Recent experimental progress in the physical description of silicic magma relevant to explosive volcanism

This paper is included in the Special Publication entitled 'The physics of explosive volcanic eruptions', edited by J.S. Gilbert and R.S.J. Sparks. Explosive eruptions of silicic magmas are the result of a complex interplay of physico-chemical processes (e.g. decompression, volatile saturation, bubble nucleation and growth, crystallization, foaming, fragmentation and annealing). These processes occur over a relatively wide range of temperature, pressure, stresses and time scales. Furthermore, substantial changes in the chemical and physical properties of the eruptive magmas are induced by decompression and degassing. Numerical modelling of eruptive processes presents us with a picture of a magma column where enormous vertical gradients in the physical state and properties of the continuous magma body in the conduit result from decompression and degassing. Adequate representation of the physico-chemical evolution of eruptive systems therefore requires a relatively detailed description of melt properties in order to be robust and generalizable. The source of property data for silicate melts is experimental investigation. In recent years much effort has been concentrated on rhyolitic melts under conditions relevant to explosive volcanism. Here several aspects of this work are highlighted. (1) The description of hydrous rhyolitic melt properties (density, surface tension, viscosity and thermal conductivity) has been greatly improved. (2) The solubility of water as well as its diffusivity in silicic melts has been investigated in detail at relatively low pressures. (3) Rheological and transport complexities of melts and magmas such as non-Newtonian melt rheology, viscoelasticity, crystal and bubble suspension rheology, and foam deformation, permeability and stability have been explored. (4) Studies of the mechanical strength of magma have been initiated. Some of the insights into the nature of explosive volcanism provided by these studies are also of a qualitative nature (e.g. the degree of equilibrium during degassing, the longevity of textural magmatic states and the mechanism of fragmentation). It is hoped that this description of experimental progress in melt properties will encourage the reader to compare the assumptions, descriptions and predictions of the modelling of eruptive processes presented in this volume with the physical nature of the magmas involved that is described herein.