Secondary electron imagery of microcracks and hackly fracture surfaces in sand-size clasts from the 1980 Mount St. Helens debris- avalanche deposit: implications for particle-particle interactions

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. 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. -from Authors