Author name: Fred Jourdan

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.

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.