Redcloud Rock Slide
By L. T. Grose, Ranger-Naturalist
At 11:10 A.M., July 15, a tremendous blast, followed by a rumbling noise, resounded around the caldera walls. It sounded much like a series of thunder claps, and evidently, everyone thought it to be thunder from a lingering dark cumulus cloud overhead. Actually this “thunder” was the collapse and slide of a section of Redcloud Cliff. A study of the slide is significant because of its being typical of the larger rock slides and mass erosion within the caldera.
A slab, estimated to be 250 x 200 x 8 feet, of the older dacite flow under and immediately to the south of the V-shaped mass of Redcloud Cliff spawled off and slid 9/10 of the way down to the lake shore. One third of the largest blocks were carried farthest by their momentum, but none of them reached the lake. Slight sorting of the debris is evident on the lower portion, the heavier blocks being followed by increasingly smaller fragments. The original, underlying talus was pushed to the shore, extending the fan to the water’s edge. Therefore, the total rock slide continued 1,000 feet from the base of Redcloud Cliff to the water’s edge. Most of the larger fragments came to rest approximately half way down, and a smaller portion were stopped by the bottle-necking effect of big and little Castle Rock formations. In these higher portions there is little or no evidence of sorting.
The slide fragments came to an unstable rest at the maximum angle of repose, in this case of angular blocks, 43 degrees. It is believed that this angle is reached only in the upper third, and that the slide curves, as the lower portion grades down to approximately 38 degrees. This upward curving is characteristic of new, unstable, and active slides. The lower slide area is semi-safe to climb over due to frictional stabilization of detritus, but the larger rocks in the upper portion are delicately balanced. Prior to this recent slide the Redcloud talus slope was not at the maximum angle of repose, otherwise more than the total additional weight of new slide material would be carried into the lake.
Distortion and dilation transformed the dacite slab into fragments ranging from 1,000 cubic feet down to rock dust. On top of the larger blocks there was loose, fresh rock powder, a characteristic of very recent, large rock slides. Many hand-sized fragments appeared integrated and competent, but crumbled easily under fist pressure. A slickenside appearance is quite obvious on the adjacent andesite spurs, however none of this resistant rock broke loose. Amazingly enough, little Castle Rocks remain none the worse for wear, in spite of much rock battering over them. Small amounts of pumice slide with the main block, but the remains of this were crushed into dust.
The causes of the larger rock slides inside the rim are a number of interesting natural processes. Redcloud Cliff faulted downward on a nearly vertical plane which affords maximum gravitational pull. Water, directly and indirectly, has helped disintegration and decomposition within a series of cracks, mostly from the top downward. Water seeped from Cloudcap dome into tension cracks near the rim and froze. Seasonal and diurnal temperature changes, alternate freezing and thawing, greatly accentuated frost wedging, which can be an effective cause of spawling. Most likely by the middle of July the ice within the cliff melted sufficiently to hold the rock together no longer. The rock collapsed after the cementing ice had melted away.
A clayey surface was seen on some of the larger rock fragments. This suggests the slow decomposition and decay of the rock itself, or the washing in of volcanic dust, either one being a function of water and ice. The rock face fractured quite evenly along semi-columnar joints. No oxidation of iron or water seepages can be seen. The uniformly light tan of the cliff face indicates hydration as a mode of decomposition. This process produces a swelling of rock parts, allowing moisture to penetrate until some decay is effected. In conclusion, the primary cause was the increasingly powerful action of frost wedging, and secondarily, the expansion and decay due to hydration.
The newly exposed cliff remains very unstable, as well as the talus slope beneath. Fragmental bits are still constantly falling off. It will be interesting to watch — from a distance.
