Geologic Time Scale: Understanding Earth's History

Introduction

• Focus on understanding Earth's history through the geologic time scale.
• Discusses methods for determining the age of rocks and fossils.
• Geologic time scale: Timeline of Earth's history based on geologic events and life forms.

Structure of the Geologic Time Scale

• Four Major Divisions:

  • Eon
  • Era
  • Period
  • Epoch

• Basis: Stratigraphy (correlation and classification of rock strata).

Eons of the Geologic Time Scale

• Precambrian Eon:

  • Hadean Eon (4.6 to 4 billion years ago):

    • Formation of solar system.
    • Earth cooled; solid crust formed.
    • Creation of oceans.

  • Archaean Eon (4 to 2.5 billion years ago):

    • Origin of life in oceans.
    • Microfossils indicate life began around 3.5 billion years ago.
    • Ocean floor covered in bacterial life.

  • Proterozoic Eon (2.5 billion years ago):

    • Start of recognizable geologic processes (plate tectonics).
    • Presence of primitive life forms.

• Phanerozoic Eon (541 million years ago to present):

  • Paleozoic Era (542 to 251 million years ago):

    • Supercontinent breakup and new supercontinent formation.
    • Life explosion (Cambrian period).
    • Major periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian.

  • Mesozoic Era (252 to 66 million years ago):

    • Known as the "Age of Reptiles" or "Age of Dinosaurs".
    • Rapid life diversification.
    • Major periods: Triassic, Jurassic, Cretaceous.

  • Cenozoic Era (65 million years ago to present):

    • Known as the "Age of Mammals".
    • Post-dinosaur extinction, smaller species thrived.
    • Major periods: Paleogene, Neogene, Quaternary.

Notable Events in Geologic Time

• Cambrian Explosion: Emergence of arthropods and chordates.
• Silurian Period: First evidence of terrestrial life.
• Carboniferous Period: Evolution of tetrapods; referred to as the "Age of Cockroaches."
• Permian Mass Extinction: Largest mass extinction event.
• Mesozoic Era: Rise of dinosaurs and first birds.
• Cenozoic Era: Emergence of mammals and early humans.

Conclusion

• The geologic time scale provides a comprehensive overview of Earth's history.
• It helps us understand the evolution of life and major geologic events.