The 3100 km2 Yellowstone caldera, Yellowstone National Park, was formed in the latest of three explosive eruptions of rhyolites and ash flow tuffs totalling 3700 km3 at 2, 1.2, and 0.6 m.y. before present. Its youthful volcanic history, widespread hydrothermal activity, intense seismicity, and extremely high heat flow, in excess of 30 time the continental average, marks the Yellowstone volcanic system as a giant caldera at unrest. Orthometric height increases of the caldera of up to 76 cm, measured from precise leveling surveys from 1923 to 1975--1977, were inverted to determine volume expansion source models for the caldera-wide deformation. For the 1923 to 1977 uplift episode, two regions of expansion were found: (1) in the northern part of the caldera near the Sour Creek resurgent dome of ~0.37 km3, and (2) in the southern part of the caldera, near the Mallard Lake resurgent dome of ~0.41 km3. Both bodies occur in the upper crust from near-surface depths to 6.0 km, but the largest volume expansions were found in the 3.0--6.0 km depth range. The southern caldera source volume, near the Mallard Lake dome, may extend down to 9.0 km.

The data are, however, unable to resolve if these two resurgent domes, separated laterally by ~40 km, are connected. From 1976 to 1987, nearly simultaneous measurements of elevation and gravity changes were made on a profile across the northern caldera during a period of net uplift. Models of the temporal gravity variation infer that the volume increase for the northern caldera source must lie about 9.0 km and involved a density perturbation greater than ¿0.002 g/cm3. The modeled volumetric soruces are in the same general locations as bodies of low P wave velocities, high seismic attenuation, and large negative Bouguer gravity anomalies. In view of the intense Quarternary volcanism, the anomalously high heat flow, and the correlation of the source volumes with the regional geophyscial anomalies, it is likely that the modeled volumetric increases were caused by migration of magmas and/or the introduction of large volumes of hydrothermal fluids into the upper crust. ¿American Geophysical Union 1990