The major lunar evolutionary stages and their defining characteristics now appear to be as follows:

1. The Beginning - (4.6 Billion years ago.) Formation of the moon was roughly contemporaneously with the Earth. Evidence suggests a beginning through accretion of relatively cold material rather than through catastrophic separation from a pre-existing and largely differentiated Earth.

2. The Melted Shell - (4.6-4.5 billion years ago.) Accretionary melting and volatile depletion of the outer 200-300 km of the moon with accompanying differentiation into a 60-70 km thick anorthositic crust and a 100-200 km thick mafic upper mantle.

3. The Cratered Highlands - (4.5-4.3 billion years ago.) Saturation of the lunar crust, once it could preserve impact structures, with impacts capable of forming craters 50-100 km in diameter.

4. The Old Large Basins and Crustal Strengthening -(4.3-4.0 (?) billion years ago.) Formation of Pre-Nectarian large basins with rapid isostatic adjustment of the crust to mass concentrations and deficiencies. Moving upward into the cooler upper crust and becoming significantly contaminated with crustal debris in the process, KREEP related liquids crystallized and formed interlocking networks of intrusions. Once solidified, these interlocking intrusions, combined with the removal of underlying urKREEP liquid, strengthened the crust.

5. The Young Large Basins - (4.0 (?)-3.9 billion years ago.) Formation of Nectarian and Imbrium age circular large impact basins in a crust strong enough to support mass concentrations and mass deficiencies indefinitely.

6. The Light-colored Plains - (3.9-3.8 billion years ago.) Lunar wide debris deposition in most of the then existing basins may have resulted from a combination of the degasing of the mantle that preceded the formation of mare basalt magmas.

7. The Basaltic Maria - (3.8-3.0 (?) billion years ago.) Surface eruption and subsurface intrusion of basaltic maria.

8. The Changing Crust - (3.0 (?) billion years ago to present.) Formation of the Procellarum volcanic ridge system and the eastern limb and western far side light-colored swirls possibly due to brief period of mantle convection. Meteor impact frequency reached approximately present values.

Apollo 11 began the process of understanding the evolution of the Moon and, indeed, the early evolution of the Earth. Now, it has been recently noted that, early in the Third Millennium, the moon mare may provide the vast and environmentally benign energy resources required by Earth and the consumables required for Martian settlement.

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