Understanding Deep Time: A Guide to Earth's Vast History

An accessible guide to geologic time, relative dating and why millions of years are difficult to visualize.

Human history is a tiny fraction

Recorded history occupies only a very small part of Earth's timeline.

Rock layers record sequence

Lower layers are often older than layers above them when the sequence is undisturbed.

Radiometric dating provides numerical ages

Certain isotopes allow scientists to estimate when rocks formed.

Mass extinctions divide major chapters

Large biological changes help define boundaries in geologic history.

Scale changes perspective

Thinking in millions of years helps explain evolution, erosion and continental movement.