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Composition & Internal Structure of the Earth

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  • Composition & Internal Structure of the Earth

    Any group of diverse materials can be classified by chemistry or physical properties. Chemistry refers to different materials, and physical properties refers to such things as whether the materials are solids or liquids, etc. Some physical layers in the earth consist of 2 chemical layers and some chemical layers consist of 2 physical layers.

    Compositional/Chemical Layering

    The chemical layering of the earth formed as part of the earth's differentiation-resulting from the heat generated during the process of planetary accretion. As a general rule, denser materials settled to the center of the earth, leaving lighter materials on top. Thus, the earth consists of successive layers of material getting less dense as you approach the surface. The major compositional layers are:

    Core is made of iron with minor amounts of nickel, and lies at the center of the earth
    Mantle is made of iron-magnesium silicates and surrounds the core. The mantle makes up the bulk of the earth.
    Crust occurs as two distinct types, oceanic crust and continental crust. Both types of crust are lighter (less dense) and contain more silica than the mantle. Oceanic crust is the crust that underlies most of the areas we call "oceans" it is thinner, is more dense, and contains less silica and aluminum and more magnesium and iron than continental crust. The lack of silica makes it darker than continental crust. Because continental crust is thicker and made of less dense material than the oceanic crust, it "floats" higher on the earth.

    Physical Layering

    Because of variations in temperature and in pressure, the materials inside the earth vary in there physical properties with depth.

    Inner Core is the central part of the iron-nickel core. It is a solid iron sphere. The reason that the iron is solid is that the pressure at the center of the earth is significantly higher than the pressure above, while the temperature is only slightly higher. While higher temperature would tend to melt materials, higher pressures tend to create solids.
    Outer Core constitutes the remainder of the iron-nickel core and is liquid. It is liquid because the pressure is lower.
    Mesosphere. The majority of the mantle from the core-mantle boundary is solid and is called the mesosphere.
    Asthenosphere. Nearer to the surface of the earth the temperature is still relatively high but the pressure is greatly reduced. This creates a situation where the mantle is partially melted. The asthenosphere is a plastic solid in that it flows over time.
    Lithosphere. Above the asthenosphere, the temperature begins to drop more rapidly. This creates a layer of cool, rigid rock called the lithosphere. The lithosphere includes the uppermost part of the mantle and it also includes all of the crust. That is, the crust is the upper part of the lithosphere, and the upper mantle is the lower part of the lithosphere.

    Isostacy

    The lithosphere "floats" on the asthenosphere. The asthenosphere accommodates the floating lithosphere by flowing. If a load is placed on the surface of the earth (like the weight of water of a large lake) the surface of the earth will sink under the added weight. Mountains are like icebergs: the higher the mountain is, the thicker the crust is underneath it, and the more displaced is the asthenosphere. Thus mountains have "roots".
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