Environmental Engineering Reference
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the crust because temperature and pressure inside the Earth increase with depth.
The temperature at the top of the mantle is 870°C; the temperature at the bottom is
2200°C. The 30-km-thick transitional layer between the mantle and crust is called
the Moho layer.
At the center of the Earth lies the core, which is nearly twice as dense as the
mantle because its composition is metallic (comprised of iron-nickel alloy) rather
than stony. Unlike the yolk of an egg, however, the Earth's core is actually made up
of two distinct parts: a 2200-km-thick liquid outer core and a 1250-km-thick solid
inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's
magnetic field. Several important points related to Earth's structure and geothermal
properties include the following:
Heat flows outward from the center as a result of radioactive decay.
The crust insulates us from the interior heat.
The temperature at the base of crust is about 1000°C and increases slowly
deeper into the core.
Hot spots are located 2 to 3 km from the surface.
c rustal P lates
Geologists have developed the theory of plate tectonics (from the Greek word for
“builder”). The theory of plate tectonics deals with the formation, destruction, and
large-scale motions of great segments of the Earth's surface (crust), called plates .
This theory relies heavily on the older concept of continental drift (developed during
the first half of the 20th century) and newer understanding of seafloor spreading,
both of which help to explain earthquakes and volcanic eruptions, as well as the
origin of fold mountain systems. Large quantities of heat (that are economically
extractable) tend to be concentrated in places where hot or even molten (magma)
rock exists at relatively shallow depths in the Earth's outmost layer (the crust). Such
hot zones generally are near the boundaries of the dozen or so slabs of rigid rock (or
plates) that form the Earth's lithosphere. These crustal plates are composed of great
slabs of rock (lithosphere) about 100 km thick and cover many thousands of square
miles (they are thin in comparison to their length and width). Geologists recognize
at least seven main plates:
African Plate (covering Africa)
Antarctic Plate (covering Australia)
Eurasian Plate (covering Asia and Europe)
Indo-Australian Plate (covering Indian subcontinent, part of the Indian
Ocean, and Australia)
North American Plate (covering North America and northeast Siberia)
Pacific Plate (covering the Pacific Ocean)
South American Plate (covering South America)
as well as several minor plates:
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