Meteorite impacts on Earth and on the Earth sciences
Blenkinsop, Tom G.
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Plate tectonics has become established during the last thirty years as a coherent explanation for the major features of the Earth today. The formation of new crust at mid-ocean ridges, the movement of relatively rigid lithospheric plates over the softer underlying asthenosphere, and the collision of plates throwing up the Earth's major mountain ranges, are familiar concepts to many people. However, recent discoveries in the Earth sciences challenge us to a newer concept of the Earth’s evolution. These discoveries indicate that large meteorite impacts have had enormous effects on the Earth, perhaps most dramatically illustrated 65 million years ago, when an impact at Chicxulub in the Caribbean may have been responsible for mass extinctions (including the dinosaurs) on a global scale. The evidence for these catastrophic events is surprisingly enigmatic in the geological record, and has been the subject of intense scientific debate and disagreement. Some of the major types of direct evidence for meteorite impacts include the craters formed by impact, and the effects of impact on rocks, including melting, the formation of shatter cones, and a whole variety of small-scale features that are revealed under the microscope. Perhaps paradoxically, these microstructures are the most unambiguous evidence for meteorite impacts, because they formed under radically greater stresses, temperatures and strain rates than those of plate tectonic processes, and are therefore quite distinct from plate tectonic microstructures. These features of meteorite impacts on Earth are illustrated by examples from several known impact sites, including the Vredefort structure in South Africa and a possible impact structure at Highbury south of Mhangura in Zimbabwe. Large meteorite impacts on Earth have significantly augmented Earth’s mineral resources since the formation of the Earth, by creating diamonds in the ultra-high pressure conditions of impact, by providing structures that trap petroleum, and possibly by creating base metal deposits. The likely environmental consequences of impacts include a searing heat wave followed by global cooling, causing episodes of mass extinction that may occur in cycles. There is some controversial evidence for the theory that the first life on Earth itself may have been transported here on meteorites from Mars. The possibility of a major meteorite impact on Earth in the near future emphasizes the dramatic nature of these recent discoveries, which are having deep impacts in the Earth sciences, possibly even constituting a scientific revolution.