Lunar Geology

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. Click on image for full size (25K GIF) Lunar Orbiter image courtesy of NASA.

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.