Simple craters
Simple craters are bowl-shaped craters (Figure 2) with diameters of up to 2-4 km, depending of the target rocks (these diameters values are only valid for terrestrial craters as the change-over diameter between simple and complex crater is also dependent of the gravity). The transition between simple and complex crater is observed at lower diameter for sedimentary rocks (around 2 km) than for crystalline rocks (around 4 km; Grieve, 1987). The crater depth, or distance between crater rim and crater floor, is about one third of the crater diameter; this is true only for well preserved craters. Rim height is about four percent of the crater diameter (Melosh, 1989). The crater is generally filled by impact breccia, the so-called “breccia lens” or “crater-fill breccia” (e.g., French, 1998).

Figure 2. Examples of well preserved simple impact structures; the Meteor Crater and the Tswaing, about 1.2 and ~1 km in diameter, respectively. a) Meteor Crater (or Barringer Meteorite Crater) occurs in Paleozoic-Mesozoic sedimentary rocks, in Arizona (U.S.A.) and was formed about 50 ka. b) Tswaing crater (or Pretoria Saltpan crater) occurs in granite and was formed about 220 ka.

Complex craters
Complex craters are characterized by a central uplift, a flat floor, and inward collapse around the rim (see Figure 3). On Earth, the diameter of complex impact craters is larger to about 2 to 4 km (Grieve, 1987); complex craters start to form on the Moon for diameters around 15-20 km (e.g., Howard, 1974). The central uplift is composed of rocks that originated below the crater floor. It represents a stratigraphic uplift of about one-tenth of the crater diameter (see, e.g., Melosh, 1989; Cintala and Grieve, 1998). Like for simple craters, complex craters are filled by a mixture of rock and mineral clasts, both shocked and unshocked, together with impact melt and material slumped in the crater from the walls and crater rim. Detailed study of the texture and composition of the different units forming the crater-fill breccia can permit the reconstruction of the successive phases of the crater formation and modification. This approach need however to be combined with numerical modeling (see Stöffler et al., 2004). Now learn about Cratering Mechanics.

Figure 3. Example of the ~93 km diameter Copernicus complex impact crater on the Moon with well-preserved central uplift and terraced walls (Credit NASA - Kipp Teague; Apollo 12 image AS12-52-7739).

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