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This picture of the lunar maria shows the difference betwen the highlands, which are heavily cratered, and the dark maria which are relatively smooth with few craters. The absence of craters indicates that the maria are a younger age than the highlands.
Exploration of these regions revealed that the maria are made of gabbro and basalt. This means that the maria were created in an episode of volcanism involving an outpouring of lava which flooded the lower regions.
source: windows to the universe