3 (generate) in the crust, the planet’s outer layer, by the decay of radioactive elements that are in all rocks. The crust, which is about 5 to 75 km (about 3 to 47 mi) thick,
4 (insulate) the surface from the hot interior, which at the core may reach temperatures from 4000° to 7000° C (7200° to 12,600° F). Where the heat is concentrated
near the surface, it can be used as a source of energy.
II. GEOTHERMAL GEOLOGY
The distance from Earth’s surface to its center is about 6,500 km (about 4,000 mi). From Earth’s surface down through the crust, the normal temperature gradient (the increase of
temperature with increase of depth) is 10° to 30° C per km (29° to 87°F per mi). 5
(underlie) the crust is the mantle, which 6 (make) of partially molten rock. Temperatures in the mantle may reach 3700° C (6700° F).
The convective (circulating) motion of this mantle rock drives plate tectonics—the 'drift' of Earth's crustal plates that 7 (occur) at a rate of 1 to 5 cm (0.4 to 2 in) per year. Where plates spread apart, molten rock (magma) rises up into the rift (opening), (solidify) to form new crust. Where plates collide, one plate is generally forced (subducted) beneath the other. As the subducted plate slides slowly downward into the mantle’s ever-increasing heat,
it melts,
9 (form) new magma. Plumes of this magma can rise and intrude into the
crust, bringing vast quantities of heat relatively close to the surface. If the magma 10
(reach) the surface it forms volcanoes, but most of the molten rock stays underground, creating huge subterranean regions of hot rock.
III. GEOTHERMAL RESERVOIRS
In certain areas, water
11 (seep) down through cracks and fissures in the crust comes in
contact with this hot rock and 12 (heat) is heated to high temperatures. Some of this
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