OK, let’s back up a bit. The Earth is not just a solid ball of stuff, but instead has layers. The outermost layer is the crust: solid rock that varies from a few kilometers thick under the ocean up to a hundred or so at its thickest under continents. The USGS site uses the great analogy of the Earth as an egg; with a crust so thin compared to the planet’s 12,700 kilometers diameter it’s like an egg’s shell.
Underneath that is the mantle, which is made of very hot but still solid (not liquid, as it’s commonly misrepresented) rock, going down to a depth of about 2,900 km. This is the white of the Earth egg.
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