Overview
This lecture covers the geologic time scale, Earth's age, division of geologic time, key events in Earth's history, and related terminology.
Geologic Time Scale Basics
- The Earth is about 4.6 billion years old.
- The geologic time scale is a timeline based on rock records detailing Earth's history.
- It divides Earth's history into hierarchical intervals: eons, eras, periods, epochs, and ages.
- Fossils are used to trace geological events from Earth's past.
- Radiocarbon dating uses carbon-14 to date archaeological and paleontological specimens.
Divisions of Geologic Time
- Eons are the largest divisions; there are four: Hadean, Archaean, Proterozoic, Phanerozoic.
- The first three eons are called the Precambrian.
- Eons are subdivided into eras, which are subdivided into periods.
- Epochs divide periods, and ages divide epochs.
Major Eons and Eras
- Hadean Eon: 4600–4000 million years ago (mya).
- Archaean Eon (4 eras): Eoarchean, Paleoarchean, Mesoarchean, Neoarchean (4000–2500 mya).
- Proterozoic Eon (3 eras): Paleoproterozoic, Mesoproterozoic, Neoproterozoic (2500–541 mya).
- Phanerozoic Eon (3 eras): Paleozoic (540–250 mya), Mesozoic (250–66 mya), Cenozoic (66 mya–present).
Phanerozoic Era Highlights
- Paleozoic Era:
- Cambrian: marine life diversified.
- Ordovician: invertebrates diversified; mass extinction.
- Silurian: first terrestrial vascular plants.
- Devonian: bony fishes diversified; mass extinction.
- Carboniferous: formation of Pangaea, first reptiles.
- Permian: major extinction, formation of Pangaea.
- Mesozoic Era:
- Triassic: first dinosaurs and mammals appear.
- Jurassic: first birds; gymnosperms and angiosperms.
- Cretaceous: extinction of dinosaurs.
- Cenozoic Era:
- Tertiary: mammals radiate, continents shift.
- Quaternary: repeated glaciation, human evolution, and rise of agriculture.
Key Geological Events (Selected)
- 3500 mya: first bacteria.
- 1900–1700 mya: early eukaryotes.
- 505 mya: first fish.
- 438 mya: early land plants.
- 245 mya: Permian mass extinction.
- 240 mya: dinosaurs appear.
- 2 mya: earliest humans.
- 1.6–0.01 mya: Ice Ages.
Key Terms & Definitions
- Fossil — Remains or traces of ancient organisms preserved in rock.
- Radiocarbon Dating — Method using carbon-14 to determine specimen age.
- Geologic Time Scale — Timeline organizing Earth's history based on rock records.
- Eon — Largest time division; contains eras.
- Era — Subdivision of an eon.
- Period — Subdivision of an era marked by rock systems.
- Epoch — Subdivision of a period.
- Age — Subdivision of an epoch.
Action Items / Next Steps
- Review divisions of geologic time (eons, eras, periods, epochs, ages).
- Memorize key events associated with each era and period.
- Complete homework or review questions on order and significance of geologic events.