Volatile controls on magma ascent and eruption

Volatiles provide the primary driving force for volcanic eruptions, thus understanding magma degassing is fundamental to understanding volcanic activity in volatile-rich arc environments. A complete picture of volatile behavior requires knowledge of not only (1) how, when and where volatiles saturate, exsolve, and accumulate in magma reservoirs and (2) how they nucleate, expand and coalesce during magma ascent; but also (3) how volatiles affect the phase relations, rheology, and fluid dynamics of the magma, and (4) how volatiles escape to and interact with surrounding wall rocks and hydrothermal systems. Together these interactions determine degassing conditions, rates of magma ascent to the Earth's surface, and, ultimately, the style and intensity of volcanic eruptions. For example, rapid closed-system degassing provides the explosivity of silicic plinian eruptions, while open-system gas escape permits passive effusion of lava domes. Variations in the details of magma decompression rates and paths create the rich variability in eruptive style that characterizes arc volcanism. Development of a fully integrated perspective on the role of volatiles in volcanic systems requires both better constraints on the time scales and dynamics of processes occurring within volcanic conduits and coupling of conduit processes to other parts of subvolcanic systems.