Absolute Age

Determining How Old Geologic Features Are

Absolute Age

• Absolute Age is the numeric age of an object or event, often stated in years before the present.

Ways to Determine Absolute Age

• Rates of Erosion

• Rates of Deposition

• Varve Count

• Radiometric Dating

Rates of Erosion• By measuring how fast a river erodes, geologists can

estimate how old it must be.• Only accurate for 10,000 – 20,000 years old.

Niagara Falls erodes at the rate of 1.3 meters per year

Erosion of Niagara Falls

Rates of Deposition

• Geologists estimate that it takes about 1,000 years to deposit 30 centimeters of sedimentary rock.

• This is not a highly accurate way of measuring age but can give useful estimates.

Varve Count

• Varves are sedimentary rock layers that occur annually.

• Caused by glaciers freezing and thawing. Just like tree rings!

(Dendrochronology)

Radiometric Dating

• A method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope.

• Most rocks contain small amounts of radioactive isotopes (parent isotopes).

• Radioactive isotopes emit particles and energy at a very constant rate.

• As the radioactive isotopes emit particles they change into new elements (daughter isotopes)

Half-Life

• The time required for one half of a sample of radioactive isotope to decay to daughter isotopes.

Start 1 2 3 4

half-life half-life half-life half-life

• Different radioactive isotopes have different half-lives.

• Long half-lives are useful for measuring older objects, while short half-lives are better used for younger objects.

Carbon-14 Dating

• Carbon-14 dating (radiocarbon dating) is used to measure the age of organic (once living) objects less than 70,000 years old.

• Rocks can be measured with this method if organic material is trapped in the rock and the organic material is dated.