Magnetohydrodynamic Description of Basaltic Lava in Active Tubes for Velocity Measurement

Abstract Basaltic lava can flow at very high velocities in active lava tubes and channels achieving rates on the order of 10 m/s. Lava tubes can transport lava up to 100 km and remain active for multiple years; which can pose as a serious geologic hazard to the infrastructure and the communities surrounding effusive volcanoes. Despite this, current methods used to monitor lava tubes and channels rely on the measurement of surface elements on top of the flow, which in the case of lava tubes requires observational points through skylights that limit the scope of measurement. In this paper we describe active basaltic lava tubes and channels as an magnetohydrodynamic (MHD) fluid to investigate the MHD induced field as a new geophysical method for measuring lava flow velocity. Basaltic lava is a low magnetic Reynolds number (Rm) MHD flow that induces a weak but appreciable magnetic field that ranges on the order of pT to µT based on the electrical conductivity, viscosity, velocity, characteristic length scale of the flow, and the strength and direction of Earth’s field relative to the flow. ANSYS Fluent is used to solve the MHD equations for a 10 m circular pipe at varying flow velocities. The results show that surface measurements of the induced field in the flow direction using a vector magnetometer are expressed as a polarity change across the center of the flow and the field intensity correlates to the velocity of the fluid. This provides constraints on both the velocity field of the flow as well as the geometry of the tube or channel.