Chapter 9 - Rates of Magma Ascent and Storage

Abstract

The manner in which a volcano erupts is largely controlled by the rate of magma ascent and the geometry of the volcanic conduit connecting the volcanic vent at the surface to the magma reservoir region in the shallow crust. Factors influencing the rate in which magma ascends to the surface during eruptions include both physical and chemical properties of the magma, such as its temperature, composition, volatile budget, and crystallinity, all of which affect its viscosity and density, as well as the permeability of the conduit walls. This chapter offers a summary of different techniques used to examine and constrain the rates of magmatic processes that occur in shallow crustal reservoirs as well as in conduits while magma ascends to the surface during eruption. Whereas compositional zoning patterns in phenocrysts and diffusion modeling indicate that relatively short-lived magmatic processes like magma mixing occur over timescales of 100–102 days, processes like magma assimilation and crystal-melt fractionation persist for 103–106 years based on studies that utilized radioactive isotopes from whole-rock or mineral separates. Thus, such timescales must also be required for the accumulation and storage of magma in the shallow crust, particularly in the case of silicic magmas emplaced as a result of voluminous caldera-forming eruptions. Techniques aimed at understanding syneruptive magma ascent involve magma extrusion rate studies as well as experimentally replicating a variety of decompression-induced mineral-melt reactions, such as bubbles, microlites, and reaction rims. Results from these studies suggest a general relationship between magma ascent rates in excess of ∼0.2 m/s and explosive eruptions. Moreover, magma ascent rates appear to be relatively consistent over a wide range of magma compositions.