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The Geological Time Scale (GTS) is the framework scientists use to divide Earth’s 4.6-billion-year history into manageable units. It’s like a calendar of Earth’s past, built from the study of rock layers (stratigraphy), fossils (paleontology), and radiometric dating.
Structure of the Geological Time Scale
The GTS is hierarchical, with Eons → Eras → Periods → Epochs → Ages.
1. Eons (largest units)
- Hadean (4.6–4.0 billion years ago): Earth forms, first crust, oceans.
- Archean (4.0–2.5 bya): First life (microbes).
- Proterozoic (2.5–0.54 bya): Oxygenation, multicellular life.
- Phanerozoic (541 mya–present): Explosion of visible life.
2. Eras of the Phanerozoic
- Paleozoic (541–252 mya): Cambrian explosion, fish, amphibians, first forests. Ends with the Permian extinction (largest in history).
- Mesozoic (252–66 mya): Age of reptiles — dinosaurs dominate. Ends with the Cretaceous extinction (asteroid impact).
- Cenozoic (66 mya–present): Age of mammals — rise of humans.
3. Periods (examples)
- Cambrian (541–485 mya): Explosion of marine life.
- Devonian (419–359 mya): “Age of Fishes.”
- Jurassic (201–145 mya): Dinosaurs thrive.
- Quaternary (2.6 mya–present): Ice ages, human evolution.
4. Epochs of the Cenozoic
- Paleogene (66–23 mya): Mammals diversify.
- Neogene (23–2.6 mya): Early hominids appear.
- Quaternary (2.6 mya–present): Includes Holocene (last 11,700 years) and Anthropocene (proposed, human impact).
Geological Time Scale Summary
| Level | Example | Time Span |
|---|---|---|
| Eon | Phanerozoic | 541 mya – present |
| Era | Mesozoic | 252–66 mya |
| Period | Jurassic | 201–145 mya |
| Epoch | Holocene | 11,700 years – present |
| Age | Meghalayan | 4,200 years – present |
How Scientists Build the GTS
- Stratigraphy: Studying rock layers.
- Fossils: Index fossils help date layers.
- Radiometric dating: Measuring isotopes (e.g., uranium-lead).
- Global correlation: International Commission on Stratigraphy (ICS) standardizes the chart worldwide.
Why It Matters
- Helps us understand evolution and mass extinctions.
- Provides context for climate change (past vs present).
- Guides exploration of oil, gas, and minerals.
- Frames the story of human evolution within Earth’s deep time.