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1 Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA
2 School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Australia
No known rocks have survived from the first 500 m.y. of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high 
18O values of zircons. The range of 18O values is constant throughout the Archean (4.4–2.6 Ga), suggesting uniformity of processes and conditions. The hypothesis of a cool early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquid- water oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.
Key Words: Archean • zircon • oxygen • isotopes • meteorites • impacts
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