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The Earth is a sphere composed of four layers. The innermost layer is the inner core, the center of the planet made of solid metal. The inner core is surrounded by the outer core, which is made of liquid metal. The outer core is surrounded by the mantle, which is...

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The Earth is a sphere composed of four layers. The innermost layer is the inner core, the center of the planet made of solid metal. The inner core is surrounded by the outer core, which is made of liquid metal. The outer core is surrounded by the mantle, which is made of rock. This rock is molten closer to the outer core and more solid closer to the crust. The final layer of the sphere is the Earth's crust, made of rocks and minerals, which floats on top of the semi-liquid mantle in pieces of various sizes, called the tectonic plates.

How did these layers form? The general theory, known as the "core accretion theory," is that heavy materials left over from the formation of the galaxy were drawn together by gravity into clumps. Earth was one of these clumps, a rough molten blob of heavy elements like iron and nickel. As the young Earth spun in its orbit around the Sun, the heaviest elements of the blob sank to the center, forming the core, while the outer surface of the planet began to cool, forming the mantle and, later, the crust.

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