When rock hits another rock, pressure waves spread from surface and from inside. When inside pressure wave reaches surface, reflection changes phase by 180 degrees. At a thin layer {spall zone} at surface, pressure wave from surface and pressure wave from reflection almost cancel, and surface pressure is near zero. However, pressure just below surface is high, and rock erupts through surface. In cosmology, expanding universe has spall zones.
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Date Modified: 2022.0224