The fracture behaviour of volcanic glass and relevance to quench fragmentation during formation of hyaloclastite and phreatomagmatism

Quench fragmentation is a non-explosive process that occurs when molten magma is super-cooled to glass upon contact with ambient water. This occurs when coherent lavas are erupted subaqueously, when they flow into water, when magma intrudes into water-saturated sediments, and when magma and water interact explosively during phreatomagmatism. Quench fragmentation also occurs alongside explosive phreatomagmatic activity. AI though products of quench fragmentation (hyaloclastite sensu stricto) have been discussed qualitatively in the volcanological literature, compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass. Based on literature from materials engineering, we present a detailed review of the processes by which glass forms, the properties of glass, and the fracture mechanics that cause it to fragment non explosively. We also consider how this can be applied to understanding the dynamics behind the volcanological processes of in-situ glass fragmentation during quenching in wet environments and phreatomagmatism. Important parameters for the occurrence of quench fragmentation are the temperature difference between the magma and the ambient water and how much the ambient water is superheated above its Leidenfrost temperature. The geometry of the lava or magma intrusion, the thermal conductivity and the thermal expansion are also of great importance. The resistance of the magma against fragmentation can be increased with the presence of crystals provided the thermal expansion of the crystals does not greatly exceed that of the glass; vesicles have the opposite effect, unless the magma is highly vesicular. This overview then provides a solid basis for further quantitative study of quench fragmentation and hyaloclastite formation. (C) 2015 Elsevier B.V. All rights reserved.