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** Geothermal energy **

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Steam rising from the Nesjavellir Geothermal Power Station in Iceland.

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*Geothermal energy* is thermal energy generated and stored in the Earth.
Thermal energy is the energy that determines the temperature of matter. The
geothermal energy of the Earth's crust originates from the original
formation of the planet (20%) and from radioactive decay of minerals
(80%).^[1]^[2] The geothermal gradient, which is the difference in
temperature between the core of the planet and its surface, drives a
continuous conduction of thermal energy in the form of heat from the core
to the surface. The adjective /geothermal/ originates from the Greek roots
/γη (ge)/, meaning earth, and /θερμος (thermos)/,
meaning hot.

Earth's internal heat is thermal energy generated from radioactive decay
and continual heat loss from Earth's formation.^[2] Temperatures at the
core-mantle boundary may reach over 4000 °C (7,200 °F).^[3] The high
temperature and pressure in Earth's interior cause some rock to melt and
solid mantle to behave plastically, resulting in portions of mantle
convecting upward since it is lighter than the surrounding rock. Rock and
water is heated in the crust, sometimes up to 370 °C (700 °F).^[4]

From hot springs, geothermal energy has been used for bathing since
Paleolithic times and for space heating since ancient Roman times, but it
is now better known for electricity generation. Worldwide, 11


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