Geothermal Resources

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The earth is a geologically active planet with distinct features such as volcanic eruptions and earthquakes. The energy required to drive these activities is the internal heat stored during earth’s formation. Because of its relatively smaller size and lower internal heat, the moon is devoid of these activities.

Geothermal resources are of four types: hydrothermal, pressurized, hot dry rock, and magma. Hydrothermal resources are the most common source of geothermal energy and are referred to as underground reservoirs containing hot water or steam. Pressurized resources are high-temperature, high-pressure brines trapped in porous rocks. Hot dry rocks (HDR) refer to solid slabs of rock that can be up to several kilometers thick. Finally, magma is the molten rock in volcanic formations rising up from deep within the mantle. Almost all geothermal sources currently being utilized are hydrothermal.

Depending on how the geothermal eruption appears, a hydrothermal source can be classified as a hot spring, a warm spring, a fumarole, or a geyser. Hot springs refer to upwelling of ground water with temperatures above that of the human body. It is called a warm spring when the temperature is lower body temperature but higher than the surrounding atmosphere. When a reservoir does not contain adequate water to seep through and is eventually converted to steam, the geothermal resource is referred to as a fumarole. Geysers are intermittent hot springs that, depending on geological conditions, erupt in regular intervals ranging from a few minutes to hours or months apart (See box “The Old Faithful”). Sixty percent of all geysers in the world are in Yellowstone National Park in Wyoming; others are scattered elsewhere, mainly in California, Italy, New Zealand, and Iceland.

Geothermal resources typically lie from between a few hundred to a few thousand meters below the earth’s surface. Deeper reservoirs are generally higher in temperature and therefore yield higher efficiencies. The cost of the power plant itself is less, but drilling costs increase exponentially with depth, increasing the overall cost of extracting energy from deeper mines. With current technologies, only reservoirs within roughly four kilometers are considered to be economically viable sources of geothermal energy.

Question: Can geothermal energy be considered a renewable source of energy? Old Faithful

Answer: The earth, like the sun, has an immense amount of stored energy which will by all accounts last billions of years into the future. Unlike solar energy, which is readily available, we must tap geothermal resources. With the current technology, only hydrothermal resources are economically suitable and these have a limited lifetime before they are exhausted. (For example, the power-generating capacity of Geysers has dropped by 40% since only a decade ago). These resources can be recharged in the future as they are gradually reheated by the internal heat of the earth and, depending on their locations, can take several decades to hundreds of years to become operational again. It has therefore been suggested that geothermal energy be considered as a sustainable resource, one whose usefulness can be prolonged or sustained by optimum production strategies and methods (Gawell, 2007). In the case of the Geysers, wastewater from a nearby community has increased the efficiency; it is estimated that at the rate of 1000 megawatts, geysers will remain sustainable for a few decades.


(1) Gawell, K., and Greenberg, G., “2007 Interim Report: Update on World Geothermal Development,” US Geothermal Energy Association, May 2007.

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

Dipippo, R., Geothermal Power Plants: Principals, Applications and Case Histories, Elsevier, 2005.

Dickson, M. H., Fanelli, M., Geothermal Energy: Utilization and Technology, Stylus Pub., 2005.

Ochsner, K, Geothermal Heat Pumps: A Guide for Planning and Installing, Earthscan Ltd, 2007.

Gupta, H. , and Roy, S., Geothermal Energy: An Alternative Resource for the 21st Century, Elsevier, 2007.

Geothermics, Direct Science Elsevier Publish. Company, publishes articles on geothermal energy resources and technologies.

Geotimes, Journal of the American Geological Institute.

Geo-heat Center Quarterly Bulletin, covers how-to articles on various geothermal applications and equipment, progress in research and development activities of direct heat utilization

Journal of Volcanology and Geothermal Research, an international journal on the geophysical, geochemical, petrological, economic, and environmental aspects of volcanology and geothermal research.

External Links

National Renewable Energy Laboratory Geothermal Energy Program (

Idaho National Laboratory Geothermal Program (

US Department of Energy Geothermal Technology Program (

California Energy Commission ((

Geothermal Resources Council (

Geothermal Energy Association (