filled with over 500 km3 of erupted rhyolitic lava. A much
smaller volume of basaltic magma has erupted, but solely
outside the margins of the caldera (Christiansen 2001;
Christiansen et al. 2007). The lack of intracaldera basaltic
volcanism is interpreted to reflect the existence of a significant
volume of silicic magma beneath the caldera that prevents
the denser, mantle-derived basaltic magma from rising
to the surface (Hildreth 1981; Hildreth et al. 1991; Christiansen
2001). The existence of a silicic upper-crustal magma reservoir
is consistent with seismic velocity and gravity anomalies as
well as with the shallow depth of the seismogenic zone beneath
the caldera (Smith and Braile 1994; Husen et al. 2004). The
magmas provide the heat that sustains the unparalleled
number of hot springs, geysers, and mud pots on the
Yellowstone Plateau (FIG. 2).