Abstract

The terrestrial record of impact events is incomplete and evolving. There are inherent biases in ages, distribution and sizes of known impact events that result from the high levels of endogenic activity on the Earth. Nevertheless, an estimated terrestrial cratering rate of (5.6 ± 2.8) × 10⁻¹⁵ km⁻² a⁻¹ for impact structures with diameters >20 km and younger than 120 Ma can be calculated and is compatible with astronomical observations. The most obvious evidence of impact is the occurrence of 156 impact structures known as of the end of 1996. Few impact structures, however, are sufficiently pristine to provide great detail concerning their original morphology. Some basic morphometric parameters, however, can be estimated. Confirmation of an impact origin for particularly terrestrial structures comes generally from the recognition of diagnostic shock metamorphic effects in the target rocks. Some 16 impact events are currently recognized in the stratigraphic column but it would appear that many others await discovery. The best documented and only global example of such events is at the Cretaceous-Tertiary (K-T) boundary. Although a number of killing mechanisms have been proposed for the attendant mass extinction, the nature of the target, in this case containing sulphates, may be the reason for the devastating effect of this event on the biosphere. In early Earth history, the high impact flux was probably a significant factor in the modification of the atmosphere, biosphere and hydrosphere and the collision of a Mars-sized object with the proto-Earth may have been responsible for the formation of the Earth’s Moon. Prompted by the association of the K-T event with a global mass extinction, it has been proposed that other mass extinctions and geological phenomena in the Phanerozoic are impact related, possibly through periodic cometary showers. At this time, there is little or no evidence for this association, although model calculations and the terrestrial cratering rate suggest that impact cannot be ignored as a forcing function for transient changes in the Earth’s atmosphere and climate. Although unequivocal evidence linking impact to climatic changes remains to be discovered, it is a statistical certainty that if human civilization exists for time-scales of hundreds of thousands of years it will be severely affected or possibly destroyed by an impact event. Given the stochastic nature of impact events, however, this could happen sooner rather than later.