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Geologic Time

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Understanding time permits assigning an age to…

  • Rocks.
  • Fossils.
  • Geologic structures.
  • Landscapes.
  • Tectonic events.

The Geologic Time Scale

Planet Earth is over 4.5 billion years old - that's a loooong time! Just as we currently divide time up by years, months, and days, Geologists divide up those 4.5 billion years into segments. The start/end point for each is usually based upon the appearance or dissappearance of a species.

Eons are the longest division of geologic time. There are four: Hadean, Archean, Proterozoic, and Phanerozoic

  • Hadean: “Hell” (4.6 - 3.8 Billion years ago). Internal differentiation of our planet, the formation of the oceans, and secondary atmosphere
  • Archean: “Ancient” (3.8 - 2.5 Billion years ago). Birth of continents and the appearance of the earliest life forms.
  • Proterozoic: “Before life” (2.5 - 0.542 Billion years ago). Development of tectonic plates; buildup of atm O2; first appearance of multicellular life
  • Phanerozoic: Paleozoic, Mesozoic & Cenozoic Eras “Visible life” (542 Ma - present). Marks the 1st appearance of hard shells. Life diversified rapidly afterward.

Eons are subdivided into Eras.

  • Paleozoic – “Ancient life.” 542 to 251 Million years ago . Life diversified rapidly.
  • Mesozoic – “Middle life.” 251 to 65.5 Million years ago . The “Age of Dinosaurs.”
  • Cenozoic – “Recent life.” 65.5 Million years ago to present. The “Age of Mammals.”

Eras are divided into shorter units called periods, and periods are divided into epochs. The times for eons and eras are the same worldwide, but periods and epochs can be different. As we get closer to the present time, the more information we have and regional differences are more apparent.

So, Where Are We Today?















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CSULA Department of Geosciences and the Environment
Pasadena City College Department of Geology    
    © Sonjia Leyva 2018