THE IMPORTANCE OF THE ALVAREZ K-PG BOUNDARY ASTEROID IMPACT HYPOTHESIS FOR IMPACT CRATERING RESEARCH (Invited Presentation)

Over 30 years ago, the general geological community greeted the evidence published by Alvarez et al. for a major asteroid impact on Earth at the end of the Cretaceous with disbelief. On the other hand, planetary scientists and cosmochemists, who were familiar with lunar samples and meteorites that show abundant evidence of impact and collisional processes, readily accepted the interpretation of an end-Cretaceous impact event. Research initiated by this controversy led to a better definition and understanding of impact evidence, particularly in terms of shock metamorphism and the identification of meteoritic components. One key result from these studies was also the acknowledgment that impacts may have influenced biological evolution on Earth. Impact cratering is a high-energy event that occurs at more or less irregular intervals. Part of the problem regarding recognition of the remnants of impact events is the fact that terrestrial processes either cover or erase the surface expression of impact structures on Earth. In some cases, ejecta have been found far from any possible impact structure. The study of these ejecta has led, in some cases, to the discovery of the source impact craters. A well known case in point is the Cretaceous-Tertiary (now Cretaceous-Paleogene) boundary, where the discovery of an extraterrestrial signature, together with the presence of shocked minerals, led not only to the identification of an impact event as the cause of the end-Cretaceous mass extinction, but also to the discovery of a large buried impact structure about 200 km in diameter, the Chicxulub structure. The first physical evidence pointing to a contribution of extraterrestrial material that was discovered was the presence of anomalously high PGE abundances in K-Pg boundary clay in Italy and other locations around the world. An aspect of impact cratering that has been underestimated for a long time is the influence of impacts on the geological and biological evolution of our own planet. The initial work by Alvarez and colleagues has lead to a revolution not only in our understanding of the impact evidence in the geological record, but also in terms of the realization that such events have had far-reaching consequences for our planet.