Geologic time
Geologic TimeGeologic time scaleRelative dating principlesAbsolute dating w/radioactivityCorrelation of rock layers 
Relative Geologic Time Scale             • The relative geologic               time scale has a               sequence of ...
Geologic time scale  EonsPhanerozoic  - last 570 million years                 Eras                 Cenozoic - recent life...
Relative Dating:  Key principles           “Telling time geologically”    Earth’s history concealed in rocks    Goal of ge...
Relative Dating:  Key principlesLaw of superposition A bed of rock is older than that above, younger than that belowPrinci...
Relative Dating:  Key principlesLaw of superposition A bed of rock is older than that above, younger than that belowPrinci...
Relative Dating:  Key principlesLaw of superposition A bed of rock is older than that above, younger than that belowPrinci...
Relative Dating:  Key principlesInclusions  Fragments of one rock enclosed in anotherUnconformities
Fragments of one rockenclosed in another
Relative Dating:  Key principlesInclusions  Fragments of one rock enclosed in anotherUnconformities  Break in time in depo...
Relative Dating:  Key principles    Unconformities                       3 types    Angular unconformity    • An angular u...
Relative Dating:  Key principles         Unconformities                         3 types  Angular unconformity  Tilted/fold...
Recipe for an angular unconformity  Deposition  Folding/Uplift  Erosion  Subsidence/more erosion
Relative Dating:  Key principles          Unconformities3 types   Angular unconformity   DisconformityLayers on either sid...
Relative Dating:  Key principles       Unconformities3 types  Angular unconformity  Disconformity  NonconformityInterface ...
Unconformity Types
Relative Dating Principles
Correlation of rock layersMatching up rocks from different placesthat are similar in age         Physical criteria        ...
Correlation of rock layersPhysical criteria   (OK for short distances)  position of a bed in sequence of beds  distinct/un...
Correlation of rock layersFossils fossil organisms succeed each other in systematic fashion any time period can be determi...
Correlation of rock layers“Index fossils” geographically wide-spread short span of time                                   ...
Overlapping Ranges of Fossils
1         2       3          4                      Kansas   Indiana   Ohio   Pennsylvaniacorrelated sequence             ...
Absolute dating w/ radioactivity                           Chemistry reviewAtommodel               ns                  ro ...
Absolute dating w/ radioactivity Example: an isotope with a half life of 1 year                       # of atoms          ...
Radioactive Decay
Absolute dating w/ radioactivity
Geologic time
Geologic time
Geologic time
Geologic time
Geologic time
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Geologic time

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Geologic time

  1. 1. Geologic time
  2. 2. Geologic TimeGeologic time scaleRelative dating principlesAbsolute dating w/radioactivityCorrelation of rock layers 
  3. 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. 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. 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. 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. 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. 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. 9. Relative Dating:  Key principlesInclusions Fragments of one rock enclosed in anotherUnconformities
  10. 10. Fragments of one rockenclosed in another
  11. 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. 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. 13. Relative Dating:  Key principles Unconformities 3 types Angular unconformity Tilted/folded sedimentary rocks overlain by younger, more flat layers Disconformity Nonconformity
  14. 14. Recipe for an angular unconformity Deposition Folding/Uplift Erosion Subsidence/more erosion
  15. 15. Relative Dating:  Key principles Unconformities3 types Angular unconformity DisconformityLayers on either side of unconformity are parallel Nonconformity
  16. 16. Relative Dating:  Key principles Unconformities3 types Angular unconformity Disconformity NonconformityInterface between sedimentary layers and metamorphic or igneous rock
  17. 17. Unconformity Types
  18. 18. Relative Dating Principles
  19. 19. Correlation of rock layersMatching up rocks from different placesthat are similar in age Physical criteria Fossils
  20. 20. Correlation of rock layersPhysical criteria (OK for short distances) position of a bed in sequence of beds distinct/uncommon minerals
  21. 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. 22. Correlation of rock layers“Index fossils” geographically wide-spread short span of time T&L Figure 8.10
  23. 23. Overlapping Ranges of Fossils
  24. 24. 1 2 3 4 Kansas Indiana Ohio Pennsylvaniacorrelated sequence Mostly nonmarine limestone
  25. 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. 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. 27. Radioactive Decay
  28. 28. Absolute dating w/ radioactivity

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