Abstract

Scanning electron microscope images of 0.5-1 mm clasts from the volcanic debris- avalanche deposit of the May 18, 1980, eruption of Mount St. Helens display two features not previously described in other volcaniclastic deposits: microcracks and hackly fracture surfaces.Microcracks characterize pyrogenic crystals (pyroxenes, amphibole, plagioclase, and quartz)as well as dacitic, andesitic, and basaltic vitric rock fragments from the avalanche debris.Microcracks commonly resemble cracks separating blocks with a jigsaw fit, as observed on an outcrop scale. Many microcracks (0.5-5 μm) show evidence of lateral displacement, separation, and intersecting relations. In addition, projecting surfaces of fragments exhibit well-developed backly texture defined by crushed areas on the surface. These areas form a dense network of smooth and concave truncating fracture surfaces that resemble fish scales. Regions of low topography on the surface of rock fragments show virtually no evidence of breakage, cracking, or abrasion in contrast with topographically higher and protruding regions. Different mechanisms appear to be responsible for the development of these two surface textures. The microcracks may have resulted from the repeated propagation of compression-rarefaction stress waves that were generated close to the mountain as it disintegrated and the initial slide blocks and later debris avalanche moved over rough topography. Hackly fractures most likely resulted from the pounding of grain-surface projections by repeated high- pressure contacts of grains that remained in close proximity during transport within the debris avalanche.

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