The Hadean eon was the earliest time period in Earth's history, spanning from 4.54 billion years ago to 4 billion years ago. It was during this time that the Earth first formed and the hypothesized planetary collision that created the Moon is believed to have occurred. Hadean rocks are extremely rare, consisting mainly of zircon grains found in Western Australia, and models of Hadean geophysics remain controversial among geologists. While the early Hadean Earth had a thick atmosphere rich in carbon dioxide and methane, oceans of liquid water eventually formed during this eon.

1. TheHadean(IPA:/heɪˈdiːən,ˈheɪdiən/hay-DEE-
ən,HAY-dee-ən)[discuss] isthefirstandoldestofthe
fourknowngeologiceonsofEarth'shistory.It
startedwiththeplanet'sformationabout
4.54Bya,[2][3] nowdefinedas(4567.30±
0.16)Mya[1] setbytheageoftheoldestsolid
materialintheSolarSystemfoundin
somemeteoritesabout4.567billionyears
old.[4] Theproposedinterplanetarycollisionthat
createdtheMoonoccurredearlyinthiseon,and
theLateHeavyBombardmentishypothesizedto
haveoccurredattheendoftheeon.TheHadean
ended4billionyearsago,asdefinedby
theInternationalCommissionon
Stratigraphy(ICS),[5] andwassucceededby
theArcheaneon.
Hadeanrocksareveryrare,largelyconsisting
ofgranularzirconsfromonelocality(JackHills)
inWestern
Australia.[6] Hadeangeophysicalmodelsremain
controversialamonggeologists:itappears
thatplatetectonicsandthegrowth
ofcontinentsmayhavestartedinthe
Hadean.[6] EarthintheearlyHadeanhadavery
thickcarbondioxide-andmethane-rich
atmosphere,buteventuallyoceansmadeof
liquidwaterformed.