Numerical modelling of stress generation and microfracturing of vesicle walls in glassy rocks

Abstract

In the absence of stress-concentrating flaws such as microfractures, vesicular glassy materials can withstand gas pressures within vesicles in excess of 100 MPa; however, vesicles within such materials are known to decrepitate explosively at much lower internal gas pressures, both in natural systems and in the laboratory. Here we present a model that 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 rapid decrease in gas pressure in the vesicles. The drop in pressure causes disequilibrium between the water in the glass and in the vesicle. Dehydration of the glass in a diffusive boundary layer around the vesicle leads to elastic shrinkage. The resulting strain generates large tensile 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 results 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; glassy submarine lavas spontaneously decrepitate upon dredging from the ocean floor (“popping rock”); vesicular glasses produced in laboratory experiments investigating vapor-melt phase equilibria have been observed to contain abundant fractures surrounding vesicles and to dehydrate at anomalously high rates.