Impact!

The environmental effects of impact events differ with respect to time (seconds to decades) and spatial (local to global) scales. Short-term localized damage is produced by thermal radiation, blast-wave propagation in the atmosphere, crater excavation, earthquakes, and tsunami. Global and long-term effects are related to the ejection of dust and climate-active gases (carbon dioxide, sulfur oxides, water vapor, methane) into the atmosphere. At the end of the Cretaceous, the impact of a >10 km diameter asteroid led to a major mass extinction. Modern civilization is vulnerable to even relatively small impacts, which may occur in the near future, that is, tens to hundreds of years.

Hypervelocity impacts of asteroids and comets have played a key role in the evolution of the Solar System and planet Earth. Geochronology, the science that investigates the ages of rocks, has become a preponderant tool for dating impact events and for assessing whether they are related in time to mass extinctions on Earth. Impact events are instantaneous compared to other geological processes and, in theory, represent easy targets for multitechnique geochronology. Yet, only a few terrestrial impact events are accurately and precisely dated. A dating campaign is urgently needed if we are to fully understand the role of impacts in Earth history.

During the formation of large impact structures, layers of melted and crushed rock (ejecta) are deposited over large areas of the Earth’s surface. Ejecta thrown farther than 2.5 crater diameters are called distal ejecta. At distances greater than ~10 crater diameters, the distal ejecta layers consist primarily of millimeter-scale glassy bodies (impact spherules) that form from melt and vapor-condensate droplets. At least 28 distal ejecta layers have been identified. Distal ejecta layers can be used to place constraints on cratering models, help fill gaps in the cratering record, and provide direct correlation between impacts and other terrestrial events.

Geochemical analysis is an essential tool for the confirmation and study of impact structures and the characterization of the various rock types involved (target rocks, impact breccias, melt rocks, etc.). Concentrations and interelement ratios of the platinum-group elements, as well as the osmium and chromium isotope systems, allow quantification of extraterrestrial components and the identification of impactor types in impact deposits. In addition, chemolithostratigraphy can reveal the possible role of impacts in environmental change throughout the geologic record. This article deals predominantly with terrestrial impact structures.

The hypervelocity impact of extraterrestrial objects causes unequivocal changes in the target due to extreme deformation rates, pressures up to hundreds of gigapascals, and postshock temperatures that may even vaporize silicates. This article introduces the basic principles of shock compression, as required to understand the formation and geological significance of shock-metamorphic effects in minerals. Special emphasis is placed on the formation of high-pressure phases such as stishovite and diamond as well as on the decomposition of carbonates.

Impact cratering is an important and unique geologic process. The high speeds, forces and temperatures involved are quite unlike conventional endogenic processes, and the environmental consequences can be catastrophic. Kilometre-scale craters are excavated and collapse in minutes, in some cases distributing debris around the globe and exhuming deeply buried strata. In the process, rocks are deformed, broken, heated and transformed in unique ways. Elevated temperatures in the crust may persist for millennia, and important chemical reactions are promoted by the extreme environment of the impact plume. Released gases may cause long-term perturbations to the climate, and impact-related phosphorus reduction may have played a role in the origin of life on Earth.

It is now universally accepted that the impact of planetesimals, asteroids, and comets has been a fundamental process throughout the Solar System. Catastrophic impact events have been instrumental in developing the early history of the planets and have caused environmental disasters throughout Earth history. A major mass extinction at the Cretaceous– Paleogene boundary has been confidently related to an impact event (Chicxulub, Mexico). While the study of impact cratering is a multidisciplinary field, mineralogical and geochemical investigations have been central since the beginning, focusing on the nature of impact-generated rocks and of the extraterrestrial projectiles as well as their interaction with geological materials. Chemical and isotopic techniques have allowed the dating of impact events and the identification of traces of meteoritic projectiles in impactformed rocks on Earth and the Moon.