While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.
Select Citation Style Copy Citation Share to social media Give Feedback External Websites Thank you for your feedbackOur editors will review what you’ve submitted and determine whether to revise the article.
External WebsitesWhile every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.
Select Citation Style Copy Citation Share to social media External Websites Thank you for your feedbackOur editors will review what you’ve submitted and determine whether to revise the article.
External WebsitesEmeritus Professor of Civil Engineering and Emeritus Director of the Geo-Heat Center, Oregon Institute of Technology, Klamath Falls, Oregon. Private consultant in geothermal engineering and civil engineering.
John W. Lund Fact-checked by The Editors of Encyclopaedia BritannicaEncyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.
The Editors of Encyclopaedia Britannica Last Updated: Aug 31, 2024 • Article History Table of Contents
Ask the Chatbot a Question
Ask the Chatbot a Question
geothermal energy, a natural resource of heat energy from within Earth that can be captured and harnessed for cooking, bathing, space heating, electrical power generation, and other uses. The total amount of geothermal energy incident on Earth is vastly in excess of the world’s current energy requirements, but it can be difficult to harness for electricity production. Despite its challenges, geothermal energy stands in stark contrast to the combustion of greenhouse gas-emitting fossil fuels (namely coal, petroleum, and natural gas) driving much of the climate crisis, and it has become increasingly attractive as a renewable energy source.
Temperatures increase below Earth’s surface at a rate of about 30 °C per km in the first 10 km (roughly 90 °F per mile in the first 6 miles) below the surface. This internal heat of Earth is an immense store of energy and can manifest aboveground in phenomena such as volcanoes, lava flows, geysers, fumaroles, hot springs, and mud pots. The heat is produced mainly by the radioactive decay of potassium, thorium, and uranium in Earth’s crust and mantle and also by friction generated along the margins of continental plates.
Worldwide, the annual low-grade heat flow to the surface of Earth averages between 50 and 70 milliwatts (mW) per square meter. In contrast, incoming solar radiation striking Earth’s surface provides 342 watts per square meter annually (see solar energy). In the upper 10 km of rock beneath the contiguous United States alone, geothermal energy amounts to 3.3 × 10 25 joules, or about 6,000 times the energy contained in the world’s oil reserves. The estimated energy that can be recovered and utilized on the surface is 4.5 × 10 6 exajoules, or about 1.4 × 10 6 terawatt-years, which equates to roughly three times the world’s annual consumption of all types of energy.
Although geothermal energy is plentiful, geothermal power is not. The amount of usable energy from geothermal sources varies with depth and by extraction method. Normally, heat extraction requires a fluid (or steam) to bring the energy to the surface. Locating and developing geothermal resources can be challenging. This is especially true for the high-temperature resources needed for generating electricity. Such resources are typically limited to parts of the world characterized by recent volcanic activity or located along plate boundaries (such as along the Pacific Ring of Fire) or within crustal hot spots (such as Yellowstone National Park and the Hawaiian Islands). Geothermal reservoirs associated with those regions must have a heat source, adequate water recharge, adequate permeability or faults that allow fluids to rise close to the surface, and an impermeable caprock to prevent the escape of the heat. In addition, such reservoirs must be economically accessible (that is, within the range of drills). The most economically efficient facilities are located close to the geothermal resource to minimize the expense of constructing long pipelines. In the case of electric power generation, costs can be kept down by locating the facility near electrical transmission lines to transmit the electricity to market. Even though there is a continuous source of heat within Earth, the extraction rate of the heated fluids and steam can exceed the replenishment rate, and, thus, use of the resource must be managed sustainably.
