METEORITE IMPACT CRATERS ON EARTH - MAJOR SHOCK-INDUCED EFFECTS IN ROCKS AND MINERALS

The basic principles of the physics of shock waves are summarised, sho wing how shock pressures, shock and post-shock temperatures, and shock durations can be estimated in the case of large meteorite impacts on Earth. In a second part, the pertinence of laboratory high-pressure dy namic experiments for simulating large meteroite impact events and for calibrating their physical conditions is discussed. It is concluded t hat most shock features are common to natural and laboratory shocks, a lthough the lifetime of experimental shocked states is shorter by seve ral orders of magnitude. Then, a review is made of the major shock eff ects observed in minerals and rocks. Quartz has been by far, the most extensively studied shock mineral. particularly, planar deformation fe atures (PDFs), interpreted as resulting from relaxations at the shock front, are unambiguous shock indicators, for shock pressures approxima tely between 15 and 35 GPa. At higher pressures, the formation of high -pressure polymorphs of SiO2 in shocked quartz is also discussed. Shoc k effects in some other selected minerals, although less extensively s tudied, are also reviewed, with special emphasis on the discovery of d iamonds at impact sites and of all the high-pressure polymorphs of oli vines an pyroxenes, including silicate perovskite, in shocked meteorit es. Finally, the controversial links between large impacts and major e nvironmental effects are discussed in a fourth part. ((C) Academie des sciences/Elsevier, Paris).