Degassing and crystallization of ascending andesite and dacite

Abstract
The prevalence of andesitic and dacitic volcanic eruptions over the past 20 years has led to a new appreciation of processes typical of magmas of intermediate composition. Extensive syn-eruptive crystallization, driven by decompression and volatile exsolution, is one such process. A water-saturated melt that is decompressed isothermally from its liquidus must crystallize in response to the diminishing capacity of the melt to retain volatiles (particularly H2O). Only rapid magma ascent allows such a melt to reach the Earth's surface without crystallizing. Intermediate rates of ascent permit varying amounts of syn-eruptive crystallization, which in turn changes magma rheology and affects continued magma progress toward the surface. Feedback among magma decompression, vesiculation, and crystallization is poorly understood, particularly with regard to the kinetics of crystallization.