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Earth’s Geologic Time Scale: A Window into Deep Time

The rocks beneath our feet hold the story of Earth’s 4.6 billion-year history. Scientists have pieced together this timeline using the geologic time scale, which divides Earth's past into eons, eras, periods, and epochs. Each division marks significant changes in the planet’s climate, landscape, and life forms.

One of the best resources for understanding geologic time is the United States Geological Survey (USGS), which provides a Major Chronostratigraphic and Geochronologic Units Chart based on the latest data from the International Commission on Stratigraphy (ICS). Below, you’ll find the chart that outlines the vast history of our planet:


Breaking Down the Geologic Time Scale

Earth’s history is divided into different time units:

  • Eons – The longest periods (e.g., the Phanerozoic Eon, which began 541 million years ago and continues today).
  • Eras – Broad divisions marked by significant changes in life and climate (e.g., the Mesozoic Era, the age of dinosaurs).
  • Periods – Smaller subdivisions highlighting major evolutionary and environmental shifts (e.g., the Jurassic Period, known for its famous dinosaurs).
  • Epochs – The most detailed breakdown, defining finer-scale changes in Earth's history (e.g., the Holocene Epoch, which began about 11,700 years ago and includes all of human civilization).

    • Earth’s Five Biggest Mass Extinctions

    Throughout Earth's history, life has been shaped by mass extinctions, catastrophic events that wiped out vast numbers of species in relatively short periods. Here are the five biggest extinction events:

    1. Ordovician-Silurian Extinction (443 million years ago)
      • Cause: A severe ice age followed by rapid global warming.
      • Impact: 85% of marine species vanished.
    2. Late Devonian Extinction (375 million years ago)
      • Cause: Oxygen depletion in oceans, possibly triggered by the expansion of land plants altering CO2 levels.
      • Impact: 75% of species disappeared.
    3. Permian-Triassic Extinction (252 million years ago) (The “Great Dying”)
      • Cause: Massive volcanic eruptions in Siberia released greenhouse gases, causing extreme climate change.
      • Impact: 96% of marine species and 70% of land species were wiped out.
    4. Triassic-Jurassic Extinction (201 million years ago)
      • Cause: Increased volcanic activity and climate shifts.
      • Impact: Cleared the way for dinosaurs to dominate the planet.
    5. Cretaceous-Paleogene Extinction (66 million years ago)
      • Cause: A massive asteroid impact in present-day Mexico.
      • Impact: Ended the age of dinosaurs, allowing mammals to rise.

    Many scientists argue that we are now witnessing the sixth mass extinction, driven by habitat destruction, climate change, and human activity.

    Why Does the Geologic Time Scale Matter Today?

    Studying Earth's past helps us understand present-day environmental challenges. The patterns of past climate shifts, ocean acidification, and biodiversity loss offer valuable insights into what could happen in the future.

    Beyond science, the geologic time scale is essential for industries like oil exploration, mining, and conservation, helping experts locate resources and understand Earth's natural processes.

    Want to Learn More? Check Out These Resources

    If you’re fascinated by Earth’s history, here are some great resources available on Amazon to dive deeper:

    (As an Amazon Associate, I earn from qualifying purchases.)

    What do you think about Earth’s incredible history? Which time period fascinates you the most? Let’s chat in the comments below!

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