Earth Science: Geologic Time Scale

Precambrian Eon

• Spans nearly 90% of Earth's history, divided into three eras: Hadean, Archean, and Proterozoic.
• Hadean (4,500 MYA): Formation of the Earth; a molten state with no solid crust.
• Archean (4,000 MYA): First stable continents formed; beginning of life with prokaryotic organisms.
• Proterozoic (2,500 MYA): Rise of multicellular organisms; oxygen levels increased due to photosynthetic bacteria.

Phanerozoic Eon

• Represents the current geological eon; characterized by the abundance of fossil records.

Paleozoic Era

• Cambrian (540 MYA): Emergence of hard-shelled organisms; first chordates and fishes.
• Ordovician (485 MYA): Major diversification event with an increase in marine life forms.
• Silurian (445 MYA): Evolution of vascular plants; colonization of land begins.
• Devonian (419 MYA): Known as the "Age of Fishes"; first amphibians appear.
• Carboniferous (359 MYA): Appearance of first reptiles and extensive forests of scale trees.
• Permian (299 MYA): Era marked by widespread deserts; significant geological changes as Pangaea forms.

Mesozoic Era

• Known as the age of dinosaurs; divided into three periods.
• Triassic (252 MYA): Concentrated land mass of Pangaea; increasing biodiversity.
• Jurassic (201 MYA): Pangaea begins to break apart; expanding dinosaur populations.
• Cretaceous (145 MYA): Continued breakup of Gondwana; significant mountain building activity.

Cenozoic Era

• Divided into three periods: Paleogene, Neogene, and Quaternary.
• Paleogene:
  • Paleocene (66 MYA): Continued continental drift; India moves toward Asia.
  • Eocene (56 MYA): Formation of the Himalayas; rise of the Alps in Europe.
  • Oligocene (34 MYA): Antarctica becomes isolated; shifts in habitats.
• Neogene:
  • Miocene (23 MYA): Emergence of the Andes mountain range; diversification of mammals.
  • Pliocene (5 MYA): Initiation of the current ice age; Mediterranean Sea formation.
• Quaternary:
  • Pleistocene (2.5 MYA): Multiple glacial cycles; major shifts in sea levels and continental positions.
  • Holocene (11,700 YA): Period of human evolution; significant global climatic changes.

Key Facts

• Earth is around 4.5 billion years old, with two main eons: Precambrian and Phanerozoic, the latter containing significant biological and geological developments.