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The internal structure of the Earth

The Earth is a roughly spherical planet with a mean radius of 6,371 km and is composed of three main regions:

a thin outermost layer called the crust, a mantle, and an inner region called the core.

The crust is about one percent of Earth's volume. It is rocky (except for small volumes of lava and magma). It is the only region of the planet that we can investigate directly by boring into it and taking samples. In continental areas, the crust's average thickness is 36 km but may be anything from 10 km to 80 km depending on the last movement of the tectonic plates in that area. The crust has an average density of about 2800 kgm/m3. The continental rocks of the crust are mostly igneous, metamorphic and sedimentary types.

The mantle , which comprises 84% of the Earth's volume and 67% of its mass, lies below the crust and is separated from it by a distinct layer called the Moho. The rocks of the upper mantle have a density of about 3300 kg/m3. The rocks in the mantle below 410 km probably consist of high pressure-temperature silicates such as simple oxide minerals. Below 670 km, down to the bottom of the mantle at 2,900 km depth, is the lower mantle. The total amount of water in the lower mantle minerals could equal five times that of Earth's oceans!

The core, has a volume of about 15% of that of the Earth and is about one third of its mass. Its radius is about a half of the Earth's. Geophysics measurements indicate that the core's composition is predominantly iron. Further evidence from this has been obtained from the study of the composition of meteorites. The core's overall density is about 13500 kg/m3 ; this is greater than that of iron due to the very high pressures in the centre of the Earth. The heat in the core was produced when the Earth was formed. The upper 2200 km or so are molten but the core is solid below that. The Earth's main magnetic field is associated with electric currents in the molten outer core.

© Keith Gibbs 2013