The Grouse Hill Dome and Flow

A short distance beyond the north rim of the caldera rises the steep-sided and wooded Grouse Hill. Diller supposed that the lava of Grouse Hill was older than the Llao Rock flow, because it seemed to have suffered more from erosion, and because it seemed to be partly overlain by the same sheet of pumice that underlies the great dacite cliffs of Llao. The first of these arguments is questionable, and the second is invalid, for the pumice on top of Grouse Hill is in reality part of the same sheet that lies above the Llao flow. There is, in fact, no way of deciding which of the two flows is the older. Probably they are of about the same age.

Form and internal structure. The culminating explosions of pumice from Mount Mazama left a thick sheet of ejecta on top of Grouse Hill. Exposures of the underlying lava are therefore comparatively rare. But the sides of the hill are steep and for long stretches they are almost precipitous. Here, the lava is perfectly exposed and the attitude of the flow planes may be mapped in detail (figure 11). In general, as Allen observed,² the banding steepens toward the summit, with the result that, in cross sections, the structure is fan-shaped. Over most of the surface the banding is either vertical or almost so, and it may even maintain this attitude to the edge of the hill. Close to the steep western margin, however, the flow planes commonly dip outward or bulge in that direction. How is this structure to be interpreted?

Grouse Hill may be divided into two parts (figure 11). At the south end there is a domical hill, approximately 3/4 mile in diameter. Here the flow planes are arranged in a crudely concentric manner about the summit. To the north lies a mesa, approximately 1/2 square mile in extent. The flow planes in this area generally trend north-northeast except close to the margins, where they are roughly concentric. Assuming that the lava was poured onto an evenly inclined slope, the thickness in the domical part must be 800 or goo feet and in the mesa part about 250 feet. These figures alone are enough to suggest, but coupled with the attitude of the flow planes they clearly indicate, that the vent lies immediately beneath the center of the dome. Apparently the first lava to escape moved sluggishly toward the north, ending in a steep, blocky front. Subsequently the lava became increasingly viscous and accumulated close to the orifice in the manner of a Pelean dome. The total amount of material extruded approximates 1/6 cubic mile.

The lava. The dominant lava is a pale-gray, minutely vesicular and glassy dacite, richly charged with phenocrysts of plagioclase, pyroxene, and hornblende. Quartz appears to be absent. In many specimens, glassy and lithoidal layers alternate rapidly, though the distribution of the two types is quite irregular. Strong fluidal banding is almost ubiquitous, and generally it is emphasized by a platy or slabby jointing. As compared with other dacite flows, the Grouse Hill lava is rarely spherulitic and lithophysae are uncommon. Basic inclusions are also scarce, though they seem to be slightly more numerous in the dome than in the flow. Otherwise there is no appreciable difference between the material of the dome and the lava extruded earlier. Streaks of black obsidian, scoriaceous bands, and layers deeply reddened by gas action are scattered at random throughout.

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