Cowabunga! Yarrabubba

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Science  12 Sep 2003:
Vol. 301, Issue 5639, pp. 1445
DOI: 10.1126/science.301.5639.1445d

When surfers find a good wave to ride, they often shout out “Cowabunga!” to express their gratitude to Mother Nature. Now Macdonald et al. are expressing similar excitement about the discovery of Yarrabubba, an atypical impact crater in the Yilgarn Craton of Western Australia. The impact event is recognized in a potassium-rich granite outcrop, a rare rock type in the Yilgarn. The K-rich minerals may have formed by impact melt remobilization. The granitic rock is fractured and contains pseudotachylites (melted fault zones) and shocked quartz, all signatures of impact. Some of the granite is brecciated and contains shatter cones, related to the shock wave front interacting with inhomogeneities in the rock fabric. The Yarrabubba crater is at least 2 billion years old and may represent the oldest terrestrial crater known. The lack of any obvious crater topography, the amount of time available to erode the impact structure, and the rock fabric and mineralogy suggest that this is a unique window into the lower part of an impact crater. — LR

Earth Planet. Sci. Lett. 213, 235 (2003).

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