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[quote][i]Originally posted by Tiger[/i] [img]http://www.lpi.usra.edu/expmoon/science/craterstructure.gif[/img] Comets and asteroids strike the Moon at a wide range of impact speeds, with 20 kilometers per second being typical. Such a high-speed impact will produce a crater that is 10 to 20 times larger in diameter than the impacting object. The detailed form of the crater depends on its size. This figure shows idealized cross-sections of the structure of small, simple craters (top) and of larger, more complex craters (bottom). Simple craters have bowl-shaped depressions and are the typical crater form for structures on the Moon with rim diameters (D in the figure) of less than about 15 kilometers. Craters on the Moon with diameters larger than about 15 kilometers have more complex forms, including shallow, relatively flat floors, central uplifts, and slump blocks and terraces on the inner wall of the crater rim. In craters on the Moon with diameters between about 20 and 175 kilometers, the central uplift is typically a single peak or small group of peaks. Craters on the Moon with diameters larger than about 175 kilometers can have complex, ring-shaped uplifts. When impact structures exceed 300 kilometers in diameter, they are termed impact basins rather than craters. More than 40 such basins are known on the Moon, and they have an important control on the regional geology of the Moon. Much of the material ejected from the crater is deposited in the area surrounding the crater. Close to the crater, the ejecta typically forms a thick, continuous layer. At larger distances, the ejecta may occur as discontinuous clumps of material. Some material that is ejected is large enough to create a new crater when it comes back down. These new craters are termed secondary craters and frequently occur as lines of craters that point back to the original crater. Material below the surface of the crater is significantly disrupted by the shock of the impact event. Near the surface is a layer of breccia (a type of rock composed of coarse, angular fragments of broken-up, older rocks). Rocks at deeper depths remain in place (and are termed bedrock) but are highly fractured by the impact. The amount of fracturing decreases as the depth below the surface increases. The energy of the impact typically causes some material to melt. In small craters, this impact melt occurs as small blobs of material within the breccia layer. In larger craters, the impact melt may occur as sheets of material. [Edited on 17-03-2003 by Tiger] [/quote]
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