These unique obsidians are found at Glass Buttes, Oregon. Spall from knapping obsidian arrowheads and other tools. Spalling and spallation both describe the process of surface failure in which spall is shed. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball bearing). Spall are fragments of a material that are broken off a larger solid body. This can occur without penetration of the plate. The impact causes the projectile to disintegrate, and generates a large number of small fragments from the aluminium ( spallation). Very high-speed photography of a small projectile striking a thin aluminium plate at 7,000 m/s. For other uses, see Spall (disambiguation).
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