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Geologic Time

Time is a funny thing. And geologists have a lot of it to deal with [less than physicists, but I won't go into that right now], so they do what most people would do with a huge chunk of something: chop it up into smaller chunks. And this is how they do it, with eons, eras, periods [also sometimes called "systems"], and epochs. There are two eons, the Precambrian, ending 544 million years ago and beginning with earth, and so little happened during this huge span of time [somewhere on the order of 4 billion years] that it's only really subdivided three times, into "chronometric eons:" the Priscoan being the oldest, going up to 4 billion years ago, the Archean stretches from then to 2.5 billion years ago, and the Proterozoic, filling out the Precambrian eon, stretching from 2.5 billion years ago to 544 million [or just over .5 billion, for the mathematically challenged] years ago.

The Phanerozoic eon is where it gets interesting: 544 million years ago to the present.

EraPeriodEpoch
CenonzoicQuaternaryHolocene
Pleistocene
TertiaryPliocene
Miocene
Oligocene
Eocene
Paleocene
MesozoicCretaceous
Jurassic
Triassic
PaleozoicPermian
Carboniferous
            Pennsylvanian
            Mississippian
Devonian
Silurian
Ordovician
Cambrian

References:
Cascades Volcano Observatory's "Geologic Time Scale"
Andrew MacRae's "Geological Time Scale"
UC-Berkeley's Museum of Paleontology's Geological Time Machine
Bill Bryson's A Short History of Nearly Everything, pages 72-74