Impact geology concerns with all the processes that take place during mutual collisions of planets, moons, asteroids, comets and all the small debris in space. These collisions occur at very high speeds on the order of 1-100 km/s and the effects are similar to explosions. The kinetic energy, pressure and temperature involved in such a collision is usually much higher than in any other geologic process (volcanism, tectonic processes, metamorphism etc.) and occurs on a much shorter timescale. This leads to the generation of various unique features, including the characteristic craters and shock features in the rocks and minerals.
For a more in-depth, yet still intuitive and easy-to-read, introduction to impact geology refer to:
French B. M. (1998) Traces of catastrophe: A handbook of shock-metamorphic effects in terrestrial meteorite impact structures. LPI Contribution No. 954. Houston, Texas, USA: Lunar and Planetary Institute. 120 p. http://www.lpi.usra.edu/publications/books/CB-954/CB-954.pdf
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