![]() The size and shape of the crater and the amount of material excavated depends on factors such as the velocity and mass of the impacting body and the geology of the surface. What influences the size and shape of a crater? Very large impact craters greater than 300 kilometers (185 miles) across are called impact basins. The structure of large craters is more complex because they collapse, forming terraces, central peaks, central pits, or multiple rings. Small craters often are simple bowl-shaped depressions. How are large craters different than small ones? Degraded craters – Craters that have become eroded due to weathering, lava flows, impacting, or downslope movement of material.Oblong craters can be created by impacts striking the surface at a very low angle. Irregular craters – Craters with irregular shapes or multiple impact craters formed at the same time.Multi-ring basins – A very large impact basin surrounded by as many as five or six circular rings of mountain chains in addition to the main basin rim.Scientists describe other types of craters as well: Galileo Image (PIA00405), produced by the United States Geological Survey, courtesy of NASA. The dark basins can be seen by the naked eye. Lava later flowed across the low floors of the basins, giving them a darker, smoother appearance than the surrounding, brighter highlands. The large circular dark areas in the image are impact basins, created as huge impactors struck the Moon. Large impact basins are also found on other planets, including Mars and Mercury. The largest impact basin on the Moon is 2500 kilometers (1550 miles) in diameter and more than 12 kilometers (7 miles) deep. Impact basins are very large impact structures that are more than 300 kilometers (185 miles) in diameter. Simple craters are small bowl-shaped, smooth-walled craters (the maximum size limit depends on the planet). Rays – Bright streaks extending away from the crater sometimes for great distances, composed of ejecta material.It can be loose materials or a blanket of debris surrounding the crater, thinning at the outermost regions. It is distributed outward from the crater's rim onto the planet's surface as debris. Ejecta – Rock material thrown out of the crater area during an impact event.It is elevated above the surrounding terrain because it is composed of material pushed up at the edge during excavation. They may have giant stair-like terraces that are created by slumping of the walls due to gravity. Walls – The interior sides of a crater, usually steep.At the same time, the rock beneath the crater rebounds, or bounces back up to add to the peak. Collapse of the material back into the crater pushes up the mound that forms the central peak. For larger craters (typically a few tens of kilometers in diameter) the excavated crater becomes so great that it collapses on itself. Central peaks – Peaks formed in the central area of the floor of a large crater.Floor – The bottom of a crater, either bowl-shaped or flat, usually below the level of the surrounding ground.The edges of these larger craters also may slump, creating terraces that step down into the crater. If an impactor is large enough, some of the material pushed toward the edges of the crater will slump back toward the center and the rock beneath the crater will rebound, or push back up, creating a central peak in the crater. Sometimes the force of the impact is great enough to melt some of the local rock. The impactor is shattered into small pieces and may melt or vaporize. The impact sprays material - ejecta - out in all directions. The shock wave fractures the rock and excavates a large cavity (much larger than the impactor). When an impactor strikes the solid surface of a planet, a shock wave spreads out from the site of the impact. Geologic processes have not erased the craters with time. The large number of craters in this region indicates that this part of the Moon is quite ancient. These are impact craters, each of which was formed when an asteroid or comet collided with the Moon's surface. This portion of the Moon is covered by numerous circular holes.
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