Impact is probably the most widespread geologic process in our solar system. All of the planets, moons, and asteroids have extensively cratered surfaces; on many, impact is the dominant process acting on the crust, and on others it is the primary mode of resurfacing. As we look to detailed mapping and exploration of the planets, an understanding of shock metamorphism is essential for both geological understanding and construction of impact-resistant structures. At the same time, the discovery of shocked minerals at the K/T boundary has highlighted the role of impacts in the evolution of life on Earth and led to an intensified search for terrestrial impact effects.
The most striking aspect of shock deformation is the extreme rapidity of the process. Whereas geologists are used to tectonic events requiring millions of years or more, an impact process can accomplish the same strain in a billionth of a second. Thus, over 20 orders of magnitude separate tectonic strain rates from those seen in shock; the gap is more than 10 orders of magnitude even for “fast” deformation (10−4/s) in conventional uniaxial presses. Typically, impactites are intensely deformed—and often chemically reactive—with densely-packed fractures, twins, or dislocations, and melting is common. Despite their obvious importance, impact processes and products are much less well studied and understood than slow-strain-rate processes.