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Geologic Time and Relative Dating

Geologic Time and Relative Dating. Geologic Time. Geologic time—beer goggles for geologists Ex. geologically recent events Earth is 4.6 billion years old The passage of this amount of time is difficult for humans to fathom Let’s think about it in a different way…. Earth Forms: noon.

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Geologic Time and Relative Dating

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  1. Geologic Time and Relative Dating

  2. Geologic Time • Geologic time—beer goggles for geologists • Ex. geologically recent events • Earth is 4.6 billion years old • The passage of this amount of time is difficult for humans to fathom • Let’s think about it in a different way…

  3. Earth Forms: noon First single celled organism: 4:00 pm First Dinosaurs: 11:00 pm First Multi-celled Organisms: 9:00 pm How about the first Humans? 30 seconds before midnight

  4. Geologic Time Continued… • Geologists have two ways of measuring geologic time • Relative age dating: Refers only to the order of events; does not assign a specific ages • Absolute age dating: Assigning an “exact” age to something

  5. Relative Dating • Using one or more of the following principles to determine the order in which things (rocks) have changed over time:

  6. Principle of Original Horizontality • Sediment is deposited in nearly horizontal layers

  7. Folded Rock Example How in the world did this happen?

  8. We start with horizontal layers… Time 1 Time passes, rocks are deformed Time 2 Erosion removes top of fold Time 3

  9. Principle of Superposition • In an undisturbed sequence of rock, the youngest rocks are always at the top and the oldest on the bottom • Ex. Cake! Youngest layers Oldest layers

  10. Principle of Crosscutting Relationships • A rock must exist before something can happen to it • Picture ex: Order of events? • Here’s a geological example…

  11. Layer H Layer G Layer F Layer E Layer D Layer C Layer B Layers A-H were deposited; dike c cuts Through layers A-H; dike B cuts dike C; Dike A cuts Dike B. Layer A is the oldest, Dike A is the youngest unit. Layer A

  12. Principle of Inclusions • An inclusion is a piece of one rock unit contained in another • Think about the game “Operation”

  13. Evolution and Fossils • Evolution in a nutshell: • Critters change with time—successive generations adapting to their environments • Fossil = remains/traces of ancient critters preserved in rock

  14. How Are Fossils Useful? • Different animals and plants have lived at different times in the past • Ex: Dinosaurs, wicked-big insects • Different fossils will appear and disappear in the same order that organisms evolved and died out—Law of Faunal Succession • Rock correlation

  15. Conformity and Unconformity • Erosion may cause rock record to be incomplete • unconformity • There are several types of unconformities:

  16. Angular Unconformity: Horizontal Sedimentary rock layers over tilted rock layers Disconformity: Erosion surface Separating horizontal sedimentary Rock layers Nonconformity: Sedimentary rocks lie On top of igneous or metamorphic rocks

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