IMPACT STRUCTURES 283 that experienced $25 GPa Shatter cones are best developed in fine-grained, structurally isotropic lithologies, such as carbonates and quartzites They occur in coarse-grained crystalline rocks but are less common and more poorly developed Impacts and Earth Evolution The basic working hypotheses for the formation of the terrestrial planets is the accretion of small bodies by collision and the subsequent growth to larger planetary embryos This early stage of planetary accretion is believed to be followed by a stage involving truly giant impacts between the embryos In the context of planetary formation, therefore, impact is the most fundamental process (see Solar System: Moon) In the case of the Earth, the last embryonic planetary collision may have resulted in the formation of the Earth’s Moon Numerical simulations of a glancing impact of a Mars-sized body with the proto-Earth result in the formation of an Earth-orbiting accretionary disk from which a Moon-like body can form The key is that the bulk of the material that forms the accretionary disc is originally terrestrial or impacting body mantle material Condensation and reaccretion result in volatile and siderophile-depleted material, which goes to form the Moon Although the impact hypothesis for the origin of the Moon is consistent with the constraints of the dynamics of the Earth–Moon system and the geochemical nature of the Moon, it remains a model The consequences of such a giant impact for the proto-Earth, however, would have been severe They would have included massive remelting and the likely loss of the original atmosphere After planetary formation, the subsequent high rate of bombardment by the remaining tail of accretionary debris is recorded on the Moon and other terrestrial planets that have preserved portions of their earliest crust This is not preserved on Earth, because of the high level of endogenic geological activity and the resultant relatively young surface of the Earth In the case of the Moon, a minimum of 6000 craters with diameters >20 km are known to have been formed during the early period of approximately 4.5–3.8 Ga There are also approximately 45 known impact basins ranging in diameter from Bailey at 300 km to the south pole Aitken basin at 2600 km Throughout geological time, the Earth has received more impacts than the Moon The Earth is a physically larger target for incoming bodies and has a much larger gravitational cross-section to attract incoming bodies There has been considerable speculation as to the potential effects of these basin-sized impacts on the Earth By analogy with the lunar case, it is likely that few terrestrial surface rocks would have survived intact through this period of heavy bombardment Basin-sized impacts on the early Earth may have also affected the existing atmosphere, hydrosphere, and the potential development of the biosphere The oldest known terrestrial impact structures are Sudbury and Vredefort They have reconstructed original diameters of approximately 250–300 km, and ages of 1.85 and 2.0 Ga, respectively Although it is expected that such large events would have had a deleterious effect on the climate and biosphere, no direct evidence is known at present These structures, however, also affected the local geology in a manner that is to human benefit Both impact structures are the sites of world-class ore deposits An extensive literature concerning the evidence for a major impact event at the K–T boundary and its association with a mass extinction in the terrestrial biosphere 65 Ma exists The unequivocal physical evidence for impact contained in K–T boundary deposits consists of planar microstructures in quartz, feldspar, and zircon, and the occurrence of stishovite, impact diamonds, high-temperature magnesioferrite spinels (believed to be vapour condensates), and various melt spherules, generally altered, but including the tektite-like glass spherules in Haiti and other Caribbean sites The chemical evidence consists of a global siderophile anomaly in K–T boundary deposits, indicative of an admixture of meteoritic material Although originally contentious, there is now little doubt that the Chicxulub structure in the Yucatan peninsula, Mexico is the K–T impact structure Chicxulub is buried by some km of younger sediments but evidence for impact, in the form of planar microstructure quartz and feldspar in deposits interior and exterior to the structure, as well as impact-melt rocks, has been documented In addition, the geochemistry of K–T tektite-like glasses from Haiti matches the mixture of lithologies found at the Chicxulub site Isotopic ages for the impact-melt rocks at Chicxulub of 64.98 Ỉ 0.05 Ma are indistinguishable from the K–T tektites at 65.07 Ỉ 1.00 Ma (see Mesozoic: End Cretaceous Extinctions) The original hypothesis for the killing mechanism for the K–T mass extinction suggested global darkening and cessation of photosynthesis due to ejecta in the atmosphere Soot has also been identified in K–T deposits and ascribed to global wildfires A considerable thickness of anhydrite (CaSO4) occurs in the target rocks at Chicxulub Impact heating of anhydrite would produce sulphur aerosols in the atmosphere Modelling suggests that these sulphur aerosols would reduce light levels below those needed for photosynthesis for 6–9 months In addition, if most of the aerosols were in the form of SO2, solar