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Why do some impact craters have flat floors?
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Meteorite crater
Four impact craters of the same size (30 km [20 miles] in diameter) imaged by spacecraft on different solid bodies of the solar system and reproduced at the same scale. They are (clockwise from upper left) Golubkhina crater on Venus, Kepler crater on the Moon, an unnamed crater on Jupiter's moon Ganymede, and an unnamed crater on Mars. The images are oriented such that the craters appear illuminated from the left; the Venusian crater is imaged in radar wavelengths, the others in visible light.
Impact craters with flat floors are classified as complex craters. While smaller, simple craters retain a parabolic bowl shape after excavation, larger craters undergo a modification stage because the initial cavity is too large to remain stable.1
In smaller complex craters, the flat floor is caused by a rebound of material from beneath the crater following the initial excavation. In larger craters, this same rebound process results in the formation of a central peak, while even larger structures may develop a raised circular ring within the crater. This modification stage also involves downward faulting, which creates terraces of material along the inner rim of the cavity.1