SHOCK METAMORPHISM 179 SHOCK METAMORPHISM P S DeCarli, SRI International, Menlo Park, CA, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction The term ‘shock metamorphism’, synonymous with ‘shock wave metamorphism’ or ‘impact metamorphism’, refers to the range of effects produced by the collision of two bodies, e.g., by the collision of an asteroid with the Earth These effects include fracturing, the formation of planar deformation features (PDF), the formation of high-pressure phases, melting, and vaporization Our knowledge of shock metamorphism, currently quite incomplete, is derived from laboratory shock experiments, static high-pressure experiments, studies of naturally impacted materials, theoretical analyses, and numerical computations It is generally accepted that the history of the solar system is one of repeated collisions between orbiting bodies Lunar craters, now widely accepted as impact craters, provide a partial record of that history Only during the past 50 years has it become evident that the Earth, because of its higher gravity, should have experienced about twice as many large craters per unit area as the Moon Most of these craters on the continental crust have been deformed, modified by erosion, and buried by sediments or volcanism Between 1960 and 2003, about 170 terrestrial impact craters were identified, and three to five newly identified craters are added to the list each year The minimum velocity of an encounter between the Earth and a body within our solar system is 11.2 km s 1, the escape velocity of the Earth Photographic measurements of meteors, the familiar shooting stars, indicate that they enter the atmosphere at velocities in the range 13–30 km s 1; this is an appropriate velocity range for encounters with asteroids Comets encounter the Earth at velocities in the range of 30 km s (short period comets) to 70 km s (very long period comets) The fate of a body entering the Earth’s atmosphere at very high velocity depends on such details as the strength and density of the body, its velocity, and its angle of entry If the body is non-spherical, details of shape and orientation must also be considered, e.g., whether an elongated body enters the atmosphere in point-first or side-first orientation To simplify further discussion, only vertical impacts of spherical bodies that are strong enough to survive passage through the atmosphere are considered The interaction of a fastmoving fragile body with the atmosphere can produce an effect equivalent to a large nuclear explosion at an altitude above 20 km Resultant pressures at the surface may knock down trees, but are too low to produce shock metamorphic effects in minerals A very large body, greater than 10 km in diameter, will not be sensibly retarded by the atmosphere Impact with the Earth will result in the formation of a large crater, greater than 100 km in diameter Only two craters larger than 100 km in diameter are known to have formed within the past 150 million years The larger of the two, the 65-million-year-old Chicxulub crater, Yucatan, Mexico, is buried under more than 300 m of carbonate rocks, and was identified in 1981 by the recognition of circular patterns in gravity and magnetic field data Shock metamorphic features in drill cores have confirmed the identification The iridium-rich Cretaceous–Teritary K–T boundary layer, which contains shock metamorphosed minerals, coincides with the mass extinction (including dinosaurs) at the end of the Cretaceous, and is believed to be associated with this impact event Many impact specialists are convinced that the environmental effects of the Chicxulub impact were the primary cause of an abrupt mass extinction However, many palaeontologists disagree They argue that the extinction was not abrupt and that there is evidence for other causes The one matter on which all agree is that the iridium-rich boundary layer serves as an excellent worldwide common time marker that will be essential to further studies of K–T extinctions Impact specialists continue to search for evidence of large impacts that could be related to other mass extinctions Smaller impact events are much more frequent, but the resultant craters are more easily eroded or obscured Four craters having diameters in the range 7–18 km have been identified as less than million years old These craters were formed by the impact of bodies in the diameter range of about 300 m to km, sufficiently large to minimize retardation by the atmosphere Atmospheric retardation becomes significant only for bodies having diameters less than about 10 m, corresponding to masses less than about 1000 tons Thus, the velocity of a stony object of 10 m diameter might be reduced from approximately 15 km s on atmospheric entry to 10 km s on impact with the Earth The impact would deposit the energy equivalent of approximately 36 000 tons of trinitrotoluene (TNT), and the resultant crater would have a diameter in the range of 100–200 m