Absolute Dating
Radiometric Methods
Correlation
Outline
Absolute dating methods are those that generate an age in years. There are two principal
methods:
- Methods relying on event in the geological record with very strong annual cyclicity:
- tree growth rings
- coral growth cycles
- varves (annual clay sediment layers)
- Methods relying on the decay of naturally occurring radiogenic isotopes:
- Carbon-14 -> Nitrogen-14
- Potassium-40 -> Argon-40
- Uranium-235 -> Lead-207
- Uranium-238 -> Lead-206
- Thorium-232 -> Lead-208
- Rubidium-87 -> Strontium-87
Radiometric dating is not without difficulties but is by far the most widespread method
applied to determing absolute ages between 100 and 4500 My. The same decay process is
responsible for the temperature gradient within the Earth (geothermal gradient). Uranium,
Thorium and Potassium are by far the most common radiogenic isotopes in the Earth's
crust. Radiocarbon is important in dating organic remains from the relatively recent
past. Bomb tritium (from atmospheric nuclear tests) has been used extensively to trace
groundwater movements over periods of tens of years.
Radiometric Methods
Correlation
Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan,
57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9