The Yellowstone Magmatic-Hydrothermal System, U.S.A.

Abstract

At Yellowstone National Park, the deep permeability and fluid circulation are probably controlled and maintained by repeated brittle fracture of rocks in response to local and regional stress. Focal depths of earthquakes beneath the Yellowstone caldera suggest that the transition from brittle fracture to quasi plastic flow takes place at about 3 to 4 km. The maximum temperature likely to be attained by the hydrothermal system is 350 to 450°C, the convective thermal output is about 5.5 x10/9 watts, and the minimum average thermal fluid is about 1800 mW/m2 throughout 2,500 km2. The average thermal gradient between the heat source and the convecting hydrothermal system must be at least 700 to 1000°C/km. Crystallization and partial cooling of about 0.082 km3 of basalt or 0.10 km3 of rhyolite annually could furnish the heat discharged in the hot spring system. The Yellowstone magmatic hydrothermal system as a whole appears to be cooling down, in spite of a relatively large rate of inflation of the Yellowstone caldera.