The breaking of magma

Abstract

SUMMARY Breadcrust blocks (bombs) on Vulcano, Sicily, show features which indicate that when they were of viscous lava they broke when stress was applied rapidly, and later flowed when it was applied slowly. If the magma of granitic intrusions behaves similarly, the perplexing “relict dykes” can be explained as due to the breaking of the magma and the injection of dykes into the fractures.