Simple crater
A small, bowl-shaped impact crater with a smooth, rounded floor and no central mountain.
A simple crater is the basic crater shape: a neat bowl surrounded by a raised rim. On Earth, craters up to about 2 kilometres across in sedimentary rock, and about 4 kilometres in hard crystalline rock, are usually simple. On the Moon, with its weaker gravity, they stay simple up to about 15 to 20 kilometres. Simple craters are deep for their size, typically around a fifth as deep as they are wide.
The bowl you see is not the bottom. During formation, loose broken rock slides back down the steep walls and collects on the floor as a lens of breccia, rock made of jumbled angular fragments. Drilling into simple craters often finds this layer, with shocked and melted rock mixed in.
Wolfe Creek Crater in Western Australia is a fine example. About 880 metres across, it was made by an iron meteorite, and heavily rusted iron fragments have been found nearby. Its floor is now partly filled with wind-blown sand. Barringer Crater in Arizona is another. To recognise a simple crater, look for a round bowl with a raised rim, steep inner walls, a fairly flat or rounded bottom and no central peak or terraces.
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Level: Explorer Ages 10 and up