--> ABSTRACT: Geothermal Features of Snake River Plain, Idaho, by David D. Blackwell; #91040 (2010)

Datapages, Inc.Print this page

Geothermal Features of Snake River Plain, Idaho

David D. Blackwell

The Snake River plain is the track of a hot spot beneath the continental lithosphere. The track has passed through southern Idaho as the continental plate has moved over the hot spot at a rate of about 3.5 cm/yr. The present site of the hot spot is Yellowstone Park. As a consequence of the passage, a systematic sequence of geologic and tectonic events illustrates the response of the continental lithosphere to this hot-spot event. The three areas that represent various time slices in the evolution are the Yellowstone Plateau, the Eastern Snake River plain downwarp, and the Western Snake River plain basin/Owhyee Plateau. In addition to the age of silicic volcanic activity, the topographic profile of the Snake River plain shows a systematic variation from the high elevations in the east to lowest elevations on the west. The change in elevation follows the form of an oceanic lithosphere cooling curve, suggesting that temperature change is the dominant effect on the elevation.

Studies of geothermal potential of the Snake River plain are complicated because of the major effects on subsurface temperatures by large-scale hydrologic flow and medium-temperature geothermal system activity. All along the margins of the Eastern and Western Snake River plains, low-temperature geothermal systems occur near the contact of the basin and/or graben-filling units and the rocks outside. Highest temperatures so far encountered in these systems are exemplified by the Boise geothermal system (temperatures of approximately 80°C). At almost all locations along the margins of the Snake River plain, temperatures of 40°-50°C can be found in holes only 200-500 m deep. Some of these systems have been demonstrated to be shallow and only of moderate temperature; however, only a small fraction of the area has been drilled and higher temperature systems may exist. Combining data from holes that test the thermal regime below the aquifer and that are generally deeper than 500 m, a thermal picture of the Snake River plain at depth can be out ined. Gradients appear to be on the order of 40°-50°C/km throughout most of the Snake River plain and its margins. These gradients correspond to heat-flow values between 100 and 120 mW/m2. In the Western Snake River plain basin, average temperature gradients are 60°-80°C/km and temperatures at a depth of 3 km have been demonstrated to be on the order of 200°C. The reason for the high temperatures is related to the low thermal conductivity sedimentary rocks in the basin because heat-flow values calculated for the holes with highest temperatures also average 100 to 120 mW/m2. Little geothermal data exist from the Eastern Snake River plain because of the ubiquitous effect of the Snake River plain aquifer and because only a few holes have been d illed below the aquifer.

West of the Island Park caldera, which is essentially unexplored at this time, no high-temperature geothermal systems have yet been demonstrated to exist in the Snake River plain. Whether such systems exist must be answered by future exploration. However, high temperatures occur at a depth of 3 km, which, if they reached the surface in fluid systems, would be appropriate to geothermal exploitation by electrical power generation. The existence of so many shallow, apparently moderate-temperature geothermal systems, makes exploration for any deeper systems more difficult. Nevertheless, the common occurrences of moderate temperatures imply a virtually unlimited geothermal resource for uses in the temperature range of 40°-100°C.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.