FEEDBACK RELATIONS BETWEEN PHYSICS, CHEMISTRY AND THERMAL BUDGET OF MAGMA DURING ASCENT AND ERUPTION

The rates and styles of physical processes such as segregation, ascent, differentiation and emplacement or eruption of magma depend on its evolving physical properties. Physical properties such as viscosity are themselves dependent on both chemical composition and temperature, leading to potentially complex feedback relations between physics, chemistry and thermal budget. Magma viscosity can vary by many orders of magnitude over likely ranges of temperature and water content even within a single conduit. Smaller, but very important, changes in density, heat capacity, thermal diffusivity and volatile solubility also occur.

Magma can be treated as a three-phase system consisting of melt, crystals and exsolved volatiles (bubbles). Volatile components diffuse out of the melt to form bubbles, while more refractory components are incorporated into crystalline phases. The compositions and relative proportions of each of these phases change in response to variations in pressure (P) and temperature (T). Although most commonly interpreted using equilibrium phase diagrams, the rapid timescales of changes in P and T relative to the timescales of chemical and thermal diffusion can lead to non-equilibrium behaviors. These include superheating, for example due to rapid near-isothermal ascent of water-undersaturated magma, and undercooling, for example due to kinetic inhibitions to crystal nucleation.

The complex feedback relations between evolving chemical composition, physical properties and thermal budget allow eruptive activity to oscillate between several different eruptive styles. For example, dacitic magmas at Santiaguito, Guatemala, are actively building lava domes, feeding thick slow-moving lava flows, and producing frequent small ash explosions. Each eruptive style reflects a different rheological regime. Domes and flows represent low strain rates affecting crystalline magmas in the brittle (spines) and ductile (flows) regimes, respectively. Explosions reflect high strain rates that exceed melt relaxation rate, crossing the glass transition and leading to brittle behavior. Unraveling internal feedbacks between physical, chemical, and thermal properties, and the role of external factors such as pre-existing conduit geometry, is a formidable task. A cartoon is presented as a first step.