Schematic cross section of the magmatic and hydrothermal systems underlying Yellowstone Caldera

Schematic cross section of the magmatic and hydrothermal systems underlying Yellowstone Caldera, showing magmatic volatiles emitted during crystallization of the rhyolitic magma and/or from basalt intrusions or convection, and the hypothesized relation with earthquake swarms on the caldera margins.  The exsolved fluids accumulate at lithostatic pressures in the ductile region of the subsurface and under very high strain rates are episodically injected into the brittle crust, where the fluid pressures are generally much lower.  The sudden increase in fluid pressure in the brittle region then triggers earthquake swarms.  Figure modified from Lowenstern and Hurwitz (2008), Shelly et al. (2013), and Hurwitz and Lowenstern (2014).