NUMERICAL MODELING OF STRESS GENERATION AND MICROFRACTURING OF VESICLE WALLS IN GLASSY ROCKS

In the absence of stress-concentrating flaws such as microfractures, v esicular glassy materials can withstand gas pressures within vesicles in excess of 100 MPa; however, vesicles within such materials are know n to decrepitate explosively at much lower internal gas pressures, bot h in natural systems and in the laboratory. Here we present a model th at quantitatively predicts the generation of microfractures in vesicle walls during cooling. Cooling of gas-bearing vesicles in glassy rock has little effect on water solubility in the glass, but leads to a rap id decrease in gas pressure in the vesicles. The drop in pressure caus es disequilibrium between the water in the glass and in the vesicle. D ehydration of the glass in a diffusive boundary layer around the vesic le leads to elastic shrinkage. The resulting strain generates large te nsile tangential stresses which can exceed the strength of the glass, causing microfracturing. Such microfractures present a possible means by which glassy rock surrounding vesicles could be weakened enough to permit explosive decrepitation at low pore vapor pressures. The result s have implications wherever hydrous vesicular glasses are formed. For example rocks formed in shallow subvolcanic intrusions or vent plugs may spontaneously disintegrate with explosive emission of vapor; glass y submarine lavas spontaneously decrepitate upon dredging from the oce an floor (''popping rock''); vesicular glasses produced in laboratory experiments investigating vapor-melt phase equilibria have been observ ed to contain abundant fractures surrounding vesicles and to dehydrate at anomalously high rates.