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
Detailed Description of the Individual Flows
Having enumerated the general features of the glowing avalanches, we may now pass to an account of the individual flows.
The Sand Creek Flows
A third series of pumice-scoria flows descended the southern slope of Mount Mazama through Kerr Notch. When this occurred the valley below the notch was occupied for a distance of approximately a mile by a thin tongue of ice. For 2/3 mile down the valley from the notch the hummocky moraines are covered by a light sprinkle of pale-buff pumice, probably washed in at a later date. Farther down the valley the cover of pumice thickens, but even for another 1/3 mile it is patchy and fine. Only at a distance of a mile from the notch do large lumps of pumice and scoria appear in profusion. Between 1 1/4 and 1 1/2 miles from the caldera rim, the mantle of pumice and scoria begins to display the patchy red color indicative of fumarolic action. The inference seems justified that the Kerr Valley glacier cannot have extended this far.
Within the next 1 1/2 miles downstream as many as 150 “fossil fumaroles” can be counted on the canyon walls. The gorge rapidly deepens, becoming vertical-sided, and is cut entirely through smoke-gray scoria and crystal ash overlain by 6 to 10 feet of pumice fall. Here the red, oxidized layer close to the top of the canyon wall is very pronounced, and the scoria begins to show a large-scale columnar jointing that becomes more marked at lower elevations. Despite the fact that the gorge is cut largely in basic scoria, the surface of the plain into which it is incised is covered only with the buff pumice flow or with the overlying pumice fall. Accordingly, the scoria flow must have been confined, as in other canyons, to a central and narrow depression.
Close to the confluence of Wheeler and Sand creeks lies the area known as the Pinnacles, undoubtedly the theater of the most intense fumarolic activity within the park. Here the canyon reaches a depth of more than 200 feet and its walls are sculptured into clusters of slender pillars. Many of the pillars are traversed by vertical cracks along which the scoria has suffered from fumarolic action, being compacted by iron oxides or by the deposition of opal and kaolin. In a few pillars, long tubular channels lead upward to circular openings at the top. These mark the passageways of rising gases, to the oxidation of which we must ascribe the red layer which follows the top of the scoria deposit and extends upward a few feet into the overlying ash.
The threefold layering of the deposits at the Pinnacles is clear from the photograph (plate 16), the white and buff pumice layer at the base contrasting vividly with the dark scoria above, and this in turn with the overlying bedded ash on the rim of the canyon. In more detail, these three layers consist of:
- Top layer of fine ejecta, 6 to 10 feet. A sample from this layer contains little material more than I mm. in diameter; only I per cent consists of fragments between 0.5 and I mm.; 20 per cent is between 0.25 and 0.5 mm., the remainder being dust less than 0.25 mm. in diameter. Excluding the dust fraction, the remainder has the following percentage composition by volume: pumice and scoria, 44; lithic chips, 32; feldspar, 17; ferromagnesian crystals, 7. Though a third of the separable fraction consists of rock particles, the total content of such material in the deposit is probably much less than one-fourth, since by far the bulk of the fine dust is made up of pulverized pumice, scoria, and crystals. That the ejecta in this topmost layer fell from the air can hardly be doubted in view of their stratification.