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Relative Time

Relative Time. Dr. David Steer. Introduction. Relative Time - How do scientists determine the sequence of geological events? Absolute Time - How do geologists figure out the ages of rocks, and hence the age of Earth?. How do you normally remember the order of events?.

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Relative Time

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  1. Relative Time Dr. David Steer

  2. Introduction • Relative Time - How do scientists determine the sequence of geological events? • Absolute Time - How do geologists figure out the ages of rocks, and hence the age of Earth?

  3. How do you normally remember the order of events?

  4. Place these events in some logical order A) MTV Founded B) Persian Gulf War C) JFK Assassinated F) Challenger Explodes E) Man lands On Moon D) World Trade Center Destroyed

  5. Oldest to Youngest 1 2 3 E) Man lands On Moon C) JFK Assassinated A) MTV Founded 4 5 6 F) Challenger Explodes B) Persian Gulf War D) World Trade Center Destroyed

  6. Three principles used to determine relative order of geological events • Principle of Superposition • Principle of Cross-cutting Relationships • Principle of Original Horizontality

  7. Superposition • Layers (beds) at the bottom of a sequence of sedimentary rocks are oldest • Rocks at the top are youngest Youngest Oldest

  8. Which layer is oldest? • Layer 1 • Layer 2 • Layer 3 • All same Layer 1 Layer 2 Layer 3 From http://www.geo.cornell.edu/geology/classes/geol326/photos.html

  9. Cross-cutting Relationships • Older rocks may be cut by younger geological features • Might be an igneous intrusion • Might be filling of an eroded feature

  10. If I use a pen to poke a hole in the stack What is the youngest event? What does the pen do to the other units?

  11. Cross-cutting Relationships Igneous rock cuts earlier layering Layered rock formed first

  12. Which rock is the youngest? A • A • B • C • D B C D

  13. Which statement is false? • D crosscuts B • D crosscuts C • C crosscuts D • B was deposited • before A A B C D

  14. Now tilt and bend the stack

  15. Original horizontality • Sedimentary rocks are deposited in horizontal layers • Tilted or folded layers indicate deformation following deposition

  16. How do we match sequences across large areas? • Correlation used to determine relative ages of rocks across continents • Correlation matches fossils and physical characteristics in sedimentary rocks

  17. Index Fossils Best correlations use index fossils that represent a short time period but are found across a large region

  18. Which statement is true? • Layer 1 is older than layer 3 • Layer 2 is the same age as layer 3 • Layer 3 is younger than layer 2 • Layer 2 is older than layer 1 Layer 1 Layer 2 Layer 3 From http://www.geo.cornell.edu/geology/classes/geol326/photos.html

  19. Place these units in order from oldest to youngest

  20. Draw this cross section Unit B: Basalt crosscuts all older units and covers a shale unit. Event F: Stream cuts partially into limestone unit. Unit L: Chemical sedimentary rock that was deposited directly on top of low silica extrusive unit. Unit M: Oldest unit is plutonic rock that was partly eroded. Units S: Sandstone that formed over the non-shell bearing limestone rock unit. Unit R: Light colored extrusive rock that completely crosscuts and covers all other units. Unit C: Small grained clastic rock deposited directly over granite.

  21. Student Examples

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