Click image to open full size
Geologic Time Scale
Geologic Time Scale
Related Worksheets
Related Cheat Sheets
The geologic time scale is the system scientists use to organize Earth's 4.6 billion year history. It helps us place major events such as the formation of the planet, the rise of life, mass extinctions, and the appearance of humans into a clear timeline. Because Earth's history is so long, geologists divide it into large blocks called eons, eras, periods, and epochs. This scale connects rocks, fossils, and radioactive dating into one shared framework.
The time scale is built by studying rock layers and using absolute ages from radiometric dating. Older rocks are usually found below younger rocks, while fossils show how life changed through time. Boundaries between major intervals often mark important planetary changes such as oxygen buildup, climate shifts, or extinction events. The result is a timeline that lets scientists compare events across continents and understand how Earth became the world we live on today.
Key Facts
- Earth formed about 4.6 x 10^9 years ago.
- Geologic time is divided hierarchically as eon > era > period > epoch.
- Age of a sample by radioactive decay can be modeled as N = N0e^(-lambda t).
- Half-life relation: t1/2 = ln(2)/lambda.
- The Precambrian includes the Hadean, Archean, and Proterozoic and covers about 88% of Earth's history.
- The Phanerozoic Eon began about 541 million years ago and includes the Paleozoic, Mesozoic, and Cenozoic eras.
Vocabulary
- Eon
- The largest division of geologic time, made up of several eras.
- Era
- A major subdivision of an eon that groups together related periods of Earth history.
- Period
- A division of geologic time within an era, often marked by important changes in rocks or life.
- Radiometric dating
- A method for finding the age of rocks by measuring the decay of radioactive isotopes.
- Mass extinction
- A relatively short interval of geologic time when a large fraction of Earth's species disappears.
Common Mistakes to Avoid
- Assuming all divisions on the geologic time scale are equal in length, which is wrong because eons, eras, periods, and epochs can differ greatly in duration.
- Thinking humans appeared early in Earth's history, which is wrong because humans occupy only a tiny fraction of the most recent part of the Cenozoic.
- Using only fossil evidence to assign exact ages, which is wrong because fossils mainly give relative ages while radiometric dating provides numerical ages.
- Believing older rocks are always deeper everywhere, which is wrong because folding, faulting, and erosion can rearrange rock layers.
Practice Questions
- 1 Earth formed about 4.6 billion years ago, and the Phanerozoic began 541 million years ago. How many billion years of Earth history passed before the Phanerozoic began?
- 2 A radioactive isotope used in dating has a half-life of 100 million years. If a rock sample has 25% of the original parent isotope remaining, how old is the rock?
- 3 Why do geologists use both rock layers and radiometric dating when building the geologic time scale? Explain what each method contributes.