a

The structure and composition of the Yellowstone-Snake River Plain (YSRP) volcanic system were analyzed using gravity data taken at over 30,000 stations in the YSRP and surrounding region. Additional constraints were provided by tomographic seismic velocity models, new heat flow and temperature information, GPS derived strain rates, earthquake locations, and chemical analyses of volcanic rocks. P-wave velocity models and velocity-density regression based on petrologic information were used to constrain a three-dimensional density models. Rheology and strength properties were calculated at selected locations and compared to earthquake focal depths. Results of this study suggest that the lower crust of the Snake River Plain (SRP) has been thickened by the addition of an underplated layer composed primarily of clinopyroxene, and having a density of 3200 kg/m 3. A mid-crustal high-density sill is composed of a series of gabbroic lenses inter-fingering with the granitic upper crust. This geometry yields a bulk composition comparable to diorite and a density of 2900 kg/m 3. The mid-crustal sill varies from 4 to 11 km in thickness, resulting in a series of SE-NW trending gravity highs observed along the axis of the SRP. The mid-crustal sill extends up to 20 km southeast of the volcanic field and causes asymmetry of the gravity field southeast of the SRP. The Yellowstone density model reveals low-density partial melt at 10 km beneath the caldera that shallows under the northeastern caldera and continues laterally 20 km north of the caldera boundary and notably extending the estimated size of the magma reservoir by ~20%. The caldera melt body has a density of 2520 kg/m 3 and a significantly lower value of 2470 kg/