Explain Crust, Mantle and Core with Chemical Composition
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4. The Inner Structure of the Earth
We have information about the inner structure of the Earth from seismic waves. When an earthquake occurs, by the measurement and examination of the seismic waves we can identify differences in density and physical state inside the Earth.
Within the Earth, different layers alter one after another with increasing depth. This alteration can be recognized according to the chemical composition and the physical properties.
Layers of Differing Chemical Composition
To determine the chemical composition inside the Earth indirect measurements are used. One way to determine composition is to measure how the density of rock changes with depth below the Earth’s surface. We can do this by measuring the speeds with which earthquake waves pass through the Earth, because they move more quickly through dense rocks than through less dense rocks. From such measurements we know that density increases with depth, but not smoothly. At some depths abrupt velocity increases indicate sudden increases in density. Knowing these different densities, we can estimate what the composition of the different layers must be.
The layers of different chemical composition outward from the centre of the Earth are as follows: core, mantle, and crust.
The Core
At the centre is the densest of the three layers, the core. It is mostly metallic iron with small amounts of nickel and other elements. The outer boundary of core is at a depth of 2,900 kilometres.
The Mantle
The thick shell of the dense, rocky matter that surrounds the core is called the mantle. The mantle consists of iron-magnesium-silicates and it is less dense than the core but denser than the outermost compositional layer, which also consists of rocky matter. The outer boundary of the mantle is at a depth from 8-70 km.
The Crust
The outermost compositional unit is the crust. While the core and the mantle have nearly constant thicknesses, the thickness of the crust is different in different places. The crust beneath the oceans, which is called oceanic crust, has an average thickness of about 8 km, whereas the thickness of the continental crust ranges from 30 to 70 km. The oceanic crust is basalt while the continental crust has a granitic composition.
The inner structure of the Earth is demonstrated in the following animation:
INSIDE THE EARTH
Layers of Differing Physical Properties
In addition to compositional layering, other changes occur within the earth. Most important, there are changes in physical properties such as rock strength and solid versus liquid. Except for the outer boundary of the core, the boundaries of the layers of different physical properties do not coincide with the boundaries of the layers of different chemical composition.
The layers of different physical properties outward from the centre of the Earth are as follows: inner core, outer core, mesosphere, asthenosphere, and lithosphere.
The inner structure of the Earth
Layers of differing chemical composition: core, mantle, crust. Layers of differing physical properties: inner core, outer core, mesosphere, asthenosphere, lithosphere. Note that the crust is a thinner layer inside the lithosphere. The oceanic crust and continental crust are different in composition.
The Inner and Outer Core
Within the core an inner region exists where pressures are so great that iron is solid despite its high temperature. The solid centre of the Earth is in the inner core. Surrounding the inner core is a zone where temperature and pressure are so balanced that the iron is molten and exists as a liquid. This is the outer core. The difference between the inner and outer cores is not one of the composition (the compositions are believed to be the same). Instead, the difference lies in the physical states of the two: one is a solid, the other is a liquid.
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