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There are two main kinds of surface found on the moon

1 Maria
  • flat
  • roughly circular, with mountain rims
  • not many craters
  • rocks are like basalt rocks on Earth (from volcanos)
  • rocks dated at 3.1 to 3.8 billion years
2 Highlands
  • not flat
  • many craters
  • not many craters
  • rocks dated 3.9 to 4.4 billion years

Even if we didn't have dates for lunar rocks based on radioactive decays, how could you tell that the rocks in the maria should be younger than the rocks in the highlands? This suggests a possible history.

4.6 billion years ago

the magma ocean



4.4 billion years ago

crust starts to form



4.4 to 4.0 billion years ago, era of the highlands

crater formation. lava flows. some craters were giant.



3.8 to 3.1 billion years ago, maria formation

lava flows into the giant craters.



http://zebu.uoregon.edu/~soper/Moon/history.html