The smooth dark areas on the Moon's surface are called maria (plural for mare; Latin for seas). These volcanic plains are made up of a rock type known as basalt, similar in composition to the rocks found in Hawaii. They cover 17 percent of the surface area of the Moon. The maria contain physical features such as pits and channels, but lack large volcanos.
This oblique photograph looks north across the southern part of Mare Imbrium. The low sun light angle and long shadows accentuate details of the surface structure. The surface in this area is mare basalt. The prominent ridges running from upper left to lower right are wrinkle ridges, formed when the mare surface sagged under the weight of several kilometers of basalt. Similaar wrinkle ridges are seen in other mare regions, including Mare Serenitatis and Mare Humorum. The prominent peak in the lower left is Mt. Lahire, which is 1.7 kilometers high.
This photo was taken by the crew of Apollo 15.
Image Credit: NASA
Credit: Lunar and Planetary Institute