Chapter 8.2

Determining Absolute Age

Rates of ErosionScientists can estimate age based on a rate of

erosionPractical only for geologic features that formed

10,000 to 20,000 years ago

Absolute Dating Methods

Ex. Niagara Falls(eroding at a rate of about 1.3 m per year)

Rates of DepositionScientists estimate the age using the rate of

sediment depositionUseful for limestone, shale, and sandstoneGenerally, 30 cm of sedimentary rocks/1,000

years

Varve CountScientists estimate age using annual layers in

sedimentary deposits (similar to rings in a tree).

Varves usually found in glacial lakes

Rocks contain small amounts of radioactive material

Isotopes = atoms of same element with different number of neutrons

Radioactive isotopes emit particles and energy from its nuclei at a constant rate.

Scientist use the breakdown or decay of isotopes to measure the absolute age of rocks

Radiometric dating is done by comparing the relative percentages of a radioactive isotope (parent) with a stable isotope (daughter).

Radiometric Dating

Half-Life: the amount of time that it takes for half the mass of a sample of a radioactive isotope to break down into the stable daughter isotope.

Scientists compare the amount of parent and daughter isotopes in rock to find its age. More daughter isotopes mean that the rock is older.

Radioactive IsotopesExamples

Uranium 238 (half-life 4.5 billion years) is good for samples >10 million years old

Potassium 40 (half-life 1.25 billion years) – used to date rocks 50,000-4.6 billion years old.

Carbon 14 dating (a.k.a. radiocarbon dating) is used to date younger rock layers (less than 70,000 years) by using the organic material found in it. (ex. wood, bones, shells)

All living organisms contain 12C and 14C. When alive, the ratio is constant. When they die, the ratio changes.

Half-life of 14C is 5, 730 years. Radioactive 14C decays to Nitrogen 14

(nonradioactive)

Carbon Dating