The earth is made of multiple layers of hardened and liquid rocks, which include, moving from the exterior of the planet toward the interior, the crust, mantle, outer core, and inner core. The crust is the hardened outer layer of rock composed primarily of silicate and iron.
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"The internal structure of Earth" by Kelvinsong is licensed under CC BY-SA.
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Pidwirny, M.
2013 Structure of the Earth. http://www.eoearth.org/view/article/156285, accessed
December 5, 2014.