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Contemporary Meteoritics |
Honorary Associate, Western Australian Museum, Francis Street, Perth, Australia
, 44 Robert Franklin Way, South Cerney, Cirencester, Gloucestershire, UK joemccall{at}tiscali.co.uk
Robert Hooke in the 17th century was the first scientist to consider the possibility of meteorite impact cratering, when looking at the lunar craters. Gilbert in the late 19th century considered it again when studying ‘Coon Butte’ (now known as ‘Meteor Crater’), Arizona, but attributed it to cryptovolcanism. Barringer early in the 20th century attributed the same crater to meteorite impact, as did Shoemaker in 1960–1963, the latter drawing on the results of recent nuclear cratering experiments. This crater and Wolfe Creek Crater in Western Australia are nowadays taken as type examples of the largest (1 km scale) terrestrial craters associated with actual meteorite fragments. A number of smaller impact craters associated with fragments were recognized in the 1930s in Estonia, Arabia, Australia and the USA; and, in 1949, 100 were formed by a shower of iron meteorites in Sikhote-Alin, Siberia. The dawn of the Space Age in the late 1950s saw an extensive search for larger craters and structures, and, because the many craters and structures of more than 1 km diameter so revealed on land had no meteoritic material accompaniment, a number of high-pressure shock indicators were defined — shatter cones, lamellations in quartz, high-pressure polymorphs of quartz (coesite, stishovite), amorphous silica (lechatelierite), diamonds and fullerenes, and impactite breccias with melt glass (suevite, Bunte breccia). About 170 such structures are now recorded, and they include structures more than 100 km across. Tektites, glassy bodies with splash forms and, in some cases, ablation flanges, found in strewn fields up to thousands of kilometres from the source structures are associated with a handful of such structures, but such associations are not the norm. One or two such structures have been located under the sea, and the Pliocene Eltanin structure, not truly craterform and situated beneath the Southern Pacific Ocean, has mesosiderite meteorite specks in the breccias. Isotopic methods have in many other cases indicated a trace extraterrestrial component. The global distribution is extremely uneven, with large populations recorded in Canada and the USA, Fennoscandia and Australia, and extensive blank regions in mid-Africa, Asia and South America. Despite this anomaly, not really satisfactorily explained, it is unlikely that the attribution of these terrestrial craters and structures will be overturned, although some may have been misinterpreted. It is suggested that the attribution of craters and the Maria on the Moon and craters on other bodies of the solar system to impact rather than volcanic agencies is less firmly founded, although entrenched.
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