Volcanoes can erupt explosively, generating high columns of ash and occasional pyroclastic flows, or they can erupt slowly forming lava flows and domes. This variation reflects significant differences in mass eruption rate (e.g., Wilson et al. 1980). Mass eruption rates in turn are controlled by the rates of magma ascent through the volcanic conduit, and the conduit size. The magma ascent rate itself is a function of the pressure in the magma storage region, the physical properties of the magma, such as its density, viscosity and crystallinity, and the resistance to flow in the conduit that connects the magma storage zone to the surface (Papale and Dobran 1994; Mastin and Ghiorso 2001; Pinkerton et al. 2002; Sparks et al. 2006). A number of important characteristics of volcanic eruptions are affected or controlled by the rate at which magma ascends from depth. For example, the bubble content (i.e., vesicularity) and the degree of crystallization that develop in the melt phase can be significantly different in rapidly vs. slowly ascended magma. In fact, the inability of rapidly ascending magma to effectively lose exsolved gas may be one factor causing an eruption to change from effusive to explosive behavior, as recently documented for the Soufriere Hills eruption on Montserrat in the West Indies (Sparks et al. 1998). Therefore, in order to better understand the processes involved and the changes that occur in volcanic eruptions, it is important to quantify the rates at which different magmas rise to the surface. The fact that the rate of magma ascent controls a number of reactions that occur in volatile-rich magmas suggests several ways to study magma ascent rates using reaction data and theoretical flow models. One reaction that is a direct result of magma ascent is the exsolution of volatiles from the melt …