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Fig. 12. Section across the Cleetwood dacite flow, showing the internal banding of the flow and its feeder, and the relations to the underlying ejecta of Mount Mazama. Section A across the east “wing” of the flow on the wall of Cleetwood Cove; section B through the center of the cove. |
No feature of the Cleetwood lava is more conspicuous than the ruggedness of its glassy crust. Near the caldera rim it bristles with pinnacles and jagged spires, some more than 100 feet in height. Bordering the Rim Road, the lava forms a veritable wilderness of crags. These are not arranged haphazard, but in more or less parallel lines separated by gullies, up to 100 yards in width and 150 feet in depth, disposed at right angles to the direction of flow. The gullies seem to be analogous in mode of formation to the transverse crevasses which develop on the surface of glaciers where they plunge over steep slopes. A quarter of a mile beyond the rim of the caldera, the lava becomes much smoother and the gullies disappear, just as the transverse crevasses on a glacier heal and close as the ice moves on to gentler gradients. We may conclude, therefore, that the old slope of Mount Mazama onto which the Cleetwood flow was erupted was much steeper near the point of discharge than farther north. As the stiff lava moved down the steeper slope, the glassy crust was rent by transverse fissures, and the great block ridges were transported on the viscous layers below at different rates. During this motion many of the blocks were tilted so that their banding is now lakeward, in the opposite direction from the banding at the base. Precisely the same phenomenon may be observed on several volcanic domes in Java, notably on Merapi and Soembing. There, viscous lava was protruded onto the sloping floors of craters and continued to pile above the vents until the domes began to slide downhill, leaving in their wake crescentic fissures and lines of crags like those on the Cleetwood flow. The evidence is lost, but it seems permissible to assume that above the feeder of the Cleetwood lava, over what is now Cleetwood Cove, a steep-sided, high dome accumulated, with a fan-shaped internal structure. When the mound of dacite had grown until it became unstable on the sloping floor, the cooler upper part began to slide downhill on the hot lava below.
