Crater rim
The raised, circular edge that runs around the top of an impact crater.
The crater rim is the ring of raised ground around a crater, like the edge of a bowl. It forms in two ways at once. The force of the impact pushes the rock layers around the hole upwards and even folds them back on themselves. At the same time, rock thrown out of the crater lands on top, making the rim higher still.
Rims are useful clues. A raised rim is one of the first signs that a round hole might be an impact crater and not a sinkhole or a pond. Geologists study the rock layers in rims because in an impact crater the layers near the top are often flipped upside down, so the oldest rock sits on top of younger rock.
At Barringer Crater in Arizona, USA, the rim rises about 45 metres above the flat desert around it, and you can see the tilted and overturned layers in the crater walls. Visitors can walk along a viewing area on the rim and look down into the crater. Over long periods, rain and wind wear rims down, so very old craters may have lost them entirely. On the Moon, crater rims cast long shadows when the Sun is low, making them easy to spot through binoculars.
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Level: Beginner Ages 7 and up