Radiometric Dating

In the early 1900's, scientists made a great discovery. They found that radioactive isotopes in rocks decay at a constant rate into a more stable element. This is significant because it meant that a geologist, by measuring the amount of an isotope contained in a rock, could estimate the age of that rock. The time it takes for the half of the material made up of the "parent element" to decay into the "daughter element" is different for different isotopes. Here are some elements that are commonly used to date rocks:

Element	Changes to	Half-life
U238	Pb206 (lead)	4,500 million years
U235	Pb207 (lead)	700 million years
Rb87	Sr87	48,600 million years
K40	Ar40	1,300 million years
C14	C12	5,730 years

A mass spectrometer is used to measure the Parent/daughter ratio, which can be used to calculate how long the rock has been around.

 

Bibliography

Igneous Rocks and Radiometric Dating Determining the 'time' in the geologic timescale. Retrieved June 7, 2001 from the World Wide Web: http://dept.kent.edu/geology/ehlab/radio_dating/radio_dating.htm