The Sand Creek Lava Flows - Crater Lake National Park

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The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

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The Climax: Culminating Explosions of Pumice and Scoria

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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.

The smoke-gray scoria layer, averaging approximately 80 feet thick. This is made up of bombs of crystal-rich, basic scoria up to 2 feet across, set in a finer matrix of pulverized scoria. Fully 90 per cent of the fraction larger than 5 mm. consists of scoria; the remainder is made up of old lava fragments. Despite the large size of the scoria bombs, few of the lithic pieces exceed even an inch across. In the finer fraction of the deposit, the content of lithic detritus is considerably higher, making up 18 and 23 per cent of the volume of two samples examined. Most of the lithic chips lie in the size range 1 to 5 mm., few occurring in the fraction less than 0.5 mm. As compared with the underlying dacite pumice, the scoria layer is generally much richer in crystals, particularly in ferromagnesian minerals. Moreover, though pyroxene predominates among the dark minerals of the pumice, hornblende predominates in the scoria. In some samples, between one-third and one-fourth of the fraction below 5 mm. in diameter is made up of crystals. How thorough the comminution of the scoria must have been is amply shown by the fact that of the material measuring less than 5 mm. in diameter, more than half consists of dust less than 0.5 mm. across.

The bottom layer of dacite pumice, also averaging approximately 80 feet in thickness. Lithic detritus makes up on an average approximately one-fourth of the volume of material less than 5 mm. in diameter, but in the entire deposit the content of such detritus is only about 10 per cent. By far the bulk of the lithic material lies in the size range 2 to 5 mm.

Of the fresh magmatic material in the deposit, bombs of pumice up to 2 feet across form between 5 and 10 per cent of the volume, the remainder consisting of pulverized pumice, much of it in the form of impalpable dust, and crystals, among which feldspar far predominates over pyroxene.

Beyond the park boundary, the smoke-gray scoria layer disappears, though many scattered bombs of scoria may be found close to the Dalles-California highway. Whereas the earlier pumice flow continued for many miles, the later scoria flow spent itself soon after reaching the flats at the base of the volcano.

From the histogram, figure 22, it will be seen that in the pumice flow near Sun Pass, approximately 40 per cent of the material less than 10 mm. in diameter measures less than 0.5 mm. Here the volume percentages of pumice, old rock, feldspar, and heavy minerals in the size range 0.25 to 10 mm. are respectively 65.7, 24.9, 8.2, and 1.2. When the coarser material is also considered, the total volume of lithic material is reduced to between 10 and 15 per cent by volume, for among the fragments more than an inch across fully 95 per cent consists of pumice. The largest lithic fragments measure 3 inches across.

We may now briefly follow the course of the Sand Creek pumice flows after they deployed onto the plateau east of Mazama. Such was their mobility and momentum that they rushed onward for another 12 miles. In their path lay Boundary Butte. Up the onset slope of this obstacle they surged for 200 feet, carrying lumps of pumice 2 feet in diameter. Swirling round this and the neighboring buttes, the flows then emptied into the canyon of Williamson River, near the present site of the village of Kirk. In doing so they must, for a time, have choked the outflow of water from the Klamath Marsh. When the channel was cleared, great quantities of pumice were carried downstream into Upper Klamath Lake.

In the vicinity of Boundary Butte, the pumice deposits are marked by flat-topped benches m to 30 feet above the general level. The road running northwest from Kirk passes through a hollow between such benches. How they were formed is not clear. Either the deposits were channeled by floodwaters in the wake of the flows, or, more, likely, the benches resulted from successive waves of pumice.

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