Geologic Time Scale Periods

The Geologic Time Scale breaks down Earth's 4.6 billion-year history into distinct periods, showcasing major geological and biological changes. Understanding these periods helps us grasp how life evolved and how Earth's landscapes transformed over time, connecting to physical geology.

1. Precambrian

   • Covers approximately 88% of Earth's history, from the formation of the Earth (about 4.6 billion years ago) to around 541 million years ago.
   • Characterized by the formation of the Earth's crust, oceans, and atmosphere, as well as the emergence of simple life forms like bacteria and algae.
   • Divided into three eons: Hadean, Archean, and Proterozoic, with significant geological and biological developments occurring in each.

2. Cambrian

   • Began around 541 million years ago and is known for the "Cambrian Explosion," a rapid diversification of life forms.
   • Marked the first appearance of many major groups of animals, including trilobites and early vertebrates.
   • Significant geological changes included the formation of shallow seas and the development of complex ecosystems.

3. Ordovician

   • Lasted from about 485 to 444 million years ago, characterized by a warm climate and high sea levels.
   • Notable for the diversification of marine life, including the first coral reefs and the appearance of the first land plants.
   • Ended with a major extinction event, which wiped out approximately 85% of marine species.

4. Silurian

   • Spanning from 444 to 419 million years ago, this period saw the stabilization of the Earth's climate and the recovery of life after the Ordovician extinction.
   • Marked by the evolution of jawed fish and the first vascular plants colonizing land.
   • Development of complex ecosystems, including the first terrestrial arthropods.

5. Devonian

   • Known as the "Age of Fishes," lasting from 419 to 359 million years ago, with significant diversification of fish species.
   • The first amphibians emerged, marking the transition from water to land.
   • Characterized by the development of extensive forests and the first seed plants.

6. Carboniferous

   • Lasted from about 359 to 299 million years ago, known for vast swampy forests that contributed to extensive coal deposits.
   • Significant evolution of amphibians and the appearance of the first reptiles.
   • Divided into two sub-periods: Mississippian (early) and Pennsylvanian (late), each with distinct geological and biological characteristics.

7. Permian

   • Spanning from 299 to 252 million years ago, this period ended with the largest mass extinction in Earth's history, the Permian-Triassic extinction.
   • Characterized by the formation of the supercontinent Pangaea and diverse terrestrial ecosystems.
   • Saw the dominance of reptiles and the first true mammals.

8. Triassic

   • Lasted from 252 to 201 million years ago, marked by the recovery of life after the Permian extinction.
   • The first dinosaurs and mammals appeared during this period.
   • Characterized by a warm climate and the formation of new ocean basins.

9. Jurassic

   • Spanning from 201 to 145 million years ago, known for the dominance of dinosaurs and the first birds.
   • Significant geological activity, including the breakup of Pangaea and the formation of new coastlines.
   • Development of lush vegetation and diverse ecosystems.

10. Cretaceous

    • Lasted from 145 to 66 million years ago, marked by the peak of dinosaur diversity and the appearance of flowering plants.
    • Significant geological changes, including the further breakup of continents and rising sea levels.
    • Ended with a mass extinction event, likely caused by an asteroid impact, leading to the demise of the dinosaurs.

11. Paleogene

    • Spanning from 66 to 23 million years ago, this period saw the rise of mammals and birds after the extinction of the dinosaurs.
    • Characterized by the development of modern ecosystems and the diversification of flowering plants.
    • Significant geological changes, including the formation of mountain ranges and the cooling of the climate.

12. Neogene

    • Lasted from 23 to 2.6 million years ago, marked by the continued evolution and diversification of mammals and birds.
    • The formation of grasslands and the expansion of savannas, influencing mammalian evolution.
    • Significant tectonic activity, leading to the uplift of mountain ranges and changes in ocean currents.

13. Quaternary

    • Spanning from 2.6 million years ago to the present, characterized by repeated glacial and interglacial periods.
    • The evolution of modern humans and significant changes in flora and fauna due to climate fluctuations.
    • Ongoing geological processes, including erosion, sedimentation, and the impact of human activity on the environment.