Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Although these look like craters, there are actually only a few small genuine impact craters in this picture, the largest of them on the left. The great oval basins are actually the summit caldera of the great Martian shield volcano Olympus Mons. Calderas form by subsidence of the summit of a volcano. The subsidence can be catastrophic, like it was for Crater Lake in Oregon, or gentle, as it is for the summit calderas on Mauna Loa and Kilauea in Hawaii, and likely was here as well. These calderas are far larger than those in Hawaii. On the far right, there are a few small oval pits and chains of small pits. These probably formed by the collapse of lava tunnels. Lava tunnels form when the top of a lava flow hardens and the interior keeps flowing. Later the roof can collapse to form pits or long trenches. Such collapsed lava tubes are also found on the earth and the Moon. The streaky textures in places are due to thin blankets of wind-blown sediment.
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Created 17 December 2007, Last Update 15 January 2020