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Determining Absolute Age

Determining Absolute Age. Chapter 8.2. Absolute Dating Methods. Rates of Erosion Scientists can estimate age based on a rate of erosion Practical only for geologic features that formed 10,000 to 20,000 years ago. Ex. Niagara Falls (eroding at a rate of about 1.3 m per year).

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Determining Absolute Age

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  1. Determining Absolute Age Chapter 8.2

  2. Absolute Dating Methods • Rates of Erosion • Scientists can estimate age based on a rate of erosion • Practical only for geologic features that formed 10,000 to 20,000 years ago Ex. Niagara Falls (eroding at a rate of about 1.3 m per year)

  3. Rates of Deposition • Scientists estimate the age using the rate of sediment deposition • Useful for limestone, shale, and sandstone • Generally, 30 cm of sedimentary rocks/1,000 years

  4. Varve Count • Scientists estimate age using annual layers in sedimentary deposits (similar to rings in a tree). • Varves usually found in glacial lakes

  5. Radiometric Dating • 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).

  6. 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.

  7. Radioactive Isotopes • Examples • 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.

  8. Carbon Dating • 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)

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