Hadean

The Hadean ( /ˈheɪdiən/) is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. As of 2016, the ICS describes its status as 'informal'. Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the 'Priscoan period', from priscus, the Latin word for 'ancient'. Other, older texts refer to the eon as the Pre-Archean. The Hadean ( /ˈheɪdiən/) is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. As of 2016, the ICS describes its status as 'informal'. Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the 'Priscoan period', from priscus, the Latin word for 'ancient'. Other, older texts refer to the eon as the Pre-Archean. 'Hadean' (from Hades, the Greek god of the underworld, and the underworld itself) describes the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies. Since few geological traces of this eon remain on Earth, there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean, so these are sometimes used in an informal sense to refer to the same periods of time on Earth. The Lunar divisions are: In 2010, an alternative scale was proposed that includes the addition of the Chaotian and Prenephelean Eons preceding the Hadean, and divides the Hadean into three eras with two periods each. The Paleohadean era consists of the Hephaestean (4.5–4.4 Ga) and the Jacobian periods (4.4–4.3 Ga). The Mesohadean is divided into the Canadian (4.3–4.2 Ga) and the Procrustean periods (4.2–4.1 Ga). The Neohadean is divided into the Acastan (4.1–4.0 Ga) and the Promethean periods (4.0–3.9 Ga). As of February 2017, this has not been adopted by the IUGS. In the last decades of the 20th century geologists identified a few Hadean rocks from western Greenland, northwestern Canada, and Western Australia. In 2015, traces of carbon minerals interpreted as 'remains of biotic life' were found in 4.1-billion-year-old rocks in Western Australia. The oldest dated zircon crystals, enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the Narryer Gneiss Terrane of Western Australia, date to 4.404 ± 0.008 Ga. This zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga—around 200 million years after the hypothesized time of the Earth's formation. In many other areas, xenocryst (or relict) Hadean zircons enclosed in older rocks indicate that younger rocks have formed on older terranes and have incorporated some of the older material. One example occurs in the Guiana shield from the Iwokrama Formation of southern Guyana where zircon cores have been dated at 4.22 Ga. A sizable quantity of water would have been in the material that formed the Earth. Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to continually escape (even to the present day) due to atmospheric escape.