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Science 24 September 1999:
Vol. 285. no. 5436, pp. 2119 - 2122
DOI: 10.1126/science.285.5436.2119
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Reports

Magma Intrusion Beneath Long Valley Caldera Confirmed by Temporal Changes in Gravity

M. Battaglia, 1* C. Roberts, 2 P. Segall 1

Precise relative gravity measurements conducted in Long Valley (California) in 1982 and 1998 reveal a decrease in gravity of as much as -107 ± 6 microgals (1 microgal = 10-8 meters per square second) centered on the uplifting resurgent dome. A positive residual gravity change of up to 64 ± 15 microgals was found after correcting for the effects of uplift and water table fluctuations. Assuming a point source of intrusion, the density of the intruding material is 2.7 × 103 to 4.1 × 103 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 hydrothermal system as the cause of uplift and seismicity.

1 Department of Geophysics, Stanford University, Stanford, CA 94305, USA.
2 U.S. Geological Survey, Menlo Park, CA 94025, USA.
*   To whom correspondence should be addressed.

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Faults and ground uplift at active calderas.
A. Folch and J. Gottsmann (2006)
Geological Society, London, Special Publications 269, 109-120
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