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Dating rocks by trails of destruction

Pakistan gneiss

Exhibition Text

This 1.85-billion-year-old gneiss and the one-million-year-old granite vein cutting through it were dated by counting the number of fission tracks in the apatite (calcium phosphate) crystals contained in each rock. Fission tracks are microscopic scars left behind when the radioactive element uranium spontaneously splits into two fragments. Since the rate of the splitting, or fission, is known, the age of the apatite crystal can be determined by counting the tracks within it.

Donated by Dr. Peter Zeitler, Lehigh University, Bethlehem, Pennsylvania

For Educators

Topic: Earth Science

Subtopic: Geologic Time

Keywords: Gneiss, Apatite, Geochemistry, Radioactive dating, Uranium

Audience: General