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Early in its history, the Moon was melted to great depths to form a "magma ocean."

The lunar highlands contain the remnants of early, low-density rocks that floated to the surface of the magma ocean.

The lunar highlands were formed about 4.4-4.6 billion years ago by flotation of an early, feldspar-rich crust on a magma ocean that covered the Moon to a depth of many tens of kilometers or more. Innumerable meteorite impacts through geologic time reduced much of the ancient crust to arcuate mountain ranges between basins.