Impact craters are divided into two main groups, based on their morphology: simple craters and complex craters. Simple craters are relatively small, with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl shape (slide #1, top). In larger craters, however, gravity causes the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20) (slide #1, bottom). The diameter at which craters become complex depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2–4 kilometers (depending on target rock properties); on the Moon, at one-sixth Earth's gravity, the transition diameter is 15–20 kilometers.