The Cleetwood Flow

At Cleetwood Cove, on the north wall of Crater Lake, the stratified lavas and pyroclastic rocks are suddenly interrupted by a dark, jagged tongue of dacite which descends from the rim to the water's edge. The caldera rim, elsewhere fairly smooth, is here so broken by bristling crags of glassy lava as to be called the Rugged Crest.

Diller was much impressed, as every geologist must be, by the fact that the lava forming the tongue on the caldera wall dips lakeward, whereas that on the outer slope generally dips in the opposite direction. This, he thought, added proof to his conclusion that Crater Lake was formed by engulfment and not by explosion, for there seemed to him no escape from the inference that the Cleetwood lava was fluent when the summit of Mount Mazama collapsed. Escaping from a vent near the rim of the caldera, some of the lava moved down the outer slope, while the remainder poured slowly backward into the newly formed depression. Finding no alternative explanation of the supposed "backflow," he was obliged to conclude that the Cleetwood lava was among the final products of Mount Mazama and the youngest of all the dacite flows.

Among the geologists who subsequently paid brief visits to Crater Lake, there were a few who found it difficult to accept Diller's interpretation of the "backflow," and some entertained the idea that faulting on the caldera wall might in some way offer an explanation. Allen, however, was the first to understand the relations correctly. He saw that if the "backflow" is regarded as an "upflow," in other words as the actual feeder of the Cleetwood dacite, all difficulties disappear. The lakeward dip of the lava on the wall does not mean movement in that direction, but exactly the reverse. When the summit of Mount Mazama collapsed, the bounding fracture sliced across the inclined conduit of the Cleetwood flow.

Beneath the flow lies a thick deposit of dacite pumice, interbedded with thin tongues of andesite. The relations are identical with those at Llao Rock, where similar beds of pumice, intercalated with andesites, underlie the great flow of dacite. The low margins of both the Llao and the Cleetwood flows were covered by ice at a later date, but their higher parts remained bare. Presumably, therefore, the two lavas were erupted at about the same time, and long before the catastrophe which produced the caldera.

The pre-eruption surface. Before the Cleetwood lava was extruded, there must have been in this vicinity a broad and shallow basin with a hummocky floor. Whether it was cut by ice or by glacial streams is not clear, for wherever the base of the lava is exposed it rests either on andesite or on pumice with no intervening layer of glacial material. Beyond the rim of the caldera, the slope at first was steep and then gradually flattened, the average angle for a mile being approximately 15°. Outward, also, the valley walls diminished in height and soon disappeared. Possibly a corrie glacier had scooped out the depression.

The feeder. It is not surprising that in the limited time at his disposal Diller decided that the lava on the caldera wall was a "backflow," for the banding conforms closely to the slope and the rugged, fissured surface suggests a recent flow, frozen in its cascade toward the lake. This appearance, as we have seen, is deceptive. The ruggedness of the surface results from erosion of the lava along joints arranged parallel and perpendicular to the banding, and from the glassy nature of the lava itself.

The contact of the feeding pipe with its walls cannot be seen, though the pipe must widen upward. In general, the banding also becomes steeper in that direction. Close to the lake shore, it lies almost horizontally or dips into the wall at low angles. Higher up, the lakeward dip is generally between 30° and 40°, and where the lava escaped at the surface the banding is vertical or even overturned. In the flow proper, the dips are generally away from the lake, at low angles near the base and at increasingly high angles in the upper parts. Both in the feeder and in the flow adjacent to it, the fluxion planes strike a p proximately east-west, right across Cleetwood Cove.

There is little difference between the dacite in the feeder and that in the flow. Much of the flow consists of jet-black and dark-gray glass, whereas the feeder is composed of slightly duller and more crystalline dacite, as might be expected considering that it cooled more slowly beneath the surface.

The flow. The present area of the Cleetwood flow is a little more than a square mile. Its southern end disappeared when the caldera was formed, but it can never have extended far toward the south, since progress in that direction was impeded by the opposing slope of Mount Mazama. Assuming an average thickness of 300 feet, approximately 1/15 cubic mile of lava was erupted.

Despite its great thickness and volume, the lava was too viscous to move much more than a mile even with the assistance of a 15° slope. On all sides it ended abruptly. Further proof of extreme viscosity may be seen at the base of the flow, where there is commonly a highly brecciated layer up to 12 feet thick, composed of black glass including sporadic blocks from the underlying pumice. Above this basal part rests a layer, averaging 6 feet in thickness, characterized by intensely convoluted flow bands, indicative of viscous drag against the blocky bottom. Above this, in the main part of the flow, the lava is paler and slightly more crystalline; the banding here is no longer contorted, but lies almost horizontally below, then sweeps up to verticality and finally turns over at the surface (figure 12). The crust of the flow, like the bottom, is a glistening black obsidian. The arrangement of the flow bands is thus identical with that seen in the Llao flow.

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.

North of the crevassed area of the flow, farther from the vent, the surface is largely concealed by pumice and scoria, and the scattered outcrops of red, brecciated lava show little fluxion. Along the steep eastern margin, however, the banding is clearly displayed, dipping inward at low angles near the base but steepening inward and upward to verticality. In a few places, the marginal lava shows a gentle outward dip. How closely these structures resemble those seen in the Grouse Hill and Llao Rock flows is apparent from a comparison of figure 12 with figures 10 and 11.

Nature of the lava. The interior parts of the Cleetwood flow are normally pale gray and locally almost pumiceous, but the top and bottom usually consist of black obsidian, and it is here that the best development of spherulites and lithophysae may be found. Throughout the mass, fluidal banding is strong, and generally it is emphasized by platy and slabby jointing. On some of the joint faces, particularly at the top of the flow, specular hematite forms a patchy coating. It is also in the higher parts that the lava has been most reddened by escaping gases. As compared with the andesites of Mount Mazama and indeed with most of the other dacites, the Cleetwood flow is poor in basic inclusions. Porphyritic plagioclase is ubiquitous, but quartz seems to be absent; the other phenocrysts are generally hypersthene and red-brown hornblende, augite either being absent altogether or occurring only as microliths in the glassy groundmass.

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