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particles are pulled together into a massive object that pulls in more particles and bigger objects to become bigger and eventually form into a spherical planet.
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After earth was hit by a large object about the size of mars the debris pulled its self together to form the moon.
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As the Earth was cooling the densest materials where pulled into the center of the Earth and the less dense material hardened around the those denser materials.
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The Earth has cooled enough for the top layer of lava to harden into rock.
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The earth is no longer being constantly bombarded by comets and astroids.