Hadean
first eon of the geological timescale, beginning with the formation of the Earth about 4.567 billion years ago
The Hadean ( hay-DEE-ən, HAY-dee-ən) is the first and oldest of the four geologic eons of Earth's history, starting with the planet's formation about 4.6 Ga (estimated 4567.30 ± 0.16 Ma set by the age of the oldest solid material in the Solar System—protoplanetary disk dust particles—found as chondrules and calcium–aluminium-rich inclusions in some meteorites about 4.567 Ga), and ending 4.031 Ga, the age of the oldest known intact rock formations on Earth as recognized by the International Commission on Stratigraphy. The interplanetary collision that created the Moon occurred early in this eon. The Hadean eon was succeeded by the Archean eon, with the Late Heavy Bombardment hypothesized to have occurred at the Hadean-Archean boundary.
Hadean rocks are very rare, largely consisting of granular zircons from one locality (Jack Hills) in Western Australia. Hadean geophysical models remain controversial among geologists: plate tectonics and the growth of cratons into continents may have started in the Hadean, but there is still uncertainty.
Earth in the early Hadean had a very thick hydride-rich atmosphere whose composition likely resembled the solar nebula and the gas giants, with mostly water vapor, methane and ammonia. As the Earth's surface cooled, vaporized atmospheric water condensed into liquid water and eventually a superocean covering nearly all of the planet was formed, turning Earth into an ocean planet. Volcanic outgassing and asteroid bombardments eventually altered the Hadean atmosphere into the nitrogen- and carbon dioxide-rich, weakly reducing Paleoarchean atmosphere.
Etymology
The eon's name "Hadean" comes from Hades, the Greek god of the underworld, referring to the hellish conditions then prevailing on early Earth: the planet had just been formed from recent accretion, and its surface is thought to have been molten lava.
The term was coined by American geologist Preston Cloud, originally to label the period before the earliest known rocks on Earth.
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This entry incorporates text from “Hadean” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.