HYDROTHERMAL DISCHARGE FROM VOLCANIC AREAS IN THE WESTERN UNITED STATES

Except in the MOR environment, voluminous, high-temperature hydrothermal discharge is commonly associated with silicic volcanism and a shallow regional water table (e.g. Yellowstone, Kamchatka, Taupo Volcanic Zone). In the western US, hydrothermal heat discharge ranges up to 6 GW (Yellowstone) and values in the 100 MW range are not uncommon (Long Valley, Lassen, Austin Hot Springs). Such large heat discharges likely imply mining of magmatic heat and, as pointed out by Clive Lister (1974), may require active fluid circulation near or within recently crystallized magma. The average hydrothermal heat discharge from some silicic magmatic systems is fairly constant on a multidecadal scale. For instance, measurements at Yellowstone and Lassen suggest relatively steady heat discharge since the early 20th century. Large seasonal variations observed at some localities over the past 30 years are due mainly to interaction with the shallow, cold hydrologic system. Mining of heat from subaerial deposits (Katmai, Loowit Hot Springs at Mt. St. Helens) can temporarily generate comparably large (100s of MW) values of hydrothermal discharge, but pronounced declines in heat output are observed on decadal scales. Relative to silicic systems, focused basaltic magmatism can provide similarly large heat inputs (e.g. 5-6 GW at Kilauea), but depth to water table tends to be greater in basaltic terranes, and hydrothermal discharge tends to be weaker and less conspicuous (e.g. a maximum of ~1 GW of subaerial hydrothermal discharge at Kilauea, mainly as low-temperature, dilute discharge near the coast). Andesitic arcs worldwide exhibit a range of length-normalized hydrothermal-discharge rates. The most important factors controlling hydrothermal discharge may be the rate of magmatic heat input and depth to water table. In the central Oregon Cascade Range, perhaps the most active part of a relatively "weak" andesitic arc (intrusion rate ~9-50 cubic km/m.y./km arc length), hydrothermal discharge (~2 MW/km arc length) is relatively hot and visible west of the arc, where the water table is shallow, and relatively cool, dilute, and inconspicuous east of the arc, where the water table is deep. Current length-normalized rates of hydrothermal heat discharge from the Cascade arc are ~10 times lower than those in the Taupo Volcanic Zone, an arc segment analogous to the more-silicic Miocene Cascades.