Peak ring
A circle of mountains inside a large impact crater, found instead of a single central peak.
In very large craters, the central uplift rises so high that it cannot support itself. It overshoots and collapses outwards, and the material settles as a circle of hills or mountains standing on the crater floor, a peak ring. Craters with them are called peak-ring basins. On the Moon they appear in craters wider than roughly 150 to 200 kilometres; on Earth, with its stronger gravity, at a few tens of kilometres.
Peak rings help scientists test how rocks behave during impacts. They also bring up very deep crust, and they can trap water and heat in fractured rock, which may have offered homes for microbes after big impacts.
The best-studied example on Earth is Chicxulub, the buried crater about 180 kilometres wide beneath Mexico's Yucatán Peninsula, linked to the extinction of the non-bird dinosaurs 66 million years ago. In 2016 a scientific drilling expedition reached its peak ring and found heavily shocked and fractured granite, rock that normally sits several kilometres down and must have been lifted up within minutes. That supported the dynamic collapse model of peak-ring formation. On the Moon, the Schrödinger basin, about 320 kilometres wide, shows a clear peak ring in spacecraft images.
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