Dacites of Redcloud and Cloudcap

The topmost lavas on the caldera wall overlooking Cloudcap Bay are glassy dacites not unlike those of the Llao, Grouse Hill, and Cleetwood flows. They were likewise erupted after the andesitic phase of Mazama's activity had come to an end. Two main flows may be distinguished: an older one to the south, which seems to have had its source beneath Cloudcap, and a much thicker flow farther north, the Redcloud flow, which escaped from a vent on the caldera wall. The relations between them are indicated on the panorama, plate 23.

The V-shaped cross section of the Redcloud lava is hardly less impressive than the great cliff of Llao on the opposite wall of the caldera. In its central part, the lava forms a precipice 600 feet in height. Some who view this massive flow from a distance are inclined to suppose that it poured down a deep, V-shaped river gorge, filling it to the brim and then spreading over the banks. A moment's reflection should, however, dispel this idea. How could a V-shaped canyon of such proportions form on the side of Mount Mazama when U-shaped glacial canyons were being carved at the same time and at the same elevation a short distance away? Surely a river could not cut such a smooth-sided gorge across bedded lavas and ashes, and least of all so close to the summit of the mountain and in the short interval between successive advances of ice.

All doubt is removed by closer inspection. Then it becomes apparent that the lava occupies a funnel-shaped explosion vent and that the feeding pipe lies at the base of the great triangular cliff. Had the lava issued from a source higher up the slope of the volcano and flowed eastward down an old river gorge, the banding would dip eastward in conformity with the valley floor. In reality, the flow planes are disposed concentrically, like a series of hollow, inverted cones one within the other. Redcloud Cliff itself is not quite vertical, but inclines steeply toward the lake, and at its base there is a projecting, right-angled buttress, 20 yards long and 75 yards across, reminiscent of the larger buttress jutting from the base of Llao Rock. Within this buttress the flow planes are beautifully displayed, standing vertically and striking parallel to the margins. The conclusion cannot be avoided that this marks the actual conduit of the Redcloud dacite.

We may now follow the margin of the lava northward from the vent along the caldera wall. Upward from the conduit, along the edge of the V-shaped part of the flow, several patches of coarse explosion breccia may be seen firmly adhering to the dacite within. The flow banding stands vertically, dips inward, or, in a few places, dips steeply lakeward, which, of course, it could not do if the lava had poured down a river valley in the opposite direction. Beyond the V-shaped part of the flow, the thickness of the lava diminishes to about 300 feet. Here, the banding at the bottom of the lava stands vertically and strikes parallel to the face of the cliff. In fact for a short distance the older, gently dipping andesites abut against a vertical wall of dacite. Clearly, therefore, either the dacite piled against an outward-facing cliff of andesite or some of it rose through a vertical fissure extending northward from the principal vent. Higher up, the banding of the dacite gradually flattens and dips away from the lake.

Close to the top of Skell Head, the margin of the dacite leaves the caldera rim and swings downhill for more than 1/2 mile. Throughout most of this stretch, it forms a line of cliffs, up to 200 feet high. Where they are highest, the cliffs are remarkably smooth and traversed by broad, almost horizontal flutings, and it comes as a surprise to find that these features are only in part the result of glacial erosion and are mainly original structures of the flow itself. Along this edge of the dacite, the lava is rich in spherulites and lithophysae lined with plates of tridymite.

Near the snout of the flow, there are several large caves, either at the base or a short distance above. One of these measures 35 feet across the mouth and 10 feet in height, and can be traced under the lava for a distance of 100 feet. It was not the escape of fluid lava from beneath a solid crust, nor was it explosion that produced these openings; they seem rather to have been caused by the rucking up of the viscous base of the lava and by shearing within the flow as layers moved differentially.

Along the southeast margin of the dacite, the banding, even at the very edge, is generally vertical and in certain places bulges outward. On the hummocky top of the flow, the planes retain steep dips and are arranged more or less concentrically. In other words, we are faced with the strange phenomenon of a thick lava flow, approximately a mile in length, in which the marginal banding rarely departs much from the vertical and the banding of the interior parts is also highly inclined over most of its surface. In view of the recentness of the eruption, the top of the flow can have been only slightly modified by erosion, and the steepness of the sides can have been increased only toward the base. If the original margins were restored, they would probably resemble those of the Watchman flow, and we should see that the banding in the lower parts lay horizontally or dipped inward at low angles, whereas in the higher parts the inward dip rapidly increased to verticality. Under the central part, the flow planes must, of course, conform with the slope of the floor, but toward the top they must curve so as to emerge at right angles to the surface. This peculiar structure has never been observed among any of the andesites of Mount Mazama except in the Watchman flow, but is typical of all the dacites. Presumably it results from shearing of layers of extremely viscous, almost solid lava over one another. In this sense the flow planes may be compared to the rising thrust planes so commonly seen in glaciers.

Although the structure of the Redcloud flow is fairly clear, that of the Cloudcap dacite is obscure. The topographic form suggests that Cloudcap itself may be a dome-shaped protrusion above a vent immediately beneath the summit. On this assumption the long flows of dacite forming Scott Bluffs may be considered as earlier products from the same source. The presumption is that the thick sheet of dacite on the caldera wall immediately below the southern extension of the Redcloud dacite is also a product of the Cloudcap vent (panorama, plate 23). At the base of this zoo-foot sheet, the lava merges into 20 feet of intensely welded and banded dacite tuff rich in lithic blocks of andesite, identical with the welded tuff of the Wineglass. Beneath this lie the blocky breccias, coarse lump pumice, and tuffs that form the Cottage Rock. Clearly, the growth of the Cloudcap dome was preceded by long and violent explosions.

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