Unraveling Earth's History: Geologic Time Insights

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… 

7. 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

8. 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

9. 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.

10. 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

11. 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

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

14. The Geologic Time Scale