Radiometric Dating and Radiometric Dating and Half LifeHalf Life

Radiometric DatingRadiometric DatingA method of dating rocks and

other materials based on the rate of radioactive isotope decay.

Radioactive isotopes emit nuclear particles at a constant rate.

As particles are emitted, the number of protons (atomic number) changes and therefore the identify of the element changes.

Radiometric DatingRadiometric DatingThe original radioactive isotope is

called the PARENT. Different and unique elements formed after particle emission are called the DAUGHTER.

For example, U-238 (Uranium isotope) decays into Pb-206 (Lead isotope)

In 4.5 billion years, half of a sample of U-238 will have been converted to Pb-206

Radiometric DatingRadiometric DatingThe emission of radioactive

particles and the resulting change into other isotopes over time is called RADIOACTIVE DECAY.

* Radioactive decay occurs independently of pressure, temperature and weathering agents.

Radiometric DatingRadiometric DatingWhen 50% of the parent material

has converted to daughter material, the ratio of PARENT:DAUGHTER is 1:1.

The time it takes for this ratio to establish is called HALF-LIFE.

Radiocarbon DatingRadiocarbon DatingCarbon-14’s half life is

significantly shorter than other radioactive parent isotopes (~ 5730 years)

Living organisms contain trace amounts of C-14.

Radioactive decay of C-14 starts only when an organism dies

Radiocarbon DatingRadiocarbon Dating

1. Cosmic rays from the Sun strike N-14 and converts it to C-14. This C-14 combines with oxygen creating radioactive CO2

2. This radioactive CO2 becomes part of the carbon cycle starting with photosynthesis.

Radiocarbon DatingRadiocarbon Dating

3. Living things contain a constant ratio of C-14 (radioactive) to C-12(stable)

◦ (1 : 1 000 000 000 000 )

4. At death, C-14 exchange stops and the remaining C-14 begins to decay into N-14

5. The change from C-14 to C-12 is the basis for radiocarbon dating

Radiocarbon DatingRadiocarbon Dating

Assumption: The rate of C-14 production

(amount and quality of cosmic rays reaching the Earth)

________________________________BOTTOM LINE: Radioactive materials break

down into other materials at known and constant rates.

WHY IS RADIOMETRIC WHY IS RADIOMETRIC DATING ONLY USEFUL DATING ONLY USEFUL FOR METAMORPHIC FOR METAMORPHIC AND IGNEOUS ROCK AND IGNEOUS ROCK AND NOT USEFUL AND NOT USEFUL FOR SEDIMENTARY FOR SEDIMENTARY ROCKS?ROCKS?

Other methods of absolute Other methods of absolute datingdating

Dendrochronology:◦The science or technique of dating

events, environmental change, and archaeological artifacts by using the characteristic patterns of annual growth rings in timber and tree trunks

Other methods of absolute Other methods of absolute datingdating

Ice Cores◦A sample from ice and snow that

froze many years ago and provides clues to the climate at the time of the freezing.

Other methods of absolute Other methods of absolute datingdating

Varves◦an annual layer of sediment or

sedimentary rock.