Earth Surface, Ejecta, Geochronology, Impact Melts, Lithology, Metamorphism (Geology), Meteorite Craters, Meteorites, Meteoritic Composition, Radioactive Age Determination, Rocks, Accuracy, Biogeochemistry, Cooling, Glass, High Temperature, Isotopes, Melting, Mineralogy, Shock Waves, Stratigraphy Systematic examination of dating results from various craters indicates that about 90% of the rocks affected by an impact preserve their pre-shock ages because shock and post-shock conditions are not sufficient to disturb isotopic dating systems.
In the other 10% of target lithologies, various geochronometers show significant shock-induced effects.
Based on apparent correspondences between periodicities observed in the marine extinction record and in the terrestrial impact record, some scientists have suggested that large meteorite impacts might be the metronome that sets the cadence of biological evolution on Earth — an unproved but intriguing hypothesis.
Nevertheless, the study of the K-T extinction and its association with one of the largest impact structures known on Earth led to renewed and widespread interest in impacts.
These features were caused by the collision of large meteorites or comets with the Earth.
They often display complex textures, where differently shocked and unshocked phases interfinger on the sub-mm scale.Therefore, the major criterion for sample selection in and around craters is the post-shock thermal regime.Based on their different thermal evolution, the following geological impact formations can be distinguished: (1) the coherent impact melt layer, (2) allochthonous breccia deposits, (3) the crater basement, and (4) distant ejecta deposits.This rate estimate is equivalent to that based on astronomical observations of Earth-crossing bodies.These rates are a factor of two higher, however, than the estimated post-mare cratering rate on the moon but the large uncertainties preclude definitive conclusions as to the significance of this observation.The EID lists only 7 or 8 such craters, and the largest in the last 100,000 years (100 ka) is the 4.5 km Rio Cuarto crater in Argentina.