Ascent and decompression of viscous vesicular magma in a volcanic conduit

During eruption, lava domes and flows may become unstable and generate dang erous explosions. Fossil lava-filled eruption conduits and ancient lava flo ws are often characterized by complex internal variations of gas content. T hese observations indicate a need for accurate predictions of the distribut ion of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressur es: those of the gas and magma phases, and that of the exterior. To solve f or these three different pressures, one must account for expansion in all d irections and hence for both horizontal and vertical velocity components. W e present a new two-dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, g as overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on ma gma viscosity and increases with the conduit radius and magma chamber press ure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the "exit" boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lav a flows spreading horizontally at the surface, the largest gas overpressure s, and hence the smallest exsolved gas contents, are achieved at the condui t walls. For zero horizontal velocity at the vent, corresponding to a plug- like eruption through a preexisting lava dome or to spine growth, gas overp ressures are largest at the center of the vent. The magnitude of gas overpr essure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth-dependent radii.