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 areas on the Moon are called maria, which means "seas" in
latin. Astronauts discovered that these regions are smooth and
shallow. Maria have few craters and are covered with a type of rock
(called basalts) which are similar to lava rocks formed by
volcanoes here on Earth. Tests showed that these lunar rocks are
between 3.1 and 3.8 billion years old.
Light-colored areas are more hilly and covered with lots of craters.
This is the "land", or terrae on the Moon. The color of
these areas comes from a type of light-colored rock called
anorthosite. This type of rock is found only in the oldest
mountain ranges on the Earth. Geologists have found that these lunar
rocks are over 4 billion years old. That's nearly as old as the solar
system itself!
Once it was known that the light areas were old and the dark maria
younger, scientists could piece together the Moon's history.
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