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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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