The two basic types of regions on the Moon: a smooth, dark mare on the left
and a heavily-cratered, light-colored highland region on the upper right.
NASA.
Lunar Geology
Looking up at the Moon, you can distinguish dark regions and light
regions. With binoculars, you can even see that the dark regions are
smooth compared to the light regions which have many craters.
Dark regions on the Moon are called maria, which is Latin for
"seas". Thus, Mare Tranquilatis is the "Sea of Tranquility". Apollo
astronauts discovered that these regions are smooth, low-lying plains
with relatively few craters. Maria are covered with a type of rock
(called basalts) similar to the dark colored rocks formed by
lava from volcanoes here on Earth. Basalts are composed of relativley
heavy elements such as iron, manganese, and titanium. Radioactive
age-dating of these rocks showed them to be between 3.1 and 3.8
billion years old.
Light-colored regions turned out to be heavily-cratered highlands
covered with a type of light-colored rock called anorthosite.
Anorthosite contains relatively lightweight elements such as calcium and
aluminum. This type of rock is found only in the oldest mountain
ranges on the Earth, and radioactive age-dating proved the lunar
anorthositic rocks were over 4 billion years old.
Once it was known that the light highlands were old and the dark maria
younger, scientists could piece together the Moon's history.
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