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Lava Tubes



This image shows a chain of small pits and ridge segments. It marks the likely site of a partially collapsed lava tube in western Mare Imbrium. Here, instead of a surface lava channel, lava flowed through a buried tunnel in the mare. After the eruption stopped, the tunnel then emptied. Where the roof of the tunnel has fallen in, we see pits. If the entire roof had fallen in, we might see a sinuous trough like other lunar rilles. Thus, it is believed that some lunar rilles mark collapsed lava tubes. These rilles are usually near other collapse pits, and some also merge with ridge-like features. Thus, uncollapsed lava tubes may still exist near some of these lunar sinuous rilles. (Lunar Orbiter image V-182-M, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

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http://antwrp.gsfc.nasa.gov/apod/ap021029.html

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