Residual gravity changes and eruption magnitudes

Abstract

Research during the last four decades has demonstrated that systematic temporal gravity changes are associated with eruptive activity. These gravity changes are caused by the free-air effect of elevation changes induced by magma bodies and by mass redistributions. Where gravity change data are accompanied by concurrent elevation control, observed gravity changes can be corrected for the free-air effect giving residual gravity changes. Residual gravity changes reflect only mass redistributions. At most volcanoes residual gravity changes can be accounted for by the addition of mass during pre-eruption time, and by the removal or addition of mass during post-eruption time. At explosive volcanoes, however, the data show mass must be removed from near surface magma systems during preeruptive time. The only reasonable mechanism acounting for these negative pre-eruption mass redistributions is displacing dense country rock or magma with gas bubbles in vesiculated magma. Application of Gauss' theorem to areal residual gravity changes and an assumed density for the displaced material, give an estimate of the volume of exsolved gas contained within the magma body. The kinetic energy released in explosive magmatic eruptions, to a first approximation, is caused by the explosive expansion of exsolved and dissolved gases contained within magma. If the density and temperature of magma, and if the amount, pressure, and composition of volatiles contained in magma bodies are known (or assumed), then the kinetic energy released by explosive gas decompression can be computed (or estimated ) using simple thermodynamic relationships. Kinetic energy estimates for explosive magmatic eruptions computed from muzzle velocities, and computed for isothermal and adiabatic gas decompression using assumed volatile contents, pressures, and temperatures give similar results. Minimum pre-eruption exsolved volatile contents estimated from negative pre-eruption mass deficits and assumed magma densities, temperatures, and pressures were used to compute predicted kinetic energies for five eruption-gravity change sequences at Pacaya, Mihara, Usu, and Mt. St. Helens volcanoes. Although the data are too few to be conclusive, they do indicate the possibility of forecasting eruption magnitudes at some volcanoes using precursory gravity changes.