An example: the Uranium-235/Lead-207 clock.
As zircon crystals form in cooling magma, they capture radioactive uranium, but they don't capture lead. There is no lead in the crystals when the clock starts. Uranium decays into lead. So when you study volcanic rock formations, you know that the lead atoms you find in zircon crystals came from the decay of uranium. Comparing the proportion of Lead-207 to the proportion of Uranium-235 in zircons can tell geologists how long ago the magma solidified into rock.
Simple, eh? Well, no. But useful.
Some naturally radioactive elements decay very quickly, and others decay much more slowly. So there are a lot of different radioisotopic "clocks."
Isotopes that decay slowly are more useful for very old rocks, and isotopes that decay more quickly are more useful for younger rocks.
Half-Life:
the Time it Takes for Half of the Original Atoms in a Sample to Decay to a "Daughter" Product
The daughter product may also be radioactive, and may decay into another product.
Parent
Daughter
Half Change in...
Carbon-14
Nitrogen-14
5730 years
Uranium-235
Lead-207
704 million years
Uranium-238
Lead-206
4,470 million years
Potassium-40
Argon-40
1,280 million years
Thorium-232
Lead-208
14,010 million years
Rubidium-87
Strontium-87
48,800 million years
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Last modified: Tue Sep 22 21:33:22 1998