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Radiometric Dating and Half Life

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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.

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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 U- 238 will have been converted to Pb-206

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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.

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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.

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Radiocarbon Dating Carbon-14s 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

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Radiocarbon Dating 1. Cosmic rays from the Sun strike N-14 and converts it to C-14. This C-14 combines with oxygen creating radioactive CO 2 2. This radioactive CO 2 becomes part of the carbon cycle starting with photosynthesis.

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Radiocarbon 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

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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.

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

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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

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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.

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Other methods of absolute dating Varves an annual layer of sediment or sedimentary rock.

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