Geology

Not of a Certain Age

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Science  28 Aug 2009:
Vol. 325, Issue 5944, pp. 1048
DOI: 10.1126/science.325_1048a
CREDIT: JUPITERIMAGES

At present, there are 174 characterized impact structures on Earth, ranging from the largest—the 2-billion-year-old Vredefort that stretches 300 km across South Africa—to about 20 or so structures smaller than the 1-km-diameter Barringer crater in Arizona; most of these are less than 1 million years old. Although these structures represent only the preserved fraction of the overall series of asteroid impacts on Earth, their age-size relationships might aid in assessing the impact flux or the relation of impacts to events such as extinctions (for example, that marking the Cretaceous-Paleogene boundary). As pointed out by Jourdan et al., however, many of the craters are in fact poorly dated. About half have only approximate ages based on stratigraphy. Moreover, although 25 of the craters have ages reported with uncertainties of less than 2%, many of these reports, on investigation, contain generous or misleading estimates; in the authors' assessment, only 11 of the 25 craters have been assigned accurate ages. Thus, it is difficult to infer much from the current crater data about periodicities or changes in impacts through time.

Earth Planet. Sci. Lett. 10.1016/j.epsl.2009.07.009 (2009).

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