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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 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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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