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Relative dating - is the approximate age of rocks, but not the exact time. Geologic dating is based on several “laws” that govern the nature of geologic activity. They are: Uniformitarianism, Original Horizontality, Superposition, Cross-cutting, and Included fragments. |
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| 1. Uniformitarianism-”the present is the key to the past” processes that shape the earth today, took place in the same ways millions of years ago. | |
| 2. Original Horizontality-sedimentary layers are always deposited in flat, horizontal layers. If we observe something different, change has taken place |
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| 3. Superposition- younger layers are deposited on top of older layers. | |
| 4. Cross-cutting- igneous dikes, sills ,intrusions, and faults are younger than the rock they cut across. |  |
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5. Included fragments-rock fragments may be older than the rock material they are in. |
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| Breaks in the geologic record occur over time. The breaks are known as UNCONFORMITIES. They are periods of time when an area was above sea level and therefore no layers of sediment were deposited. | |
| There are three types of unconformities. They are: | |
| ANGULAR UNCONFORMITY - new materials are deposited on top of tilted sedimentary layers. |
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| DISCONFORMITY - level layers are eroded, then new sediments are deposited on top of those. |  |
| NONCONFORMITY - sedimentary layers are deposited on top of eroded igneous or metamorphic rock. | |
| Sometimes we can compare rocks from different areas to see if they were formed at the same time. This can be in a small area, or global as in comparing rocks from different continents. This is critical when studying such topics as plate tectonics. To correlate rocks, we depend on: key beds - widespread sedimentary layers from a period of time that are easy to recognize index fossils - widespread fossils that were abundant but only existed for a few million years. Geologic columns - complete stacks of rock layers in order by age. | |