The Role of Volcanic Geology in the Exploration for Geothermal Energy

Abstract

High-temperature geothermal resources occur primarily in zones of volcanism along spreading ridges, above subduction zones, at intraplate melting anomalies, and at slab windows. In these regions, conductive transfer of thermal energy is greatly enhanced by magma movement and hydrothermal circulation. Favorable geothermal targets occur where repeated injection of basalt into the lower crust creates long-lived magmatic systems within which partial melting and crustal assimilation produce intermediate and silicic magmas. In mountainous volcanic areas, hot springs tend to occur at relatively low elevations at the distal ends of outflowing tongues of hot water. Frequently the primary hydrothermal upflow is offset toward the youngest silicic vents, which represent the surface expression of the magmatic heat source. Efficient geothermal exploration in volcanic terrain should therefore be based upon geological mapping, geochronology, and petrology that together define the Time- Space-Volume-Composition (TSVC) characteristics of the young volcanic rocks. These TSVC data provide the interpretive framework for integrating complementary hydrogeochemical and geophysical surveys into a comprehensive pre-drilling geothermal evaluation that includes assessment of volcanic risk.