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For centuries, scientists have pondered how the Earth came to be, it’s size and shape, and what its “foundations” are composed of. Most agreed that it was wrought out of some combination of the four or five basic elements (earth, air, wind, water, fire, and in some cultures, metal) while others argued that it was either a disc or a globe. In time, these debates were all resolved to most people’s satisfaction; however, it was not until a relatively short time ago that we’ve come get a clear picture of what the Earth really looks like inside and out. The Structure of the Earth, if viewed from the inside, is similar in appearance to a layer cake. These layers can be defined by either their chemical or their rheological (i.e. having to do with flowing matter) properties. There are four layers in total, consisting of the crust, mantle, outer core, and inner core.
The outer crust of the Earth is solid, made up of silicate material, and consists of the continents and the ocean basins. The crust has a variable thickness, being 35-70 km thick where the continents lie and 5-10 km thick in the ocean basins. Directly beneath this is the mantle, which is the thickest layer of the Earth at roughly 2900 km from top to bottom. The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust, but due to high temperatures, the silicate material is sufficiently ductile that it is able to flow. This causes convection which is believed to cause the motions of the tectonic plates on the surface. Then there’s the outer core, which is liquid and composed of iron and nickel, is 2300 km thick and far more viscous than the mantle. It is also believed that convection of the outer core, combined with the Corioliseffect, is responsible for the Earth’s magnetic field. Finally, there is the inner core, composed almost entirely of iron, is about 1200 km thick and completely solid.
Our scientific understanding of Earth’s structure is based on ongoing observations of topography and bathymetry (the study of underwater depths, such of ocean floors), observations of rock in outcrops, samples brought to the surface by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the Earth’s gravity field, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth’s core. However, it is unlikely human beings will ever be able to explore the interior of the Earth and get a firsthand account of how it is made up, simply because the heat and pressure and its extreme depths are more than any human being or man-made object can withstand.
Currently, the deepest excavations on Earth are in South Africa where mining companies have dug 3.5 km into the Earth to extract gold.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.