![]() Pat McGovern, Walter Kiefer (colleagues at the Lunar and Planetary Institute) and I were intrigued by this correspondence. ![]() When the Moon’s topography was mapped with laser altimetry (first by Clementine in 1994, then at greater resolution by the Japanese Kaguya spacecraft and NASA’s Lunar Reconnaissance Orbiter mission), it showed clusters of many small volcanoes occurring on topographic highs that are quasi-circular, with low relief and shield-shaped. Scattered within the ancient maria are numerous small volcanic constructs, previously believed to be the only manifestation of central-vent volcanism on the Moon. The maria formed as this geologic process of massive high-volume eruptions built up stacks from the thin, fluid flows which extend for hundreds of kilometers. We understand lunar lavas to be similarly fluid, having erupted in thin sheet-like flows onto the airless surface of the Moon. ![]() On Earth, when such rocks are molten, the resulting magma has a very low viscosity (i.e., they are very fluid, spreading onto flat surfaces in thin sheets). They are rich in iron and magnesium and poor in silica. Rocks from the maria are basalts, the most common type of igneous rock in the Solar System. Many of these small volcanic features tend to be clustered in provinces concentrated on the western near side. Orbital images showed us a landscape of domes, small cones, sinuous lava channels (rilles) and collapse pits – surface features created by volcanic activity. Before the Apollo 11 astronauts visited Mare Tranquillitatis in 1969, we understood that the dark maria of the Moon were volcanic lava plains. The large-scale shield volcanoes so prominent on Mars, Venus and Earth were believed to be absent on the Moon. Our understanding of lunar volcanism has been informed and shaped both by images and samples. Earth’s shield volcanoes range in size from a few to more than 200 km for the Big Island of Hawaii, the extent of its base on the sea floor beneath the surface of the Pacific Ocean. In geology, a shield volcano is a volcanic construct with a broad, low profile made up primarily of thin lava flows with little ash deposits. Come home with your shield, or on it – Spartan women to their husbands, marching off to war.įrom the giant Olympus Mons shield on Mars (600 kilometers across and 27 km high) to the large volcanoes of Venus, shield-building was thought to be a common expression of volcanism on all rocky Solar System bodies the Moon appeared to be a conspicuous exception.
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