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Concept
Hadean
Summary
The Hadean (heɪˈdiːən,_ˈheɪdiən ) is the first and oldest of the four known geologic eons of Earth's history. It started with the planet's formation about 4.54 Bya, now defined as (4567.30 ± 0.16) Mya set by the age of the oldest solid material in the Solar System found in some meteorites about 4.567 billion years old. The proposed interplanetary collision that created the Moon occurred early in this eon, and the Late Heavy Bombardment is hypothesized to have occurred at the end of the eon. The Hadean ended 4 billion years ago, as defined by the International Commission on Stratigraphy (ICS), and was succeeded by the Archean eon.
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: it appears that plate tectonics and the growth of continents may have started in the Hadean. Earth in the early Hadean had a very thick carbon dioxide- and methane-rich atmosphere, but eventually oceans made of liquid water formed.
"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.
The term was coined by American geologist Preston Cloud, after the Greek mythical underworld Hades, 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.
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.
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