Hadean

Hadean eon
Geologic timescale of the Precambrian
(millions of years ago)

The Hadean is the geologic eon before the Archean. It extends back to the Earth's formation, and ended roughly 3.8 billion years ago (3800 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 3.8 billion years ago 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.

Atmosphere and oceans

A sizeable quantity of water would have been in the material which formed the Earth. Water molecules would have escaped Earth's gravity until a radius of about 40% of the current size, and water (and other volatile elements) would have been retained after that point. Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser noble gases in the modern atmosphere suggests that something disastrous happened to the early atmosphere.

Part of the young planet is theorized to have been disrupted by the impact which created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy carbon dioxide atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO₂ atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO₂ from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.

Study of zircons have found that liquid water must have existed as long ago as 4400 Ma, very soon after the formation of the Earth. This requires the presence of an atmosphere.

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)	Archean	Proterozoic	Phanerozoic

Hadean eon
Cryptic	Basin Groups	Nectarian	Lower Imbrian

See also

• History of Earth - The first sections describe the formation of the earth
• Solar nebula - For details of the pre-Hadean development of the solar system
• Formation and evolution of the Solar System

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