Overview
This lecture covers the major time periods of Earth's history, outlining the four geological eons and the significant events that shaped the planet and life.
Geological Time and Eons
- Earth's age is estimated at about 4.57 billion years.
- Geologists divide Earth's history into four eons: Hadean, Archean, Proterozoic, and Phanerozoic.
- Eons help break down Earth's vast timeline into manageable periods.
Hadean Eon (4.54–4 billion years ago)
- Early Earth was hot, with no continents, frequent volcanism, and constant asteroid impacts.
- The Hadean eon is named after the Greek underworld, Hades, due to harsh conditions.
- Around 4.4 billion years ago, a Mars-sized body (Theia) collided with Earth, forming the Moon.
- Asteroid bombardment persisted until about 3.8 billion years ago, likely preventing life from developing.
- No rocks remain from this eon except for durable zircon crystals.
Archean Eon (4–2.5 billion years ago)
- Heavy asteroid impacts decreased, allowing life to emerge.
- Oldest fossils (3.48 billion-year-old stromatolites) show early single-celled cyanobacteria.
- Large continents formed through the collision of microcontinents, creating stable cratons.
- By the end, Earth had oceans and continents, but its atmosphere had much less oxygen and life was unicellular.
Proterozoic Eon (2.5 billion–541 million years ago)
- Photosynthetic cyanobacteria increased atmospheric oxygen, leading to the Great Oxidation Event.
- Banded Iron Formations (BIFs) formed from oceanic deposition of iron oxides during this oxygen rise.
- Anaerobic lifeforms suffered mass extinction due to oxygen.
- Unicellular eukaryotes evolved via endosymbiosis; first multicellular life and animals appeared (Ediacara Fauna).
- Supercontinents (Nuna, Rodinia) formed and broke up, causing climate changes such as "Snowball Earth."
Precambrian and Cambrian Explosion
- The Hadean, Archean, and Proterozoic are together called the Precambrian.
- Precambrian ended 541 million years ago with the Cambrian explosion, a rapid increase in animal diversity.
- All major animal groups trace ancestors to the Cambrian period.
Phanerozoic Eon
- Began after the Cambrian explosion and continues today.
Key Terms & Definitions
- Eon — The largest division of geological time, comprising multiple eras.
- Hadean — The first eon, marked by extreme conditions on early Earth.
- Cratons — Ancient, stable parts of continental crust.
- Great Oxidation Event — The rapid increase in atmospheric oxygen about 2.4 billion years ago.
- Banded Iron Formations (BIFs) — Layers of iron-rich rock from increased oceanic oxygen.
- Ediacara Fauna — The oldest known animal fossils.
- Supercontinent — A large landmass formed from the merging of continents.
Action Items / Next Steps
- Review the characteristics and significance of each eon.
- Prepare to study the Phanerozoic eon and its subdivisions in the next session.