The geologic time scale is how scientists divide up Earth’s history into various time periods. It divides Earth’s past into ever-smaller units: eons, eras, periods, epochs, and ages.
Evidence for these divides comes from rocks, fossils, important biological changes, geological events, and numerical ages based on techniques such as radiometric dating.
It is easiest to think of geologic time as a giant history book where the eons are the largest chapters. Those chapters are divided into eras, periods, epochs, and ages, which are progressively smaller divisions of the chapters.
Hierarchy of Geologic Time
The normal order of size is:
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Eon
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Era (band)
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Epoch Period
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Age (years)
For example, we live today in the Phanerozoic Eon, Cenozoic Era, Quaternary Period and Holocene Epoch.
This hierarchy enables scientists throughout the world to communicate in a common scientific language when major events in the history of the Earth occurred.
Eons Major
Earth’s history is divided into four major eons:
Hadean Eon
This covers the first period of Earth’s history, beginning with the birth of the planet about 4.6 billion years ago.
Archean Era
In the Archean, the crust of the Earth was formed, along with the oceans and early living forms.
Proterozoic Aeon
During this long period of time, great changes occurred in the atmosphere of the Earth and the development of more and more complicated life.
Phanerozoic Era
The Phanerozoic begins about 541 million years ago and extends to the present day and provides an abundant record of complex animal and plant life.
Fossils become especially significant in ordering the later periods of geologic time.
Major Periods
The Phanerozoic Eon is subdivided into three key eras:
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Paleozoic Era: Time of great marine life diversification and evolution of life on land.
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Mesozoic Era: Often connected with dinosaurs and the rise of early mammals and birds.
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Cenozoic Era: The era that started after the great extinction about 66 million years ago and that covers the present day and the substantial diversification of mammals.
A key observation is that these borders are not just equal blocks of time. They often correspond to big changes seen in the geological and fossil record of Earth.
How Geologists Divide Up Time
Scientists combine several types of evidence.
Relative dating looks at rock layers and fossils to determine whether geological events happened before or after other events. Fossils have been instrumental in the development of the relative geologic time scale.
The radioactive decay of isotopes in appropriate minerals and rocks offers numerical ages by radiometric dating. This technique allows scientists to date the geologic record in millions and billions of years.
Personally, I find this mixture crucial because scientists are not estimating Earth's past from one form of data. The timeline is developed and refined using a combination of geological and physical techniques.
Significance
Scientists classify Earth’s past to understand:
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Life: Evolution and Extinction
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Climate change & ocean
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Formation and migration of the continents
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Great Geological Events
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Correlation of fossils with strata
It essentially gives scientists a common timetable for piecing together how Earth became the globe we know today.
FAQs (Frequently Asked Questions)
1. What is the biggest subdivision of geologic time?
In the geologic time hierarchy, the greatest generally used formal division is an eon.
2. What is the eon we are in?
We are now in the Phanerozoic eon.
3. What is the period we are living in?
We are in the Cenozoic era.
4. How do scientists date ancient rocks?
Radiometric dating is a technique used by scientists which analyzes radioactive isotopes and their decay products in appropriate minerals.
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