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Chapter 8
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itarianism A principle that geologic processes that occurred in the past can be explained by current geologic processes. Volcanism and Erosion Uniform p.185
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Relative Age The age of an object in relation to the age of other objects p.186
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The law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed. Law of Superposition p.187
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A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time. Un ities p.189 conform
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Absolute Age The numeric age of an object or event, often stated in years before the present. Using a process like “radiometric dating” or “carbon dating”
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Rates of Deposition In general, about 30 cm of sedimentary rock are deposited over a period of 1,000 years. However, a flood can deposit many meters of sediment in just one day.
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Radiometric Dating A method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) and a stable (daughter) isotope. Half-Life: the time required for half of a sample of a radioactive isotope to break down.
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Carbon -14 Dating Plants absorb Carbon during Photosynthesis Scientists compare the carbon isotopes 12 C and 14 C (radioactive) This can be used to determine the ages of wood, bones, shells and other organic remains that are less than 7,000 years old.
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Paleontology The scientific study of fossils.
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Trace Fossils Fossilized evidence of past movement of an animal such as tracks, footprints, borings and borrows.
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Index Fossils A fossil that is used to establish the age of rock layers because it is: Distinct – features different from other fossils Abundant – occurs in fairly large numbers Widespread – present in scattered rocks Existed only for a short span of geologic time
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