The Boise geothermal system lies in an area of high heat flow along the northern margin of the western Snake River plain. Exploratory drilling for petroleum and geothermal water, seismic reflection profiling, and regional gravity data permit construction of a detailed structure section across the western plain. A faulted acoustic basement of volcanic rocks lies at depths of 2,400 to 6,000 ft (730-1,830 m) beneath late Cenozoic lacustrine and fluvial deposits in the center of the plain. Volcanic rocks of the acoustic basement are typically basalt out in the plain, but the acoustic basement along the north margin in the vicinity of Boise is largely silicic volcanic rock. Geologic mapping and geothermal well data have provided information on the late Cenozoic geologic units nd structures important to the understanding of the Boise geothermal system. The main geothermal aquifer is a sequence of rhyolite layers and minor arkosic and tuffaceous sediment of the Miocene Idavada Volcanics. The aquifer is confined by a sequence of impermeable basaltic tuffs. The aquifer has sufficient fracture permeability to yield 150°-170°F (65°-76.6°C) hot water for space heating at a rate of 600 to 1,200 gpm from wells drilled in the metropolitan area, north of the Boise River. In this area the rhyolite lies at a depth of 900-2,000 ft (274-610 m). Artesian pressure typically lifts water to an elevation of about 2,760 ft (840 m). A conceptual model of recharge assumes percolation driven by the topographic head to a depth of more than 7,000 ft (2,135 m) benea h the granitic highlands northeast of the city. Heated water convects upward through northwest-trending range-front faults.

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