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Geologic Time

Geologic Time. Mr. Litaker. Objectives. State the principle of uniformitarianism. Explain how the law of superposition can be used to determine the relative age of rocks. Apply the law of crosscutting relationships to determine the relative age of rocks. Uniformitarianism.

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Geologic Time

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  1. Geologic Time Mr. Litaker

  2. Objectives • State the principle of uniformitarianism. • Explain how the law of superposition can be used to determine the relative age of rocks. • Apply the law of crosscutting relationships to determine the relative age of rocks.

  3. Uniformitarianism • In the late 1700’s, James Hutton published his Theory of the Earth. Hutton put forth the idea of… • UNIFORMITARIANISM: the same forces which change Earth’s surface now, such as volcanism and erosion, were at work in the past

  4. Earth’s Age • Hutton’s ideas raised serious questions about Earth’s age. Until that time, most scientists thought Earth was only 6,000 years old, and all Earth’s features had formed at the same time. • Geologists now estimate that Earth is about 4.6 billion years old

  5. RelativeDating • Principle of Original Horizontality: sedimentary rocks left undisturbed will remain in horizontal layers • Law of Superposition: in sedimentary rock, each layer is older than the one above it and younger than the one below it. • Relative Dating: tells us the sequence of events, NOT how long ago they occurred. What came 1st, 2nd, 3rd… 

  6. Cross-Cutting Relationships • Principle of Cross-Cutting Relationships: when a fault cuts through a layer of rock, the fault is youngerthan the rocks affected • The rock layers must have been there 1st in order to fragment or break, so intrusions (faults) must be younger than the layer they intrude upon • Relative Dating Video

  7. Absolute Dating Methods • Absolute Age: numeric age of an object, as established by a process, such as radiometric dating • 1. Rates of Erosion: scientists use this method to estimate absolute age • only practical for geologic features which formed within the past 10,000 to 20,000 years • less dependable for older features because rates of erosion can vary

  8. Absolute Dating Methods • 2. Rates of Deposition: estimate of absolute age by calculating the rate of sediment deposition. • not always accurate because not all sediment is deposited at the same rate • provides only an estimate of absolute age.

  9. Absolute Dating Methods • 3. Radiometric Dating: comparing the remaining percentage of an element’s nuclei with one which has not decayed • Nuclei of the same element emit particles at a constant rate • Half-Life: time required for half of an element’s nuclei to decay

  10. Absolute Dating Methods • Scientists measure the concentration of an original element’s nuclei against one which has decayed in a rock, and use the known decay rate of that element to determine the absolute age of the rock. • Radiometric Dating Video

  11. 4. Carbon Dating • Atoms of the same element which have different numbers of neutrons are called isotopes • All living organisms contain carbon-12 and carbon-14 isotope. • So, living organisms contain carbon atoms with 12 neutrons and other carbon atoms with 14 neutrons

  12. Carbon Dating • Once a plant or animal dies, the ratio of 14C to 12C begins to change. • To find the age of a sample of non-living organic material, scientists take the ratio of 14C to 12C and then compare this with the ratio of 14C to 12 C known to exist in a living organism. • Carbon Dating Videog

  13. The Geologic Time Scale

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