Convection of magma in volcanic conduits as a degassing mechanism at active volcanoes

Convection of magma in the volcanic conduit is a hypothesis that has been proposed to explain long-term, continuous, high rates of gas emission and heat loss at non-explosive, passively degassing volcanoes. I investigate this hypothesis using a geochemical and fluid dynamic approach focusing research at two passively degassing volcanoes, Popocatépetl (Mexico) and Villarrica (Chile) where conduit convection is thought to be the dominant degassing mechanism. Popocatépetl has been the site of voluminous passive degassing accompanied by minor eruptive activity from late-1994 until the time of writing. Villarrica has been characterized by continuous, open-vent, passive degassing from a summit lava lake, with negligible eruption of magmatic material, for the past 17 years. I present a detailed analysis of the chemistry and magnitude of volcanic gas emissions as well as the major and volatile element chemistry of the magma at the two volcanoes. These analyses provide constraints on (1) the magmatic contribution to the emitted volatiles and (2) the physical parameters (i.e. density and viscosity of the magma) that control convection. These data on the physical parameters of the Popocatépetl and Villarrica magmas combined with analytical fluid dynamic equations for convection of magma in volcanic conduits show that magma convection is a feasible mechanism to provide the observed magnitude of gas emissions considering reasonable conduit sizes and magma ascent velocities at both volcanoes. Further calculations of the Reynolds number and convective velocity scale are presented for magmas having a wide range of viscosity, buoyancy flux, and conduit size, including both cylindrical and dike-shaped conduits. Results indicate that convection of magma in the conduit is likely at Villarrica for conduit radii r = 1 to 5 m. In addition, convection of magma in the conduit is plausible at Popocatépetl for conduit radii r = 2 to 10 m and magma with or without a separate, pre-existing volatile phase. Convection of magma in the volcanic conduit can explain long-term, non-explosive degassing behavior at Popocatépetl, Villarrica, and probably many other volcanoes for which gas emissions far exceed the quantity of gas that could be derived from erupted magma.