Lunar Geologic History

  1. Formation From Mars-sized Impact of Protoearth (4.6 Billion Years Ago)
    Evidence for this comes from the fact that composition of lunar rocks is very much like that of the Earth's mantle. Also, there is evidence that the moon is moving away from the Earth very slowly, and so must have been much closer in the distant past. After the collision the moon is very close to the Earth (within an Earth-diameter or two).

  2. Period of Heavy Bombardment (4.6 - 4.0 Billion Years Ago)
    Scattered pieces of rock left over from the formation of the Moon and from the rest of the solar system collide with the moon and heavily crater the surface. Very large impacts form gigantic basins. The Moon slowly moves away from the Earth because of tidal forces.

  3. Formation of the Maria (3.9 - 3.1 Billion Years Ago)
    Basaltic lava flows seep into the basins to form the low dark plains seen on the side of the Moon that faces the Earth. Tides from the Earth force the lava-filled basins to point to the Earth, as the Moon continues to move away.

  4. Subsequent Light Cratering (3.1 Billion Years Ago - Present)
    Meteors continue to hit the moon, and crater the surface. Some large craters like Copernicus were formed as late as 0.9 Billion Years Ago. The Moon moves to its present location.