EXCESSIVE DEGASSING OF IZU-OSHIMA VOLCANO - MAGMA CONVECTION IN A CONDUIT

Excess degassing of magmatic H2O and SO2 was observed at Izu-Oshima vo lcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1 x 10(9)(4) kg/s using emission rates of magmatic H2O and S O2, and H2O and S contents of the magma. The minimum total volume of m agma degassed during the 27-month period is estimated to be 2.6 x 10(8 ) m3. This volume is 20 times larger than that of the magma ejected du ring the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a mag ma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed m agma spheres through a conduit filled with degassed magma. This proces s is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles.