How do geologists date rocks? Radiometric dating!

http://geomaps.wr.usgs.gov/parks/gtime/radiom.html

Explanation of how geologists date rocks (to determine the ages of the Earth, fossils, mineral deposits, and other geologic events) and processes using radiometric dating techniques based on radioactive isotopes of carbon and potassium/argon.

by National Park Service (Cooperator) and [Western] Geology and Geophysics Science Center (Sponsor)