Geologic time scale — Science definition
The timeline scientists use to divide Earth's 4.6-billion-year history into nested divisions, from largest (eons) down to eras and periods.
Where it is taught
- Geologic Time & the Rock Record — Earth’s History, Earth & Space Science, Grade 9
Questions using geologic time scale
Problem 1
Which lists the units of the geologic time scale from LARGEST to smallest?
- Eon, era, period
- Period, era, eon
- Era, eon, period
- Period, eon, era
Answer and explanation
Eon, era, period
Eons are the biggest divisions, then eras, then periods — like books, chapters, and pages.
Problem 2
Explain why the geologic time scale's boundaries are not evenly spaced in years.
Answer and explanation
The divisions were originally defined by changes in the fossil record rather than by arithmetic, so a boundary marks a major turnover in life — often a mass extinction — whenever it happened. Absolute dating later attached years to those boundaries, revealing intervals of very different lengths. The scale reflects the history of life rather than a convenient subdivision of time.
Response criteria
- State a scientifically accurate conclusion.
- Show the data, mechanism, or calculation that supports it.
- Include units, conditions, or limits where relevant.
Problem 3
The geologic time scale is divided mainly on the basis of
- round numbers of years
- major changes in the fossil record
- the colour of rocks
- continental positions
Answer and explanation
major changes in the fossil record
Boundaries mark substantial changes in life, many of them mass extinctions.