Eon (geology)

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For the Gnostic word, see aeon. For other uses, see Eon (disambiguation).

In general usage, an eon (sometimes spelled aeon) is a period of time arbitrarily designated by humans. Geologists refer to an eon as the largest subdivision of time on the geologic time scale. For example, the Phanerozoic Eon, which is about 550 million years long, covers the period of time during which animals with hard shells that fossilize would have been abundant.

An eon is composed of several geologic eras, which in turn are composed of geologic periods, which are composed of geologic epochs. We are currently in the Phanerozoic Eon, the Cenozoic Era, the Neogene Period, and the Holocene epoch. Formerly, only one eon existed besides the Phanerozoic: the Precambrian. More recently, the Hadean, Archean, and Proterozoic "eras" of Precambrian time have been considered eons. The geologic timescale in terms of eons, eras, and periods looks like this:

ImageSize = width:800 height:100 PlotArea = left:65 right:15 bottom:20 top:5 AlignBars = justify

Colors =

id:neogene   value:rgb(0.99215,0.8,0.54)
id:paleogene value:rgb(1,0.7019,0)
id:cretaceous   value:rgb(0.5,0.764,0.1098)
id:jurassic      value:rgb(0.302,0.706,0.5)
id:triassic    value:rgb(0.403,0.765,0.716)
id:permian   value:rgb(0.404,0.776,0.867)
id:carboniferous     value:rgb(0.6,0.741,0.855)
id:devonian  value:rgb(0.6,0.6,0.788)
id:silurian  value:rgb(0.694,0.447,0.714)
id:ordovician      value:rgb(0.976,0.506,0.651)
id:cambrian  value:rgb(0.984,0.5,0.373)
id:neoproterozoic    value:rgb(0.792,0.647,0.583)
id:mesoproterozoic    value:rgb(0.867,0.761,0.533)
id:paleoproterozoic    value:rgb(0.702,0.698,0.369)
id:eoarchean    value:rgb(0.5,0.565,0.565)
id:paleoarchean    value:rgb(0.6,0.592,0.569)
id:mesoarchean    value:rgb(0.698,0.65,0.6)
id:neoarchean    value:rgb(0.796,0.804,0.784)
id:ediacaran     value:rgb(0.918,0.847,0.737)
id:cryogenian    value:rgb(0.863,0.671,0.667)
id:tonian        value:rgb(0.796,0.643,0.424)
id:stratherian   value:rgb(1,1,0.8)   # light yellow
id:calymmian     value:rgb(1,1,0.8)   # light yellow
id:orosirian     value:rgb(1,1,0.8)   # light yellow
id:rhyacian      value:rgb(1,1,0.8)   # light yellow
id:siderian     value:rgb(1,1,0.8)   # light yellow
id:ectasian      value:rgb(1,1,0.8)   # light yellow
id:stenian      value:rgb(1,1,0.8)   # light yellow
id:cenozoic   value:rgb(1,1,0)
id:mesozoic   value:rgb(0.5,0.6784,0.3176)
id:paleozoic  value:rgb(0.5,0.7098,0.835)
id:phanerozoic value:rgb(0.7019,0.886,0.819)
id:proterozoic value:rgb(0.8,0.85,0.568)
id:archean   value:rgb(0.6,0.6784,0.6745)
id:hadean value:rgb(0.4,0.4,0.4)
id:black  value:black
id:white  value:white

Period = from:-4567.17 till:0 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-4500 ScaleMinor = unit:year increment:100 start:-4500

Define $markred = text:"*" textcolor:red shift:(0,3) fontsize:10

PlotData=

align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5)

bar:eon

at:      0   align:right  $markred
at:   -542   align:left   $markred shift:(2,3)
from: -542   till:    0   text:Phanerozoic  color:phanerozoic
from:-2500   till: -542   text:Proterozoic  color:proterozoic
from:-3800   till: -2500  text:Archean      color:archean
from: start  till: -3800  text:Hadean       color:hadean

bar:era

from:  -65.5 till:    0   text:C~z shift:(0,1.5)        color:cenozoic
from: -251   till:  -65.5 text:Meso~zoic shift:(0,1.5)  color:mesozoic
from: -542   till: -251 text:Paleo~zoic shift:(0,1.5)  color:paleozoic
from: -1000  till:  -542  text:Neoprote-~rozoic shift:(0,1.8) color:neoproterozoic
from:-1600   till:  -1000  text:Mesoproterozoic color:mesoproterozoic
from:-2500   till: -1600  text:Paleoproterozoic color:paleoproterozoic
from:-2800   till: -2500  text:Neo-~archean shift:(0,1.5)     color:neoarchean
from:-3200   till: -2800  text:Meso-~archean shift:(0,1.5)   color:mesoarchean
from:-3600   till: -3200  text:Paleo-~archean shift:(0,1.5) color:paleoarchean
from:-3800   till: -3600  text:Eoar-~chean shift:(0,0.5) color:eoarchean fontsize:6
from:start   till: -3800  color:white

bar:period

fontsize:6
from:   -23.03 till:    0    color:neogene
from:  -65.5 till:   -23.03  color:paleogene
from: -145.5   till:  -65.5  color:cretaceous
from: -199.6   till: -145.5  color:jurassic
from: -251   till: -199.6    color:triassic
from: -299   till: -251      color:permian
from: -359.2   till: -299    color:carboniferous
from: -416 till: -359.2      color:devonian
from: -443.7 till: -416      color:silurian
from: -488.3   till: -443.7  color:ordovician
from: -542   till: -488.3    color:cambrian

from: -630   till:  -542  text:Ed. color:ediacaran
from: -850   till:  -630  text:Cryo-~genian color:cryogenian shift:(0,0.5)
from: -1000  till:  -850  text:Ton-~ian color:tonian shift:(0,0.5)
from: -1200  till:  -1000 text:Ste-~nian color:mesoproterozoic shift:(0,0.5)
from: -1400  till:  -1200 text:Ect-~asian color:mesoproterozoic shift:(0,0.5)
from: -1600  till:  -1400 text:Calym-~mian color:mesoproterozoic shift:(0,0.5)
from: -1800  till:  -1600 text:Stath-~erian color:paleoproterozoic shift:(0,0.5)
from: -2050  till:  -1800 text:Oro-~sirian color:paleoproterozoic shift:(0,0.5)
from: -2300  till:  -2050 text:Rhy-~acian color:paleoproterozoic shift:(0,0.5)
from: -2500  till:  -2300 text:Sid-~erian color:paleoproterozoic shift:(0,0.5)
from: start  till:  -2500 color:white

The term eon has sometimes been used informally as a synonym for millennium.

References

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