Jack Hills
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The Jack Hills are located in the Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, and comprise an 80 km long northeast-trending belt of folded and metamorphosed supracrustal rocks.
Sedimentary siliciclastic rocks, interpreted as alluvial fan-delta deposits, are the major lithology. Minor mafic/ultramafic rocks and banded iron formation (BIF) are also found in the sequence. The overall sequence is generally considered to be a granulite gneiss, which has undergone multiple deformations and multiple metamorphic episodes. The protolith age of the Narryer Gneiss Terrane is variable, but generally considered to be in excess of 3.6 Ga (billion years).
Detrital zircons with ages >4000 Ma have been found in these rocks and a 4,404 +/- 8 Myr zircon was found at Eranondoo Hill (Wilde et al., 2001), the oldest dated material on Earth; the date is in the Cryptic era of the Hadean eon. They were found within part of the 3.6-3.8Ga supracrustal sequence. These zircons are considered most likely to have been placed into these rocks by erosion of older material.
The importance of this interpretation is that, in order for the rocks of the Jack Hill to contain detrital zircons, the Earth must have firstly been cool enough to support liquid water on the surface, if not a water ocean; that there must have been some kind of temporary crust, most likely very thin, on the surface of the Earth, and not a magma ocean as postulated for the earliest phase of the Earth's history.
[edit] References
- Wilde SA, Valley JW, Peck WH and Graham CM (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, v. 409, pp. 175-178.
- Zircons are Forever
- Western Australia's Jack Hills. NASA Earth Observatory newsroom. Retrieved on 2006-04-28.
- Lindsey, Rebecca (March 1, 2006). Ancient Crystals Suggest Earlier Ocean. NASA Earth Observatory. Retrieved on 2006-04-28.
