Hadean
The Hadean ( hay-DEE-ən, HAY-dee-ən) is the first and oldest of the four known geologic eons of Earth's history, starting with the planet's formation about 4.6 billion years ago (now defined as 4567.30 ± 0.16 million years ago set by the age of the oldest solid material in the Solar System found in some meteorites about 4.567 billion years old), and ended 4.031 billion years ago. 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 further altered the Hadean atmosphere eventually into the nitrogen- and carbon dioxide-rich, weakly reducing Paleoarchean atmosphere.
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