The interior structure of the Earth is layered in spherical shells, like an onion. These layers can be defined by either their chemical or their rheological properties. Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth’s deep interior.
|0–60||0–37||Lithosphere (locally varies between 5 and 200 km)|
|0–35||0–22||… Crust (locally varies between 5 and 70 km)|
|35–60||22–37||… Uppermost part of mantle|
|100–200||210-270||… Upper mesosphere (upper mantle)|
|660–2,890||410–1,790||… Lower mesosphere (lower mantle)|
The core, beneath the mantle, is the deepest and hottest layer of the Earth. It is made almost entirely of metal. The core is made of two layers: the outer core, which borders the mantle, and the inner core. The inner core is shaped like a ball.
Outer core is made mostly of iron and nickel. Iron and nickel are two important metals found everywhere on the planet. (On the surface of the Earth, these metals are found in solid form.) Iron and nickel in the outer core form an alloy, or a mixture of metallic elements. The outer core is approximately 2,300 kilometers (1,430 miles) thick. The alloy of the outer core is very hot, between 4,000 and 5,000 degrees Celsius (7,200 and 9,000 degrees Fahrenheit).
The inner core is made mostly of iron. It is approximately 1,200 kilometers (750 miles) thick. Although the iron is extremely hot—between 5,000 and 7,000 degrees Celsius (9,000 and 13,000 degrees Fahrenheit)—the pressure from the rest of the planet is so great that the iron cannot melt. For this reason, the inner core is mostly solid.
In early stages of Earth’s formation about four and a half billion (4.5×109) years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation (see also the iron catastrophe), while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron (80%), along withnickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials). Some have argued that the inner core may be in the form of a single iron crystal.
The mantle is one of the three main layers of the Earth. It lies between the innermost layer, the core, and the thin outermost layer, the crust. The mantle consists of hot,dense, semisolid rock and is about 2,900 kilometers (1,802 miles) thick.
Layers of the mantle
- Lithosphere. The thin outermost shell of the upper mantle is similar to the crust, though cooler and more rigid. Together with the crust, this layer is called the Earth’s lithosphere.
- Asthenosphere. The lithosphere is actually broken up into several large pieces, or plates. They “float” on a softer mantle layer called the asthenosphere. Their very slow motion is the cause of plate tectonics, a process associated with continental drift,earthquakes, volcanoes, and the formation of mountains.
- Upper mantle. Below the asthenosphere lies another layer, stronger and more solid than the asthenosphere. All layers below the crust down to a depth of about 670 kilometers (416 miles) are known as the upper mantle.
- Lower mantle. The rest of the mantle between the upper mantle and the core is known as the lower mantle. It is denser and hotter than the upper mantle.
Earth’s crust is divided into 15 major tectonic plates: the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca plates. Tectonic plates actually slide around on the mantle, causing earthquakes, mountain formation,continental drift, volcanoes, and other geologic activity on the crust.
The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt. The thicker crust is continental crust, which is less dense and composed of (felsic) sodiumpotassium aluminium silicate rocks, like granite. The rocks of the crust fall into two major categories – sial and sima (Suess,1831–1914). It is estimated that sima starts about 11 km below the Conrad discontinuity (a second order discontinuity). The uppermost mantle together with the crust constitutes the lithosphere. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is known as the Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, in oceanic crust, there is a chemicaldiscontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted onto the continental crust and preserved as ophiolite sequences.