The Last Weak Explosions
After the pumice and scoria flows had been discharged, most of the energy of the volcano was spent. Nevertheless, enough gas remained in the magma chamber to cause a few dying explosions.
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Plate 17. Fig. 3. Pumice deposits on Diamond Lake highway. At the base, 4 feet of coarse, unbedded pumice-flow deposits containing a charcoal log (near hammer). Above, bedded,
fine pumice fall. The dark band near the top is the pink zone caused by oxidation of fumarolic gases rising from the pumice flow at the base. This indicates that the fine pumice
fell on the flow while it was still hot. |
The concluding eruptions cannot have been violent, for the ejecta are fine and limited in distribution. Most of the material was blown above the vents and dispersed by winds, just as the initial deposits of pumice were scattered. But unlike the earlier pumice fall, the final fall contains few large lumps of pumice, and is much richer in both lithic fragments and crystals. Probably most of the pumice blown out by these last explosions represents material that fell back into the craters during the first explosions, to be hurled out a second time. Much of the ejecta consists of dark-brown and reddish crystal-rich scoria and small fragments of old lava. The high content of lithic detritus is only to be expected, for after the great pumice and scoria explosions the conduits must have been greatly enlarged and their walls must have been subject to inward slumping.
Most of the ejecta fell near the caldera rim, especially on the northeast side, where they form banks up to 50 feet in thickness. Some material was blown westward and may be seen resting on top of the pumice flows along the Rogue River (plate 17, figure 3). Compared with the earlier pumice falls, the youngest ash is much finer, better sorted, and richer in crystals and lithic fragments. Typical samples are represented by the histograms (figure 21, Rugged Crest; figure 25, no. 360; figure 26, nos. 230c,
36a).
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Fig. 21. Samples of pumice fall. Samples numbered 252 to 258 collected on Dalles-California highway at the following distances from Crater Lake: 24, 25, 28.5, 30.5, 37, 45, and 60 miles. Wikiup sample,
60 miles NNE.; Lakeview highway sample, 48 miles NE. In samples from Dalles-California highway, content of lithic fragments omitted, but usually less than 1 per cent. |
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Fig. 25. Histograms of pumice deposits. All but the first from pumice flows.
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Fig. 26. Histograms of pumice-scoria flows in Annie Creek canyon
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The fine ash overlying the pumice-scoria flows in the canyons of Castle, Annie, Sun, and Sand creeks cannot be a product of these eruptions, for it is restricted to the central parts of the valleys. For this reason, it is interpreted as the result of settling of the finer constituents that rose into the air as the avalanches swept down the valleys. The thickness of such ash locally reaches 50 feet, but is usually between 5 and 20 feet. It fell immediately or soon after the flows came to rest.
Similar in mode of origin is the fine glass dust which overlies the pumice flows near Chemult. No scoria flows reached this far from the source, and consequently there is no crystal-rich ash; on the other hand, there must have been violent turbulence at the snouts of the pumice flows, and great clouds of fine glass dust must have risen into the air, to settle slowly on the coarse deposits.
The volume of the final "ash" fall is probably of the order of 0.25 mile.
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