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 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.