A Large Asteroid, a Major Extinction?

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Science  05 May 2000:
Vol. 288, Issue 5467, pp. 773
DOI: 10.1126/science.288.5467.773b

Asteroid impacts are now recognized as having shaped Earth's history. Early impacts likely formed the moon, modified the composition of Earth's crust and mantle, and influenced the formation of its atmosphere and life; more recently, impact of a 10-kilometer diameter asteroid at the end of the Cretaceous (65 million years ago) may have led to a mass extinction that altered the course of evolution. It has been difficult to identify large impacts on Earth—and thus to reconstruct this important history—because many of the diagnostic rocks are weathered and altered, and craters, such as that at Chicxulub, are rapidly buried.

Mory et al. suggest that a large impact structure (120 km in diameter) lies buried in Western Australia. The crater is covered by Lower Jurassic and younger sediments but retains a diagnostic geophysical signature. Drill cores into the structure returned heavily shocked minerals and rocks, and Permian fossils. Precise dating has not yet been achieved, but the age range between Permian and Lower Jurassic encompasses two major extinctions, and the size of this crater places it, after Chicxulub, as the second largest impact identified in the Phanerozoic.—BH

Earth Planet. Sci. Lett.177, 119 (2000).

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