Geologic Time

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Geologic Time Scale

Geologic time is measured in billions of years.  
The earth is about 4.6 billion years old. 
Geologic time is divided into eons, eras, epochs, and periods.  
There are two eons:  Phanerozoic Eon and the Precambrian Time or Eon.  

USGS Homehttp://geology.er.usgs.gov/paleo/geotime.shtml

The Geologic Time Scale

Sedimentary strata icon

For a more detailed version of the geologic time scale, see the University of California (Berkeley) Museum of Paleontology's excellent Web Geological Time Machine. For a review of the concept of geologic time and how it is measured, see the USGS publication "Geologic Time."

Phanerozoic Eon (544 million years ago to present)

Cenozoic Era (65 million years ago to present)

Quaternary Period (1.8 million years ago to present)
Holocene Epoch (8,000 years ago to present)
Pleistocene Epoch (1.8 million to 8,000 years ago)
Tertiary Period (65 to 1.8 million years ago)
Pliocene Epoch (5.3 to 1.8 million years ago)
Miocene Epoch (23.8 to 5.3 million years ago)
Oligocene Epoch (33.7 to 23.8 million years ago)
Eocene Epoch (55.5 to 33.7 million years ago)
Paleocene Epoch (65 to 55.5 million years ago)

Mesozoic Era (248 to 65 million years ago)

Cretaceous Period (145 to 65 million years ago)
Jurassic Period (213 to 145 million years ago)
Triassic Period (248 to 213 million years ago)

Paleozoic Era (544 to 248 million years ago)

Permian Period (286 to 248 million years ago)
Carboniferous Period (360 to 286 million years ago)
Pennsylvanian Period (325 to 286 million years ago)
Mississippian Period (360 to 325 million years ago)
Devonian Period (410 to 360 million years ago)
Silurian Period (440 to 410 million years ago)
Ordovician Period (505 to 440 million years ago)
Cambrian Period (544 to 505 million years ago)

Precambrian Time (4500 to 544 million years ago)

Proterozoic Era (2500 to 544 million years ago)

Vendian Period (544 to 650 million years ago)

Archaean Era (3800 to 2500 million years ago)

Hadean Time (4500 to 3800 million years ago)

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http://geology.er.usgs.gov/paleo/glossary.shtml#hadean

Hadean

The earliest subdivision of the Precambrian, spanning the time between the formation of the Earth, about 4.5 billion years ago, and the start of the Archaean era, 3.8 billion years ago. This interval predates the period of true geologic time since no rocks of this age are known on Earth, with the exception of a few meteorites

Archaean

The middle era of Precambrian time, spanning the period between 3.8 and 2.5 billion years ago. Life arose on Earth during the early Archaean, as indicated by the appearance of fossil bacteria in rocks thought to be about 3.5 billion years old. Its name means "ancient."

Proterozoic

The final era of the Precambrian, spanning the time between 2.5 billion and 544 million years ago. Fossils of both primitive single celled and more advanced multicellular organisms begin to appear in abundance in rocks from this era. Its name means "early life."

Paleozoic

An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, spanning the time between 544 and 248 million years ago. The word Paleozoic is from Greek and means "old life."

Mesozoic

An era of geologic time between the Paleozoic and the Cenozoic, spanning the time between 248 and 65 million years ago. The word Mesozoic is from Greek and means "middle life."

Cenozoic

An era of geologic time from the beginning of the Tertiary period (65 million years ago) to the present. Its name is from Greek and means "new life."

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USGS http://pubs.usgs.gov/gip/geotime/divisions.html

Major Divisions of Geologic Time

The major divisions, with brief explanations of each, are shown in the following scale of relative geologic time, which is arranged in chronological order with the oldest division at the bottom, the youngest at the top.

Diagram showing divisions of geologic time

See also:  http://www.ucmp.berkeley.edu/help/timeform.html