Magma intrusion beneath Long Valley caldera confirmed by temporal changes in gravity

Precise relative gravity measurements conducted in Long Valley (California) in 1982 and 1998 reveal a decrease in gravity of as much as -107 +/- 6 mic rogals (1 microgal = 10(-8) meters per square second) centered on the uplif ting resurgent dome. A positive residual gravity change of up to 64 +/- 15 microgals was found after correcting for the effects of uplift and water ta ble fluctuations. Assuming a point source of intrusion, the density of the intruding material is 2.7 X 10(3) to 4.1 X 10(3) kilograms per cubic meter at 95 percent confidence. The gravity results require intrusion of silicate magma and exclude in situ thermal expansion or pressurization of the hydro thermal system as the cause of uplift and seismicity.