Figure 36.5 Mare basalt.

This 1.5-kg specimen of mare basalt was returned to Earth by the Apollo 15 astronauts in 1971. The small holes covering about a third of the surface are a result of gas dissolved under pressure in the lava from which the rock solidified. Bubbles formed when the lava reached the airless lunar surface, and some of them were frozen in place as the rock cooled. (NASA)

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