Geologic time

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  • 1. Geologic time
  • 2. Geologic TimeGeologic time scaleRelative dating principlesAbsolute dating w/radioactivityCorrelation of rock layers 
  • 3. Relative Geologic Time Scale • The relative geologic time scale has a sequence of eons eras periods epochs – but no numbers indicating how long ago each of these times occurred
  • 4. Geologic time scale EonsPhanerozoic  - last 570 million years Eras Cenozoic - recent life 0-66.4 m.y.a (mammals, humans) Mesozoic - age of middle life 66.4-245 m.y.a (dinosaurs, 1st scrawny mammals)    Paleozoic - ancient life 245-570 m.y.a (fish, trilobites, clams, corals, ferns )Precambrian - from birth of Earth up to before complex life forms developed (algae, bacteria, some fossils without shells like jellyfish)
  • 5. Relative Dating:  Key principles “Telling time geologically” Earth’s history concealed in rocks Goal of geology: unraveling Earth’s history Principle time keeping devices:Relative dating - putting rocks/events in proper orderAbsolute dating - determining event’s actual time
  • 6. Relative Dating:  Key principlesLaw of superposition A bed of rock is older than that above, younger than that belowPrinciple of original horizontalityPrinciple of cross-cutting relationshipsInclusionsUnconformities
  • 7. Relative Dating:  Key principlesLaw of superposition A bed of rock is older than that above, younger than that belowPrinciple of original horizontality Sedimentary layers deposited flatPrinciple of cross-cutting relationshipsInclusionsUnconformities
  • 8. Relative Dating:  Key principlesLaw of superposition A bed of rock is older than that above, younger than that belowPrinciple of original horizontality Sedimentary layers deposited flatPrinciple of cross-cutting relationships Faults & dikes are younger than beds they crossInclusionsUnconformities
  • 9. Relative Dating:  Key principlesInclusions Fragments of one rock enclosed in anotherUnconformities
  • 10. Fragments of one rockenclosed in another
  • 11. Relative Dating:  Key principlesInclusions Fragments of one rock enclosed in anotherUnconformities Break in time in deposition • An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed. (Conformable layers: deposited w/out interruption)
  • 12. Relative Dating:  Key principles Unconformities 3 types Angular unconformity • An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred. Disconformity • A disconformity is when two sedimentary rock layers are separated by an erosional surface. Nonconformity • A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.
  • 13. Relative Dating:  Key principles Unconformities 3 types Angular unconformity Tilted/folded sedimentary rocks overlain by younger, more flat layers Disconformity Nonconformity
  • 14. Recipe for an angular unconformity Deposition Folding/Uplift Erosion Subsidence/more erosion
  • 15. Relative Dating:  Key principles Unconformities3 types Angular unconformity DisconformityLayers on either side of unconformity are parallel Nonconformity
  • 16. Relative Dating:  Key principles Unconformities3 types Angular unconformity Disconformity NonconformityInterface between sedimentary layers and metamorphic or igneous rock
  • 17. Unconformity Types
  • 18. Relative Dating Principles
  • 19. Correlation of rock layersMatching up rocks from different placesthat are similar in age Physical criteria Fossils
  • 20. Correlation of rock layersPhysical criteria (OK for short distances) position of a bed in sequence of beds distinct/uncommon minerals
  • 21. Correlation of rock layersFossils fossil organisms succeed each other in systematic fashion any time period can be determined by fossil content “principle of fossil succession”
  • 22. Correlation of rock layers“Index fossils” geographically wide-spread short span of time T&L Figure 8.10
  • 23. Overlapping Ranges of Fossils
  • 24. 1 2 3 4 Kansas Indiana Ohio Pennsylvaniacorrelated sequence Mostly nonmarine limestone
  • 25. Absolute dating w/ radioactivity Chemistry reviewAtommodel ns ro e ct # protrons = atomic #, el defines the element nucleus protrons # neutrons can vary: neutrons “isotopes”
  • 26. Absolute dating w/ radioactivity Example: an isotope with a half life of 1 year # of atoms Parent DaughterNow 1000 0Next year 500 5002 yrs from now 250 7503 yrs from now 125 87510 yrs from now 1 999
  • 27. Radioactive Decay
  • 28. Absolute dating w/ radioactivity