Hadean

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Hadean eon
Geologic timescale of the Precambrian
(millions of years ago)
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The Hadean refers to the geologic eon before the Archean. It extends back to the Earth's formation, and ended roughly 3800 million years ago (Ma), though the date varies according to different sources. The name "Hadean" derives from Hades, Greek for "unseen" or "Hell" and suggesting the underworld or referring to the conditions on Earth at the time. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks. W. B. Harland later coined an almost synonymous term: the "Priscoan period". Other older texts simply refer to the eon as the Pre-Archaean, while during much of the 19th and 20th centuries, the term "Azoic" (meaning without or before life) was commonly used.

Hadean rocks

In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and Western Australia. The oldest known rock formations (the Isua greenstone belt) comprise somewhat altered sediments from Greenland dated around 3800 Ma by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of Western Australia are much older. The oldest dated zircons date from about 4400 Ma - very close to the hypothesized time of the Earth's formation. The Greenland sediments include banded iron beds. They contain possibly organic carbon and quite possibly indicate that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.

The late heavy bombardment happened during Hadean times and affected the Earth and the Moon.

Subdivisions

Since few geological traces of this period remain on Earth there are no official subdivisions. However, several major divisions of the lunar geologic timescale occurred during the Hadean, and so these are sometimes used unofficially to refer to the same periods of time on Earth.

Precambrian
Hadean eon Archean eon Proterozoic eon Phanerozoic eon

Hadean eon
Cryptic Basin Groups Nectarian Lower Imbrian

References

Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison [Wgeology.wisc.edu] – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (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.
Wyche, S., D. R. Nelson and A. Riganti (2004) 4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton, Australian Journal of Earth Sciences Volume 51 [Zircon ages from W. Australia - Absract] Accessed Jan. 10, 2006

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