AimAim: How do geologists determine the absolute age of a rock?

I. Radioactive DatingRadioactive Dating

A. Absolute AgeAbsolute Age – exact age in years

1. IsotopeIsotope – same element that has different number of neutrons in nucleus. Ex: Carbon12 (C12) and Carbon 14 (C14)

2. Radioactive isotopeRadioactive isotope – when the atom has fewer or more neutrons in the nucleus. A radioactive isotope will break down naturally into a lighter element called a decay productdecay product.

3. Decay productDecay product – the substance produced after a radioactive isotope breaks down. The decay product is stable and will not turn into anything else.

Radioactive Isotope

Decay Product

Half Life(years)

C14 N14

5.7 x 103

K40 Ar40 or Ca40

1.3 x 109

U238 Pb206

4.5 x 109

Rb87 Sr87

4.9 x 1010

B. Half LifeHalf Life – the time taken for the activity of decay to reduce the total amount of radioactive substance in a material to half its original amount.

1. If you know the original content of a radioactive material and compare it to the present content of the same radioactive material, you can then determine the age of the material.

2. The half-lives of radioactive isotopes are different for different substances.

a. Some substances have short half-lives (ex: C14) and are good for dating recent organic remains (1,000 to 50,000 years)

C14

100% Radioactive

1 Half Life 2 Half Lives

3 Half Lives 4 ½ Lives

100% Radioactive C14

C14

1 Half Life

5,700 years

2 Half Lives

11,400 years

3 Half Lives

17,100 years 22,800 years

C14

C14

C14C14

N14 N14

N14 N14

4 ½ Lives

http://www.colorado.edu/physics/2000/isotopes/lifetime.html

http://vcourseware5.calstatela.edu/VirtualDating/files/2.0_HalfLife.html