Eon (geology)

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In general usage, an eon (sometimes spelled æon) is an arbitrarily designated period of time. 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 Quaternary 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:

<timeline> 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

</timeline>

Greek root

The English word eon is derived from the Koine Greek word aion.

Although a proposal was made in 1957 to define an aeon to be a unit of time equal to one billion years (1 Ga), the idea was not approved as a unit of scientific measure and is seldom used for a specific period of time. Its more common usage is for any lengthy or indefinite period of time. The origin is from the Greek root "aion" for "age" or "life force." A similar Latin word "aevum" for age is still present in words such as longevity and medieval. [1]


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See also

References

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