• Much of our knowledge of Earth's interior comes from the study of earthquake waves that penetrate Earth and emerge at some distant point. In general, seismic waves travel faster in solid elastic materials and slower in weaker layers. Further, seismic energy is reflected and refracted (bent) at boundaries between compositionally or mechanically different materials. By carefully measuring the travel times of seismic waves, seismologists have been able to determine the major divisions of Earth's interior.

• The principal compositional layers of Earth include (1) the crust, Earth's comparatively thin outer skin that ranges in thickness from 3 kilometers (2 miles) at the oceanic ridges to over 70 kilometers (40 miles) in some mountainous belts such as the Andes and Himalayas; (2) the mantle, a solid rocky shell that extends to a depth of about 2900 kilometers (1800 miles); and (3) the core, an iron-rich sphere having a radius of 3486 kilometers (2166 miles).

• Earth's outer mechanical layer, including the uppermost mantle and crust, form a relatively cool, rigid shell known as the lithosphere (sphere of rock). Averaging about 100 kilometers in thickness, the lithosphere may be 250 kilometers or more in thickness below older portions (shields) of the continents. Within the ocean basins the lithosphere ranges from a few kilometers thick along the oceanic ridges to perhaps 100 kilometers in regions of older and cooler crustal rocks.

• Beneath the lithosphere (to a depth of about 660 kilometers) lies a soft, relatively weak layer located in the upper mantle known as the asthenosphere ("weak sphere"). The upper 150 kilometers or so of the asthenosphere has a temperature/pressure regime in which a small amount of melting takes place (perhaps 1 to 5 percent). Within this very weak zone, the lithosphere is effectively detached from the asthenosphere located below.

• The crust, the rigid outermost layer of Earth, is divided into oceanic and continental crust. Oceanic crust ranges from 3 to 15 kilometers in thickness and is composed of basaltic igneous rocks. By contrast, the continental crust consists of a large variety of rock types having an average composition of felsic rock called granodiorite. The rocks of the oceanic crust are younger (180 million years or less) and more dense (about 3.0 g/cm3) than continental rocks. Continental rocks have an average density of about 2.7 g/cm3 and some have been discovered that exceed 4 billion years in age.

• Over 82 percent of Earth's volume is contained in the mantle, a rocky shell about 2900 kilometers thick. The boundary between the crust and mantle represents a change in composition. Although the mantle behaves like a solid when transmitting earthquake waves, mantle rocks are able to flow at an infinitesimally slow rate. Some of the rocks in the lowermost mantle (D’’ layer) are thought to be partially molten.

• The core is composed mostly of iron with lesser amounts of nickel and other elements. At the extreme pressure found in the core, this iron-rich material has an average density of nearly 11 g/cm3 and approaches 14 times the density of water at Earth's center. The inner and outer core are compositionally similar; however, the outer core is liquid and capable of flow. It is the circulation within the core of our rotating planet that generates Earth's magnetic field.

• Temperature gradually increases with depth in our planet's interior. Three processes contribute to Earth's internal heat: (1) heat emitted by radioactivity; (2) heat released as iron solidifies in the core; and (3) heat released by colliding particles during the formative years of our planet.

• Convective flow in the mantle is thought to consist of buoyant plumes of hot rock and downward flow of cool, dense slabs of lithosphere. This thermally generated convective flow is the driving force that propels lithospheric plates across the globe.

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