Asteroid Impacts

Mars, Earth, Venus and Mercury are all affected by asteroids deflected from the asteroid belt and comets falling off the Kuiper belt, but when the Earth is viewed from space it betrays little or no cratering by large impacts, despite the fact it is located in the trajectory of both asteroids and comets. This impression however is more apparent rather than real and constitutes the consequence of the coverage of the Earth's crust by oceans over some 2/3 of the surface. The other major factor is the dynamic nature of the Earth, including the accretion and subduction of tectonic plates as well the intensive erosional processes, which obscure its impact history. Thus asteroid impact records on Earth are mostly concealed and are the subject of extensive exploration, using structural, geophysical, petrological and geochemical methods. It is the stable cratons which cratons contain the best preserved impact records, including exposed and filled-in craters and deep-seated impact-rebound dome structures. Documentation of the impact records of the Australian continent and marine shelves includes impact ejecta/fallout units, exposed impact structures, buried impact structures, meteorite craters and ring and dome geophysical anomalies of unproven origin. The identification of impact structures and impact ejecta/fallout deposits is fraught with complications. Initial references to circular morphological and drainage patterns, round lakes and oval depressions may provide a hint to possible underlying ring or dome structures, requiring field tests or drilling. Where impact structures have been destroyed, the presence of impact ejecta/fallout in sediments allows further insights into the impact history of the Earth.