Complex crater
A larger impact crater whose floor has collapsed and rebounded, giving it a flat floor, terraced walls and a central peak or ring.
Above a certain size, a crater cannot keep its steep bowl shape. Gravity makes the walls slump inwards in giant steps, and the floor springs upwards in the centre, rather like the splash when a drop hits water, but frozen in rock. The result is a complex crater: wide and comparatively shallow, with a flat floor, terraced walls and a central peak or, in bigger examples, a ring of peaks. On Earth the change from simple to complex happens at about 2 to 4 kilometres across, depending on the rock.
Complex craters reveal what lies beneath, because the central uplift brings up rock from deep down. They also hold much more impact melt, which can fill the floor as a sheet.
The Nördlinger Ries in southern Germany is a famous example. About 24 kilometres across and nearly 15 million years old, it has a flat floor where the town of Nördlingen stands and an inner ring of hills made of uplifted crystalline rock. On the Moon, Tycho, about 85 kilometres wide, shows terraced walls and a central peak that you can pick out with a small telescope. Look for terraces, a flat floor and something raised in the middle.
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Level: Explorer Ages 10 and up