THE TERRESTRIAL IMPACT CRATERING RECORD

Approximately 130 terrestrial hypervelocity impact craters are currently known. Due to variations in preservation and in geologic knowledge, this sample is biased towards young (< 200 Ma), large (> 20 km) craters on the cratons of Australia, Europe (including the former U.S.S.R.) and North America. The rate of discovery of new craters is 3-5 craters per year. Although modified by erosion, terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies, such as the Moon. Terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters, which can be used to constrain interpretations of lunar samples and models of crater formation. Due to erosion and its effects on form, terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects. These include: shatter cones, microscopic planar deformation features, solid-state and fusion glasses, high pressure polymorphs and whole rock melting and Vaporization. Shock recovery experiments indicate that these features occur over shock pressures of greater-than-or-equal-to 5 GPa to > 100 GPa. Terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact-induced fracturing. They include gravity and magnetic lows and reductions in seismic velocity. The gravity anomalies are seldom greater than approximately 30 mGal, due to the limiting effects of lithostatic pressure on fracturing. At large complex craters, the gravity signature may include a central relative gravity high, due to uplift, and short wavelength central magnetic anomalies, due to a variety of processes. Much current work is focused on the effects of impact on earth evolution. Previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from Cretaceous-Tertiary boundary sites as resulting from a major impact. Suggestions that other biological boundaries in the Stratigraphic record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record. By analogy with the lunar record and modelling of the effects of very large impacts, it has been proposed that biological and atmospheric evolution of the Earth could not stabilize before the end of the late heavy bombardment approximately 3.8 Ga ago. The present terrestrial cratering rate is 5.4 +/- 2.7 x 10(-15) km-2 a-1 for a diameter greater-than-or-equal-to 20 km. This represents a local threat on historic time scales. On a global scale, a major impact sufficient to cripple human civilization severely will occur on time scales of approximately 10(6) a.