THE FIRST 2 BILLION YEARS OF THE MOON
Radioactive dating of the lunar rocks points to an origin much like that of the earth. Most of the lunar material probably came from accreting planetesimals (small solid bodies) that were part of the contracting material forming the solar system. Nearly all the original crust was lost when the moon underwent a global melting to a depth of at least several hundred kilometers, followed by chemical differentiation or separation shortly after the moon's formation. A few of the rock samples retu rned to the earth are about 4.6 bi Ilion years old, the same age as the earth (even though earth rocks no longer exist that are older than 3.5 to 3.8 billion years).
The highland areas are apparently about 4.0 to 4.3 billion years old. After its formation, the global crust of the moon was continually modified by the impact of material from elsewhere in the solar system. The cratering record preserved in the early crustal units represents a distinct phase of intense cratering, which began to decline rapidly about 3.8 billion years ago. Although volcanic processes may have operated during this early period, the surface history of the moon is primarily that due to cratering. As mentioned earlier, this phase in the earth's history has been almost completely erased. Impact cratering continues today, but at a drastically reduced rate from what it must have been billions of years ago.
The next stage in lunar history was dominated by the formation of dark mare plains, which cover about 17 percent of the lunar surface, favoring the earthward side. These structures are relatively thin ponds of basaltic lava totaling less than 1 percent of the volume of the crust. Apollo rocks from maria suggest that the major outpouring of lava occurred between 3.9 and 3.2 billion years ago. Although some mare deposits may be as young as 2 billion years, there appears to have been no extensive igneous activity on the lunar surface for the last 3 billion years. Thus the shaping of the present lunar terrain is almost the opposite of that of the earth's-the moon dominated by cratering and the earth by volcanic and tectonic activity.
