The major time periods of Earth are the Eons, which are further subdivided into Eras, Periods, Epochs, and Ages. Here's a breakdown of the major ones with a focus on the most commonly referenced:
Major Divisions of Geologic Time
Geologic time is divided into hierarchical units. The broadest division is into Eons. Eons are divided into Eras, Eras into Periods, Periods into Epochs, and Epochs into Ages.
Eons
Eons represent the largest intervals of geologic time. There are four eons:
- Hadean Eon: The earliest eon, representing Earth's formation and early development.
- Archean Eon: Characterized by the emergence of the first single-celled life.
- Proterozoic Eon: Marked by the evolution of more complex life forms and the buildup of atmospheric oxygen.
- Phanerozoic Eon: The current eon, characterized by abundant and diverse life.
Eras of the Phanerozoic Eon
The Phanerozoic Eon is further divided into three major eras, which are most commonly referred to when discussing geologic time:
- Cenozoic Era: (66 million years ago to present) - Known as the "Age of Mammals," characterized by the evolution and diversification of mammals, birds, and flowering plants.
- Mesozoic Era: (251.9 to 66 million years ago) - The "Age of Reptiles," best known for the dominance of dinosaurs.
- Paleozoic Era: (538.8 to 251.9 million years ago) - Characterized by the Cambrian explosion of life, the development of fish, amphibians, and reptiles, and the formation of supercontinents.
Table of Eras within the Phanerozoic Eon
| Era | Time Span (Millions of Years Ago) | Key Characteristics |
|---|---|---|
| Cenozoic | 66 to 0 | Rise of mammals, birds, flowering plants, and humans. |
| Mesozoic | 251.9 to 66 | Age of dinosaurs; evolution of early mammals and birds. |
| Paleozoic | 538.8 to 251.9 | Cambrian explosion, rise of fish, amphibians, and reptiles. |
These eras are further subdivided into periods, epochs, and ages, providing a more detailed timeline of Earth's history. The Geologic Time Scale offers a comprehensive resource for exploring these divisions in depth.
