Telling Time at Grand Canyon National Park

The geologic timescale, relative versus numeric dating, and how radioactive decay, volcanic ash beds, cross-cutting dikes and detrital zircons put ages on Grand Canyon's rocks.

The tilted Proterozoic rocks of the Grand Canyon Supergroup, 1,255 to 729 million years old, record the assembly and breakup of the supercontinent Rodinia and some of Earth's early life.

Grand Canyon's mile of rock looks like a complete record, but more time is missing from it than preserved. The Great Unconformity, the Great Nonconformity, the Great Angular Unconformity, and what fills the gaps.

The Vishnu Basement Rocks at the bottom of the Grand Canyon are metamorphic and igneous rocks 1,840 to 1,375 million years old, formed as volcanic island chains collided with ancestral North America.