumice and entered sand at a depth of 55 feet.

At Kirk Station, close to the south margin of the avalanche deposit, the pumice is 47 feet thick and rests on so-called "chalk rock," probably Pliocene diatomite.

There are few available records of wells drilled through the pumice flow east of the Southern Pacific Railroad. On the Fordyce or Ryan Ranch, a short distance south of Lenz Station, three wells penetrated the deposits and entered coarse sand at depths between 18 and 30 feet. Farther east, near the Old Military Crossing, on the edge of Klamath Marsh, the pumice flow is 66 feet thick and is underlain by massive basalt. According to George Hartley, wells sunk near the center of Klamath Marsh pass through layers of pumice interstratified with sediments even to a depth of 350 feet. Doubtless these thin pumice layers represent products of much earlier eruptions.

In order to determine the thickness of the pumice and scoria flows closer to the base of Mount Mazama, three wells were drilled west of the Dalles-California highway. The first was 1 mile south of the northwest corner of the Klamath Indian Reservation, at an elevation of 4700 feet. Down to a depth of 80 feet the well passed through smoke-gray pumice and scoria, rich in crystals and small lithic fragments. Approximately 10 feet from the surface, the ejecta were stained bright pink by fumarolic action. From 80 to 93 feet, the well penetrated white lump pumice; from 93 to 96 feet, it traversed water-worn volcanic sand and gravel.

A second well, 5 miles to the south, passed through the following layers: 0-5 feet, crystal-lithic ash with fine pumice; 5-20 feet, pink dacite pumice; 20-80 feet, pumice mixed with smoke-gray scoria and crystal ash; 86-121 feet, coarse white dacite lump pumice; 121-125 feet, pumice-free volcanic sand and gravel.

The third well was sunk 8 miles farther south, near the bank of Scott Creek, approximately 3/5 mile west of the Dalles-California highway. In the upper part of this well, the smoke-gray scoria and pumice were abnormally rich in small lithic fragments, to such an extent in fact that certain layers contained as much as 75 per cent of such ejecta. From 50 feet down to 75 feet, basic scoria was absent and lithic fragments were rare. In this range, the ejecta consisted almost entirely of coarse dacite pumice lightly mixed with crystals. From 75 feet to the bottom of the well at 100 feet, there were rapid alternations of fine, granular pumice and water-worn volcanic sand. Presumably, therefore, the base of the pumice-scoria flows may here be placed at 75 feet; the underlying deposits represent the products of older pumice eruptions mingled with river-borne sediment.

We are now in a position to make an estimate of the volume of the pumice-scoria flows. First, consider the area east of the park. From the foregoing well logs, and taking into consideration the probable pre-pumice form of the valleys that debouch from the park onto the plateau to the east, it may be calculated that approximately 4 to 4.5 cubic miles of material were laid down by the glowing avalanches. How much pumice escaped from the Klamath Marsh into the Williamson River and so was washed into Upper Klamath Lake cannot be determined.

Within the limits of the park, the volume of the pumice-scoria flows, prior to erosion, approximates 1 cubic mile. In the valley north of Timber Crater and in the depression bordering Diamond Lake and Lake Creek, the volume may be about 0.6 cubic mile. In the valley of the Rogue River down to the Union Creek junction, and in National, Copeland, Bybee, and Crater creeks west of the park boundary, the total volume is estimated to have been 0.75 cubic mile prior to erosion. Finally, between the Union Creek junction and the snout of the Rogue River flow near McLeod, the original volume must have been close to 0.3 cubic mile, whereas in the valley of Annie Creek south of the park boundary there may have been approximately 0.2 cubic mile of pumice. What can never be known is the amount of pumice that was washed into the Upper Klamath Lake below Fort Klamath, and the amount that was carried down the Rogue River below McLeod. It is likely, however, that this missing volume was not great.

The total volume of the pumice and scoria flows is therefore about 7 cubic miles. Almost certainly, it is not less than 6 nor more than 8 cubic miles. Of this volume, at most 20 per cent consists of old lithic detritus. In other words, of the 17 cubic miles that disappeared from the top of Mount Mazama, only a small fraction is to be found in the deposits of the glowing avalanches. The proportion of lithic detritus in the preceding pumice fall is even less. The significance of these figures in connection with the origin of the caldera needs no emphasis.

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