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Earth’s Geothermal Gradient

Geothermal gradient is the rate of change in temperature with respect to increasing depth in Earth’s interior.  This gradient reflects heat flowing from the Earth’s warm interior to its surface and drives the flow of geothermal energy. The geothermal gradient can vary in different places across the planet, but, a typical value is 25˚ Celsius per 1 kilometer depth (25°C/km).1R. Wolfson, “Energy from Earth and Moon” in Energy, Environment, and Climate, 2nd ed., New York, NY: W.W. Norton & Company, 2012, ch. 8, pp. 204-224 In areas where the geothermal gradient is greater, geothermal energy is most viable. For example, in north-central Nevada, geothermal gradients are as great as 64°C km−1 per kilometer in bedrock.2Hose, R. K., & Taylor, B. E. (1974). Geothermal systems of northern Nevada (No. 74-271). US Geological Survey, https://doi.org/10.3133/ofr74271.

Areas of High Geothermal Gradient

Locations with high geothermal gradients are favorable for geothermal systems. Let’s examine some scenarios that are conducive to high geothermal gradients.

High Tectonic Activity

Places with high tectonic activity usually contain higher geothermal gradients. Subduction and convergent plate boundaries are areas with high geological activity, and these regions tend to have deep fault zones, accompanied by rifting and magmatic intrusions.

The Ring of Fire is an area around the Pacific Ocean where 75% of the volcanoes are located and 90% of the earthquakes occur.3“Ring of Fire”, n.d., National Geographic, https://www.nationalgeographic.com/science/article/ring-of-fire Subduction zones have formed here where the Pacific Plate is subducting beneath continental and younger ocean plates. Subduction zones allow for the rise of magmatic material to the top layer of the crust, which creates a higher geothermal gradient.

Volcanic arcs and oceanic trenches partly encircling the Pacific Basin form the so-called Pacific Ring of fire, a zone of frequent earthquakes and volcanic eruptions.
Volcanic arcs and oceanic trenches partly encircling the Pacific Basin form the so-called Pacific Ring of Fire, a zone of frequent earthquakes and volcanic eruptions. The trenches are shown in blue-green. The volcanic island arcs, although not labelled, are parallel to, and always landward of, the trenches.4Wikimedia image caption: https://commons.wikimedia.org/wiki/File:Pacific_Ring_of_Fire.svg

Radioactive Element Concentration in Crustal Rock

Heat flow is also greater in areas with granitic or volcanic crust because these rocks contain the greatest concentration of radioactive elements found in Earth.5William, G. E. (2010). Geothermal Energy: Renewable Energy and the Environment (pp. 1-176). Boca Raton, FL: CRC Press.

Thinning of Earth’s Crust

Map of the Great Basin in the western United States

Another geologic scenario that facilitates greater heat flow is where the Earth’s crust and uppermost part of the mantle (the lithosphere) is thinner. For example, thinning is present at mid-ocean ridges. Thinning of the crust make the Great Basin of the western United States a world-class geothermal province with more than 400 known geothermal systems.6Ayling, B., 2020, Renewable energy underground – searching for hot water and hot rocks in the western USA, presentation University of Nevada, Reno, https://gbcge.org/seminars/discover_science/ In this case, the high heat flow is present because thinning allows convective heat transfer to more effectively conduct heat to shallower depths (a higher geothermal gradient is present).7Sclater, J. G., Parsons, B., & Jaupart, C. (1981). Oceans and continents: similarities and differences in the mechanisms of heat loss. Journal of Geophysical Research: Solid Earth86(B12), 11535-11552. The normal faults typical of these regions are great assets for geothermal projects as they provide pathways for fluids to circulate. It is no surprise that Nevada is an area where numerous geothermal projects have been developed over the years.

Hotspots

A hotspot is an area of intraplate volcanism (e.g. Hawaii or Yellowstone National Park), or especially vigorous volcanism along a plate boundary (e.g. Iceland). These hotspots are caused by mantle material rising as mantle plumes: narrow upwellings in the Earth’s mantle from within the earth through the process of advection. The active hotspot volcanism is typically constrained to small area, on the order of 100 km in diameter.8Steinberger, B., & O’Connell, R. J. (1998). Advection of plumes in mantle flow: implications for hotspot motion, mantle viscosity and plume distribution. Geophysical Journal International132(2), 412-434.

Unanticipated High Geothermal Gradients

Some areas have high heat flow but the reasons may not be fully understood. For example, the Gulf Coast region of Texas has the highest overpressure and geothermal temperature gradient in sedimentary basins in the state. For example, there are wells in south Texas that have a geothermal gradient of ~50°C/km.9Daigle, Hugh, 2022, December 6, Lecture 3.3: Deep geothermal wells, https://youtu.be/r9JLsDQygxg?si=bmNtasrZ0pAk-A04 Sediments are at higher pressures than normal because the sediments were buried quickly and not allowed to dewater normally. High heat flow appears to accompany much of the overpressure, but the relationship is poorly understood. Some targeted explanations include: hot fluids expelled from overpressured sediments that migrate upward through faults, causing high heat flow anomalies; salt diapirs, common in this region, impact the 3-D thermal picture of a target due to salt’s relatively high thermal conductivity. Further work is needed to understand and decouple the relation between overpressure and high geothermal gradient in the Gulf Coast.10Wisian, K., Bhattacharya, S., & Richards, M. (2023). The Texas Geothermal Resource: Regions and Geologies Ripe for Development in The Future of Geothermal in Texas: Contemporary Prospects and Perspectives.

Areas of Low Geothermal Gradients

Areas with low geothermal gradients, can be found in areas that lack magma intrusions such as along continental margins that lack geological activity, such as the eastern part of the United States. Drilling would need to go deeper in areas with low geothermal gradient to reach the equivalent gradient of active areas.

We have seen how locations with high geothermal gradients are favorable for geothermal systems. But it is still possible to construct a productive geothermal system in areas of low geothermal gradients—more on that later.

The Impact of Rock Type on Geothermal Gradient

The specific rocks that are part of a geothermal system can impact the geothermal gradient of the system. Rocks can have a negative or positive effect. For example, rocks with insulators such as shales, and dry sand decrease the geothermal gradient of the system. Halite, a conductor, has the highest thermal conductivity.11Dr. Ken Wisian, 2024, pers. comm.

How Can We Measure Geothermal Gradient?

Because the geothermal gradient varies with location, it is typically measured by determining the bottom-hole temperature after borehole drilling. However, temperature logs obtained immediately after drilling are affected due to drilling fluid circulation. Thus, many recorded temperatures may inaccurately assess the geothermal gradient in an area if bottom-hole temperatures have not stabilized in advance of measurements. This problem with temperature data is important to keep in mind during geothermal exploration.

Data for geothermal gradient can be readily available in areas with large oil and gas exploration activity. For example, ~30,000 wells have geothermal gradient data in Texas and these data are archived at the Railroad Commission of Texas.12Daigle, Hugh, 2022, December 6, Lecture 3.3: Deep geothermal wells, https://youtu.be/r9JLsDQygxg?si=bmNtasrZ0pAk-A04

Images: “Yellowstone Caldera header” by U.S. National Park Service; “The Pacific Ring of Fire” by Gringer via Wikimedia; “Great Basin Map” by Kmusser at Wikimedia Commons