Three fragmentation mechanisms for highly viscous magma under rapid decompression

Analysis of the behavior of viscous magma under short-term tensile stresses demonstrates that the strain rates anticipated in the eruptive stages of collapse-related events may be high enough under certain circumstances to provide fragmentation of the melt by brittle failure. Mechanisms of fragmentation of a highly viscous vesicular magma by the propagation of an unloading wave, by layer-by-layer vesicle bursting (fragmentation wave propagation) and by rapid filtration gas flow are outlined. Fragmentation scenarios involving possible combinations of the three fragmentation mechanisms are evaluated for two models of magma (with isolated and with connected pores). Experimental results: fur magma fragmentation under high decompression rates appear to support such mechanisms. These mechanisms of highly viscous magma fragmentation by rapid decompression probably have their greatest application in cases of explosive disintegration of lava domes and cryptodomes caused by collapse events. (C) 2000 Elsevier Science B.V. All rights reserved.