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 see that there are 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". So Mare Tranquilatis is the "Sea of Tranquility". Apollo
astronauts discovered that these regions are smooth, low-lying plains
with relatively few craters. Maria get their color from a type of
rock (called basalts) similar to the dark colored rocks
formed by lava from volcanoes here on Earth. Basalts are composed of
relatively heavy elements such as iron, manganese, and titanium.
Tests showed these lunar rocks are between 3.1 and 3.8 billion years
old.
Light-colored regions turned out to be hilly regions with many craters
and 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 geologists have found
that the lunar anorthositic rocks are over 4 billion years old.
Once it was known that the light regions were old and the dark maria
younger, scientists could piece together the Moon's history.
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