Comment on "Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga".

Harrison et al. (Reports, 23 December 2005, p. 1947) proposed that plate tectonics and granites existed 4.5 billion years ago (Ga), within 70 million years of Earth's formation, based on geochemistry of >4.0 Ga detrital zircons from Australia. We highlight the large uncertainties of this claim and make the more moderate proposal that some crust formed by 4.4 Ga and oceans formed by 4.2 Ga.