Geologic Time Scale

Geologic Time Scale – High-Level Framework

• Earth’s age: $4.6 \text{ Ga}$ (billion years)
• Hierarchy of time units (largest ➜ smallest): Eon → Era → Period → Epoch
• Two super-divisions:
  • Precambrian Time (makes up $\approx 88\%$ of Earth history)
  • Phanerozoic Eon (visible life)

Major Divisions & Boundaries (Ma = million years ago)

• Precambrian: $4600$–$541 \text{ Ma}$
• Paleozoic Era: $541$–$252 \text{ Ma}$
• Mesozoic Era: $252$–$66 \text{ Ma}$
• Cenozoic Era: $66 \text{ Ma}$–Present

Precambrian Time ($4600$–$541 \text{ Ma}$)

• Formation of Earth from gas & dust; molten surface cools ➜ crust
• Early atmosphere: $\text{CO}<em>2$, $\text{N}</em>2$, $\text{SO}<em>2$; later dominated by $\text{N}</em>2$
• First life: single-celled prokaryotes (cyanobacteria) ➜ oxygen release (Great Oxidation)
• Late Precambrian (Proterozoic): supercontinent Rodinia; global “Snowball Earth” ice ages
• Appearance of aerobic eukaryotes & first multicellular organisms

Paleozoic Era ($541$–$252 \text{ Ma}$)

• Six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian & Pennsylvanian), Permian
• Early breakup of Rodinia ➜ Gondwana & Laurentia; later assembly of Pangaea
• Key biological milestones:
  • Cambrian Explosion: rapid diversification of marine invertebrates
  • First fishes (Ordovician)
  • First land plants (Silurian)
  • First amphibians (Devonian); first reptiles (Mississippian)
• Late Permian mass extinction ("The Great Dying"): $\approx 90\%$ marine, $\approx 70\%$ terrestrial species lost

Mesozoic Era ($252$–$66 \text{ Ma}$)

• Periods: Triassic, Jurassic, Cretaceous
• Tectonics: gradual rifting of Pangaea ➜ increased coastlines, warmer climates
• Atmospheric $\text{CO}_2$ high, fostering greenhouse conditions
• Dominant life: reptiles (dinosaurs); first small mammals & birds; gymnosperms abundant
• End-Cretaceous (K-Pg) asteroid impact ➜ mass extinction of non-avian dinosaurs & many marine taxa

Cenozoic Era ($66 \text{ Ma}$–Present)

• Periods ➜ Epochs:
  • Paleogene: Paleocene, Eocene, Oligocene
  • Neogene: Miocene, Pliocene
  • Quaternary: Pleistocene, Holocene
• Major mountain building: Himalayas, Rockies, Alps
• "Age of Mammals": rapid adaptive radiation into land, air & sea niches
• First hominids (Pliocene); modern humans appear Pleistocene
• Late Quaternary megafaunal extinctions $\approx 10{,}000$–$8{,}000$ years ago linked to climate warming & human hunting

Key Mass Extinctions (Five Big Events)

• End-Ordovician ($443 \text{ Ma}$)
• Late Devonian ($372–359 \text{ Ma}$)
• End-Permian ($252 \text{ Ma}$) – largest
• End-Triassic ($201 \text{ Ma}$)
• End-Cretaceous ($66 \text{ Ma}$)

Quick Recall Tips

• "Pumpkin Pie Can Make Diners Quite Happy" ➜ Paleozoic periods in order: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
• "Tri-Jur-Cre" ➜ Mesozoic periods: Triassic, Jurassic, Cretaceous
• 88 % of history = Precambrian; 12 % = Phanerozoic
• Boundary ages to memorize: $541$, $252$, $66$ (Era transitions)