1. 2/25/20161 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. 2/25/20162

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 2/25/20163

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 2/25/20164

5. Relative Dating  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 1 st, 2 nd, 3 rd …  Relative Dating : tells us the sequence of events, NOT how long ago they occurred. What came 1 st, 2 nd, 3 rd …  5

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

8. Absolute Dating Methods  Absolute Age: numeric age of an object, as established by a process  Absolute Age: numeric age of an object, as established by a process, such as radiometric dating  1. Rates of Erosion  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

9. Absolute Dating Methods  2. Rates of Deposition  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.

10.  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

11.  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 Radiometric Dating Video 112/25/2016

12. 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

13. 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 Carbon Dating Videog

14. 14 The Geologic Time Scale