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Figure 1. A summary
of the geothermal systems in the Great Basin. The study area is focused in the
NE area of the basin, in SE Idaho. From McCurry and Welhan (2012).
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Lots of magmatic heat resides below the surface around the
Snake River Plain region in Idaho, as evidenced by the Yellowstone hot spot and
regional volcanism. However, SE Idaho seems to lack obvious signs of thermal
activity at the surface (Figure 1). The presence of the hot spot and other
volcanics in the area should provide a reasonably good source for geothermal
energy. If there is at least some magma body residing in the shallow crust to
produce geothermal resources, then we expect to see some type of response at
the surface (think hot springs like at Yellowstone). However, the expression of
these geothermal resources at the surface in SE Idaho is not as strong as
expected.
Three hypotheses are presented in this paper to explain this
phenomenon. The first hypothesis states that there are no easily accessible magmatic
heat sources in the area. This may be due to a lack of any magmas near the earth’s
surface and instead are located too deep within the earth for us to access or
detect. Also, it could be that any magmas that were once close to the surface
had already erupted, preventing us from using them as a heat source today. Hypothesis
1 is unlikely because geotechnical seismic work indicates a significant magma
storage exists in the mid- to upper-crust. This indicates that there is at
least some magmatic fluid in the “shallow” crust.
The second hypothesis is that there is physically accessible
magmatic heat but the amount of heat available is relatively low. This could be
due to a low permeability layer (or in other words, a rock layer that prevents
heat or fluids from travelling through it), preventing us from sensing the heat
at the surface. Hypothesis 2 is also unlikely because previous work has
demonstrated that the H2O content in the magma was 2-6%, which is comparable to
other magma systems in the Basin and Range, and indicates the magma is not dry.
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Figure 2. A conceptual
model for the China Hat dome field and Blackfoot Reservoir rift zone. Modified from Autenrieth et al. (2011). This figure
illustrates the movement of magma through faults toward the NE, away from the
source. Original paper details the abbreviations. From McCurry and Welhan (2012).
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The third hypothesis states that there are geothermal
systems in the area, but we don’t see them as well at the surface because the
heat is reduced or diverted away. For example, a large, shallow water aquifer
below the surface could absorb some of the heat that migrates toward the
surface. Also, there may be fractures below the surface that allow the heat to
migrate along the fracture paths away from the original magma source. Such a
scenario may produce heat signs somewhere else in the area. Hypothesis 3 is
favored due to the presence of a large groundwater system in the area that
could dilute or divert thermal responses from deeper high-temperature magmatic
fluids. Additionally, the study area contains west-dipping faults in the
subsurface, allowing for magmatic fluids to travel away from its source (Figure
2).
Recent volcanic fields (less than 2.6 Million years old) in
SE Idaho point towards a significant storage of magma and heat energy in the
upper crust between 2 and 15 km deep. This region may be a strong candidate for
future hydrothermal exploration work. However, the presence of a broad aquifer
in the subsurface poses challenges to studying this type of resource where
migration of magmatic heat is involved.
Paper: McCurry, M., and Welhan, J. (2012)Do Magmatic-Related Geothermal Energy Resources Exist in Southeast Idaho? GRC Transactions V36, p699-707.
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