Radiometric Dating Chapter 18 Geology

Absolute Dating Gives a numerical age Works best with igneous rocks difficult with sedimentary rocks Uses isotopes –Radioactive –Different number of neutrons (than stable atom)

Carbon 12 ( 12 C) –Atomic mass is on periodic table Carbon 14 ( 14 C) –2 “extra” neutrons –radioactive

Half-Life New1 Half- Life 2 Half- Lives 3 Half- Lives 4 Half- Lives Activity The time required for the Parent material to decrease by one-half

How long is a half-life?

ParentDaughterHalf life Potassium 40Argon by Rubidium 87Strontium by Uranium 235Lead by Carbon 14Nitrogen 146,000 years Sodium 22Aluminum 2715 hours

14 Carbon Only accurate for ages less than 100,000 years Parent gets too small to accurately measure

Preparing a rock for mass spectrometer

Mass spectrometer measures D/P ratio

Half life problem If you begin with 80 grams of 14 C after a time, 20 grams are left. How old is the sample?

Another half-life problem 1,000 grams of radioactive element is in a rock when it is formed. The element’s half-life is 2 million years. After a time, 125 grams of the original element remain. How old is the rock?

Using daughter/parent ratio 1) D/P ratio 2) figure out number of half-lives –Use graph 3) multiply number of half-lives by the time of one half-live Example: 240 g 14 N ; 8 g 14 C 14 C 14 N one half life= 6000 years