Where are the geothermal resources in Southeast Idaho?

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). 

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.

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). 

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.

http://www.researchgate.net/publication/235654596_Do_Magmatic-Related_Geothermal_Energy_ResourcesExist_in_Southeast_Idaho/file/79e41512510a13e1c4.pdf

GPS Used To Predict Height of Volcanic Plumes

 Photo of eruption Grímsvötn, May 2011. (SPR AFP/Getty Images) 

Scientists in Iceland have developed an innovative method to predict volcanic plume height using changes in magma pressure.  Using a series of anchored GPS stations, they were able to detect inflation of the Grímsvötn volcano before its eruption in May 2011 and the sudden co-eruptive subsidence.

These GPS stations, originally installed to monitor the movement of tectonic plates over time, continuously recorded position and elevation during the pre-eruptive and eruptive phases of the explosion.  They found that the volcano expanded radially approximately 20 inches in the hour preceding the eruption and sank 10 inches during the initial eruptive phase, resulting in tilting of one hundredth of a degree.  By tracking the change in volume of the volcano, the scientists were able to track the change in pressure of the magma chamber during the eruption.  This process is similar to a tire deflating – as air (or in the case of a volcano, magma) leaks out the tire pressure decreases. 

Based off this change in pressure, the total change in volume of the magma chamber was .027 km3, about 10x smaller than the total erupted volume. Pumice is extremely porous and can contain over 50% void space; this is a result of bubbles being trapped in the solidifying melt as they expand due to decreased pressure at the volcanic vent relative to the magma chamber. The scientists were then able to determine the rate of the eruption using the overall duration and amount of ejected material.  The mass eruption rate is directly correlated with the height of the plume; the faster the eruption rate, the taller the plume. Using this novel method, they estimated that the plume height peaked at 15 km, which compares favorably to satellite imagery.

Column height is an important parameter to predict where an ash cloud will travel.  The heat produced by jet engines cause the ash particles to melt, gunking up the engine and causing the plane to stall.  Currently, the protocol for avoiding this is to completely close airspace.   The eruption of Grímsvötn, which lasted for seven days, caused airspace closures in northern Europe, grounding approximately 900 flights.  Better predictions of column height would result in more accurate hazard maps and fewer plane delays. This new method to estimate plume height uses real time data that does not rely on satellites.  Aviation experts can then better seed their models to create smaller no fly zones due to more accurate predictions of ash movement.

Original Article: Hreinsdóttir, Sigrún, Freysteinn Sigmundsson, Matthew J. Roberts, Halldór Björnsson, Ronni Grapenthin, Pórdur Arason, Thóra Árnadóttir et al. "Volcanic plume height correlated with magma-pressure change at Grimsvotn Volcano, Iceland." Nature Geoscience (2014).