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What is geothermal energy?A simple definition: Geothermal energy is heat within the earth. The word geothermal comes from the Greek words geo (earth) and therme (heat).

Geothermal energy is a renewable energy source because heat is continuously produced inside the earth. People use geothermal heat for bathing, to heat buildings and to generate electricity.

The slow decay of radioactive particles in the earth’s core, a process that happens in all rocks, produces geothermal energy.

The earth has four major parts or layers:

• An inner core of solid iron that is roughly 1,500 miles in diameter.

• An outer core of hot, molten rock called magma that is about 1,500 miles thick.

• A mantle of magma and rock surrounding the outer core that is approximately 1,800 miles thick.

• A crust of solid rock that forms the continents and ocean floors that is 15 to 35 miles thick under the continents and three to five miles thick under the oceans.

Scientists have discovered that the temperature of the earth’s inner core is approximately 10,800 degrees Fahrenheit, which is as hot as the surface of the sun. Temperatures in the mantle range from about 392°F at the upper boundary with the earth’s crust to approximately 7,230°F at the mantle-core boundary.

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The earth’s crust is broken into pieces called tectonic plates. Magma comes close to the earth’s surface near the edges of these plates, which is where many volcanoes occur. The lava that erupts from volcanoes is partly magma. Rocks and water absorb heat from magma deep underground. The rocks and water found deeper underground have the highest temperatures.

What is a geothermal resource?Geothermal resources are reservoirs of hot water that exist at varying temperatures and depths below the surface of the earth. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam, and very hot water that can be brought to the surface for use in a variety of applications, including electricity generation, direct use and heating and cooling.

Benefits of geothermal energy• Renewable: Through proper reservoir management, the rate of energy extraction can be balanced with a reservoir’s natural heat recharge rate.

• Baseload: Geothermal power plants produce electricity consistently, running 24 hours per day, seven days per week, regardless of weather conditions.

• Domestic: U.S. geothermal resources can be harnessed for power production without importing fuel.

• Small Footprint: Geothermal power plants are compact; using less land per GWh than coal, wind or solar PV with center station.

• Clean: Modern, closed-loop geothermal power plants emit no greenhouse gases; Geothermal power plants consume less water on average over the lifetime energy output than the most conventional generation technologies.

(Sources: U.S. Energy Information Association, U.S. Office of Energy Efficiency and Renewable Energy.)

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