MPE - Geologic Time Scale Lesson

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Geologic Time Scale

Geologic time scale chartOver the past 150 years scientists have studied stratigraphic columns of rocks from around the world and have correlated rock units and broken them into time units. The result of these studies led to the creation of the Geologic Time Scale, which breaks down relative geologic time into known units of absolute age. While the relative age and events have been known for quite some time, the numeric ages in the right hand column were not known until recently.

When discussing the history of the Earth, it does no good to think in terms of days, months, years, or even centuries, so geologist have come up with divisions of time to describe the major time periods of history on Earth.

The Eon is the largest unit of time in the geologic time scale and usually refers to billions of years (It is interesting to note that some scientists have started the category Super Eon which includes 2 or more eons). There are four major Eons. From oldest to youngest include:

  • Precambrian - Haden or Pre-Archean
  • Archean - Ancient Life
  • Precambrian - Proterozoic Early Life
  • Phanerozoic - Visible Life

Eons are further divided into eras. An era is a major division of time that subdivides an eon and has a long and distinct period of history with a particular feature or characteristic.   The eras shown on the chart to the side from the Phanerozoic Eon include the following Eras from oldest to youngest:

  • Paleozoic (means ancient life)
  • Mesozoic (means middle life, also called the age of dinosaurs)
  • Cenozoic (means recent life, also called the age of mammals)

Eras are further divided into periods. A period is a major division of geological time that is a subdivision of an era and is itself subdivided into epochs. Periods are generally named for specific localities. Defined periods of the Phanerozoic eon include the following from oldest to youngest:

The Paleozoic Era:

  • Cambrian
  • Ordovician
  • Silurian
  • Devonian
  • Carboniferous (in the U.S. this is further divided into Mississippian and Pennsylvanian)
  • Permian

The Mesozoic Era:

  • Triassic
  • Jurassic
  • Cretaceous

The Cenozoic Era:

  • Tertiary
  • Quaternary

Periods are further subdivided into epochs, and epochs into ages.

Complete the Which is Older? activity below:

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IMAGES CREATED BY GAVS OR OPENSOURCE