40 Dacite Pumice

The Geology and Petrography of Crater Lake National Park, 1902






The dacitic eruptions of Mount Mazama were generally accompanied by more or less pumice. Pl. IX, A, shows a large amount of pumice which underlies the Llao Rock dacite and which is, therefore, somewhat older than that flow. It is arranged in layers, probably due to the assorting influence of the atmosphere. Under the main body of the flow the sheet of pumice is sometimes wanting or thin and irregular. It is possible that this pumice belongs to the earlier part of the eruption which gave rise to Llao Rock.

The summit of Llao Rock is composed of fragmental material, approximately 120 feet in thickness, of which the lower 90 feet is pumice resting on the surface of the dacite. It is clear that this body of pumice is later than the Llao Rock flow, although it may belong to the final portion of the same eruption. The layer of white, pinkish, or yellowish pumice of the ordinary dacitic type (136) on the summit of Llao Rock is overlain by 20 feet of more or less pumiceous material, which is heavier and much darker in color. The fragments, like those of the layer below, are small, being rarely as large as 6 inches in diameter. Some of them (143) are black, rich in phenocrysts of hornblende and feldspar, and are doubtfully of a dacitic nature. There can be no question that the darker layer represents the concluding stage of the final explosive eruption of Mount Mazama. It is one of the largest masses of this sort of material found anywhere about the rim crest, and is of unusual importance on account of its peculiarities, for it is markedly unlike any of the dacitic lavas.

Along the crest above Danger Bay, near Sentinel Rock, there is a succession of pumice and glacial deposits. There are two deposits of what appear to be glacial gravels succeeded by layers of very pumiceous material, recording the fact of alternating glaciation and volcanic eruption. The lapilli of the two pumice layers appear to be the same. Farther to the northwest, beyond Cloud Cap, a thin sheet of tuffaceous dacite appears at the top of the lower pumice.

Tuff with much pumiceous fragmental material is widespread in the Crater Lake region, and is so abundant in places as to indicate a great explosive eruption along the final events in the history of Mount Mazama. Pumice Desert is a treeless tract north of the rim, in which pumice prevails. Fragments are usually from 1 to 4 inches through, although some nearly a foot in diameter are common in places, and occasionally larger ones appear. The largest mass of dacite seen (138) is 10 feet in diameter. It is very pumiceous and yet distinctly fragmental. Near by was a fragment 3 feet in diameter, rich in hornblende. Such fragments are not abundant, although black sand (134) composed chiefly of feldspar and hornblende is abundant everywhere. Here and there may be seen occasional chunks of andesite. From Mount Thielsen to Red Cone, a distance of over 10 miles, nothing but fragmental material was seen near the trail. As Red Cone is approached some of the red and black lapilli washed from the cone appear upon the surface to overlie the dacitic pumice and tend to give a wrong impression as to their order of eruption. The sheet of pumice, although not indicated upon the map, covers by far the larger part of the flows, but ledges are in general sufficiently numerous to determine approximately the distribution of the various kinds of lava. Along the southern border of the rim the greatest thickness of pumice, about 10 feet, is exposed at the top of Dutton Cliff. The largest fragments at that place are not more than a few inches in diameter and appear to be the farthest from the rim. The peculiar black lapilli rich in crystals of hornblende are numerous. Specimen 142 was collected from a piece 18 inches in diameter. Near the same place, in 1883, I found a bowlder-like bomb of the same size as that noted above. It had a blackened shell which was cracked to a little depth, and the cracks stood open like those of a bursted apple. The outside was very hard and glistened (148) as if melted and glazed. Within the rock was very porous and pumiceous (147). Near by another specimen had a thin, reddish, cracked rind with a more crystalline granitic center. Specimens 150 and 151 also were found along the northern rim of the lake and belong to this peculiar ejectamenta.

On the broad divide north of Crater Peak is a moraine composed largely of andesitic material. It is covered with dacitic pumice in such a way as to mark the relative age of the glaciation and eruption.

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