Radiometric Dating and Half Life Radiometric Dating A 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 Dating The 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 U238 will have been converted to Pb-206 Radiometric Dating The 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 Dating When 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 Dating Carbon-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 Dating 1. 2. Cosmic rays from the Sun strike N-14 and converts it to C-14. This C-14 combines with oxygen creating radioactive CO2 This radioactive CO2 becomes part of the carbon cycle starting with photosynthesis. Radiocarbon Dating 3. Living things contain a constant ratio of C-14 (radioactive) to C-12(stable) ◦ 4. 5. (1 : 1 000 000 000 000 ) At death, C-14 exchange stops and the remaining C-14 begins to decay into N14 The change from C-14 to C-12 is the basis for radiocarbon dating Radiocarbon 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 DATING ONLY USEFUL FOR METAMORPHIC AND IGNEOUS ROCK AND NOT USEFUL FOR SEDIMENTARY ROCKS? Other methods of absolute dating 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 dating 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 dating Varves ◦ an annual layer of sediment or sedimentary rock.