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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 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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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA