Dominant time scale for brittle fragmentation of vesicular magma by decompression

Abstract

Brittle fragmentation was examined in a vesicular material analogous to magma, in this case, maltose syrup with bubbles. All the key time scales for magma fragmentation are controlled in the experiment using a rapid decompression facility. It was found that the onset of fragmentation can be well characterized, using the Maxwell relaxation time Tr and the decompression time Δtdec, in the case where sufficiently large stress is generated in the material. As the ratio Δtdec/Tr increases from less than unity to over fifty, the response of the specimen changes from brittle fragmentation to ductile expansion without fragmentation. During the transition, the specimen exhibits small ductile deformation before the onset of fragmentation. The transition occurs even though the stress at the bubble wall is the same. The results suggest that Δtdec/Tr is the controlling parameter not only for the onset of, but also for the variation of magma fragmentation by decompression. Copyright 2008 by the American Geophysical Union.