Imbrium Flow Map

Lava flows within the lunar mare are quite large. Shown here is a map of 3 "young" lava flows in Mare Imbrium. These flows apparently record three separate eruptions within a period of ~500 million years over 2.5 billion years ago. The oldest group is the largest. Its furthest point lies about 750 miles (~1200 km) from the inferred vent in the lower left corner. The second group then buried parts of the first group. It extends for a distance of about 375 mile (~600 km). Finally, the youngest group is also the smallest. It is ONLY some 250 miles (400 km) in length. (NOTE: some areas contain a mixture of flows from the first and second flow groups and mapped here as "mixed.") No active Earth volcanoes have lava flows anywhere near this length. Still, a few older eruptions are of similar size. Due to their size, these features are called Flood Basalts. One example is the Columbia River Flood Basalts in the northwestern U.S. They extend from Idaho into the Pacific Ocean. Most of these flows formed about 16 million years ago, but some erupted as recently as 6 million years ago. The biggest flows are over 188 miles (300 km) long, and they collectively cover over 102,500 suqre miles (164,000 square kilometers). Thus, the Columbia River Basalts are nearly the same size as the youngest and smallest of the basalt flows in Mare Imbrium. (Map after figure 4.26 in the Lunar Sourcebook; based on Schaber, 1973)

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