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What evidence shows that the Moon was volcanically active long ago?
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Moon
(Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.
Evidence that the Moon was volcanically active in the distant past is primarily derived from the study of lunar samples and surface features. Key indicators include:
- Mare Basalts: Samples returned by Apollo astronauts confirmed that the dark, flat plains known as maria are composed of basalt, a rock formed from ancient surface lava flows that congealed after flooding giant impact basins.12
- Volcanic Features: The lunar surface contains rilles, which are trench-like valleys. Sinuous rilles, such as Schrƶterās Valley, resemble winding river channels and are believed to be flow channels created by low-viscosity lava. Additionally, there are rimless craters surrounded by dark halos that appear to have been formed by volcanic eruptions.342
- Compositional Evidence: Some rock samples contain vesicles (gas bubbles) and pyroclastic glass (volcanic ash), indicating that the lavas contained dissolved gases.2
- Radiometric Dating: Age dating of lunar samples reveals that these volcanic outpourings occurred hundreds of millions of years after the formation of the lunar highlands, suggesting they were driven by radioactive heating within the Moon's interior.25