A method of dating a rock by measuring the proportions of radioactive parent istopes to daughter products.
|Isotope||Half-Life(yr)||Daughter Product||Parent abundant in:|
|40K||1300 Myr||40Ar||Feldspar, mica|
|238U||4500 Myr||206Pb||Uranium ores, zircon|
|235U||704 Myr||207Pb||Uranium ores, zircon|
|87Rb||48800 Myr||87Sr||Potassium-rich minerals|
|14C||5730 yr||14N||Organic matter, dissolved limestone|
Atoms have a nucleus which contains protons and neutrons which are surrounded by electrons. An element may have a varying number of neutrons, producing isotopes. Some of the isotopes produced may be unstable- radioactive isotopes. Radioactive isotopes change over time to become stable (radioactive decay). For example, 235U decays to form 207Pb.
A half life is the time it take for half of the parent product to decay into its daughter product.
image from http://rubble.phys.ualberta.ca/~doug/G221/Radiometric/radgraph.gif
The half life for a particular element is constant. If the number of radioactive parents atoms present when a rock was formed is known and the number present now, the age of the rock can be calculated using the decay constant.
Radioactive decay is random and is unaffected by temperature or pressure.
Errors in Radiometric Dating