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Geology LibreTexts

2.2: Structure of the Earth

The structure of the earth has been classically divided into four major grous. The crust, the mantel, and the outer and inner cores have all been defined by their unique chemical properties based off of studies of volcanic and seismic activity as well as mass estimates of the earth that have been able to determine the densities of the different layers. The way these layers interact with each other has significant implications to volcanic, seismic, and electromagnetic activity that have characterize earth and have contributed to the dramatic changes over geological time. The uppermost layer of the mantle and the crust tend to act together as a rigid shell. Together they are called the lithosphere, the "sphere of rock". The lower level of the mantle is called the asthenosphere and it is softer and weaker, particularly in its upper portion where a small amount of melting can occur. It is at this level where the model of plate tectonics suggests that horizontal movement can occur as a result of convection of heat upward from the Earth's core. Contributing to volcanic activity and continental drift over time. Modeling the core of the Earth must rest upon even more indirect evidence. Its observed that the metallic meteorites have cores of iron and nickel, and this correlates with other evidence that suggests that the Earth's core is similarly composed of iron and nickel. Modeling the density of the center of the Earth yields densities of about 14 times that of water, which could be obtainable by compressing iron and nickel, but not surface type rocks. An iron core also gives us a circulating electrical conductor, which could provide the necessary mechanism for creating the Earth's magnetic field.

 

The earth consists of either three layers, or five layers depending on the model used.  The first model divides the Earth based on its mineral framework into the crust, the mantle and the core.  The crust is the outermost layer and is very thin and solid.  There are two variants of crust, the oceanic and continental crusts.  The oceanic crust is mainly composed of basalt and is denser and thicker than continental crust, which is made of granite.  More about how these two forms of crust interact will be discussed in 2.5.  The buoyant crust floats on top of the iron and magnesium rich layer: the mantle.  This massive layer takes up roughly 84% of the Earth's volume and is composed of slightly molten rock, constantly being squeezed by the pressure which increases significantly as one approaches the center of the earth called the core.  Discovered in 1906, the metallic core is incredibly dense and thus most seismic waves are unable to pass through.

 

The second model divides Earth based on its strengths and properties into the lithosphere, the asthenosphere, the mesosphere, the outer core, and the inner core.  The lithosphere consists of the crust and uppermost section of the mantle.  Composed mostly of stone, this layer varies in width around the globe, but is responsible for plate tectonics.  However, the lithosphere floats atop the weak asthenosphere, the next layer that is relatively thin and barely melted.  As the pressure increases moving inward, the rock melts more, slowly rotating outwards and inwards in the lower mantle, also known as the mesosphere.  The rock in the outer core is a fluid and is dense enough to where certain waves cannot pass it, unlike the mantle which all seismic waves pass through.  The rotation of the outer core is what creates Earth's magnetic field.  Eventually the pressure is so great that the liquid rock gets compacted into a solid again in the inner core.

 

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