Geology lecture 15

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  • Relative dating techniques and approaches have been around for a long time
  • We can infer that if we can observe processes now, they have always occurred. We have evidence of some things in the past that we cannot find today
  • If you keep depositing stuff, the older stuff is going to be at the bottom and the newer stuff is going to be on the top
  • You tend to deposit chucks of sediment over large areas
  • Anything that is cutting through things is younger than the thing it is cutting through
  • Geology lecture 15

    1. 1. Geologic Time Chapter 12
    2. 2. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12
    3. 3. Geologic Time• Magnitude of Earth’s past is amazing.• Discovering this forever altered our perception of ourselves within nature & the Universe. Chapter 12
    4. 4. Geologic Time• Understanding time permits assigning ages to… • Rocks. • Fossils. • Geologic structures. • Landscapes. • Tectonic events. Chapter 12
    5. 5. Geologic Time• Prior to late 1600s, geologic time was thought to = historical time. • Archbishop James Ussher, Ireland, 1654. • He added up generations from Old Testament. • Determined Earth formed on Oct 23, 4004 BCE. Chapter 12
    6. 6. Geologic Time• Scientists began to find clues to a more ancient Earth… • Nicolaus Steno (1638–1686; danish physician). • Observed marine fossils high in mountains. • Deduced they were ancient animals. • Processes of lithification & uplift suggested long time periods. fossil shark tooth Chapter 12
    7. 7. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12
    8. 8. Geologic Time2 ways to date geological materials: 1. Relative age – Based on order of formation. • Qualitative method developed 100s of yrs ago. • Determine older vs. younger relationships. Chapter 12
    9. 9. Geologic Time2 ways to date geological materials: 1. Relative age – Based on order of formation. • Qualitative method developed 100s of yrs ago. • Determine older vs. younger relationships. 2. Numerical (absolute) age – # of yrs since an event. • Quantitative method developed recently. • Numerical age assigned. Chapter 12
    10. 10. Relative vs. Absolute1. Relative ages assign event order.2. Numerical ages assign dates to events. Chapter 12
    11. 11. Relative Age• Logical tools are useful for defining relative age.• Principles of: 1. Uniformitarianism 2. Superposition 3. Original horizontality 4. Original continuity 5. Cross-cutting relations 6. Inclusions 7. Baked contacts. Chapter 12
    12. 12. Geologic Time1. Uniformitarianism – “The present is key to the past”. • Physical processes we observe today have always operated the same way (ie. in the geological past). • Modern processes help us understand ancient events. Chapter 12
    13. 13. Defining Relative Age2. Superposition. • In an undeformed sequence of layered sed. rocks… • Younger rocks on top; older rocks below. Chapter 12
    14. 14. Relative Age3 & 4. Horizontality and continuity. • Seds deposited in horizontally extensive layers. • Erosion then dissects once-continuous layers. • Flat-lying layers unlikely to have been disturbed. Chapter 12
    15. 15. Relative Age5. Cross-cutting relations. • Younger features cut across older features. • Faults, dikes, erosion etc., must be younger than the material that is faulted, intruded, or eroded. • E.g. a volcano cannot intrude rocks that aren’t there yet. Chapter 12
    16. 16. Relative Age6. Inclusions – a rock fragment within another. • Inclusion is older than surrounding material. • Eg.: • Igneous xenoliths – Country rock that fell into magma. Chapter 12
    17. 17. Relative Age7. Baked contacts. • Thermal (contact) metamorphism occurs when country rock is intruded by igneous rock. • Baked rock is older than the intruded rock. Chapter 12
    18. 18. Relative Age• Determining relative ages empowers geologists to unravel complicated geologic histories. Chapter 12
    19. 19. Geologic History• Deposition of horizontal strata below sea level in order 1 8 (old to young). *Horizontality & continuity* Chapter 12
    20. 20. Geologic History• Igneous intrusion of a sill. *baked contact* Chapter 12
    21. 21. Geologic History• Intrusion solidified into sill• Tectonic compression Chapter 12
    22. 22. Geologic History• Compression results: • Folding (inference: layers had to exist to be folded). • Uplift (above sea level) & erosion.• Intrusion of a pluton. *baked contact/cross-cutting* Chapter 12
    23. 23. Geologic History• Extension -> normal faulting. • Faulting cross-cuts pluton & rock layers. Chapter 12
    24. 24. Geologic History• Dike intrusion. • Dike cross-cuts everything (even normal fault). Chapter 12
    25. 25. Geologic History• Erosion to present landscape. • Removed volcano and cuts down the dike top. Chapter 12
    26. 26. Geologic History• Relative ages help to unravel a complicated history.• Those rules permit one to decipher this diagram! Chapter 12
    27. 27. Geologic History• Test yourself at home: Chapter 12
    28. 28. Geologic History Geologic HistoryA cross-section through the earth reveals the variety ofgeologic features. View 1 of this animation identifies avariety of geologic features; View 2 animates the sequenceof events that produced these features, and demonstrateshow geologists apply established principles to deducegeologic history. For more information, see Section 12.4Principles for Defining Relative Age starting on p.418 andFigure 12.5 in your textbook. Chapter 12
    29. 29. Fossil Succession• Fossils (organism traces) can be preserved in sedimentary rocks. • Useful for relative age determination. • Several fossil types will occur as an assemblage. • Fossils are time markers. Chapter 12
    30. 30. Fossil Succession• Species evolve, exist, and then go extinct. • 1st appearance to extinction dates rocks. • Fossils succeed one another in a known order. • A time period is recognized by fossil assemblage. Chapter 12
    31. 31. Fossil Succession• Fossil range – first to last appearance. • Each fossil has a unique time range. • Overlapping ranges provide distinctive time markers. • Also index fossils (unique).• Permit correlation of strata. • Locally to globally. Chapter 12
    32. 32. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12
    33. 33. Unconformity• An unconformity is a time gap in the rock record. • Causes: non-deposition or erosion. Chapter 12
    34. 34. Unconformities3 Types: 1. Disconformity – parallel strata bracketing non-deposition. • Due to an interruption in sedimentation. • Can be difficult to recognize. Chapter 12
    35. 35. Disconformities Chapter 12
    36. 36. Unconformities3 Types: 2. Nonconformity – Metamorphic/igneous rocks overlain by sedimentary strata. • Igneous/metamorphic rocks exposed by erosion. • Sediment deposited on eroded surface. Chapter 12
    37. 37. Nonconformity Chapter 12
    38. 38. Unconformities3 Types: 3. Angular unconformity – represents a big gap in time. • Horizontal rocks deposited, then deformed (i.e. titled/folded). • Then eroded. • Then sediments horizontally deposited on erosion surface. Chapter 12
    39. 39. Types of Unconformity Types of UnconformityThis animation shows the stages in the development ofthree main types of unconformity in cross-section, andexplains how an incomplete succession of strata provides arecord of Earth history. View 1 shows a disconformity, View2 shows a nonconformity and View 3 shows an angularunconformity. Chapter 12
    40. 40. Unconformities• Earth history is in strata.• Missing strata = missing history.• The Grand Canyon: • Thick strata. • Many gaps (red). • Partial record of geologic past. Chapter 12
    41. 41. Stratigraphic Correlation• In 1793, William “Strata” Smith noted strata could be matched across distances. • Similar rock types in a similar order. • Rock layers contained same distinctive fossils.• He made the 1st geologic map (of the UK). Chapter 12
    42. 42. Stratigraphic Correlation• Stratigraphic columns depict strata in a region. • Drawn to portray relative thicknesses. • Rock types depicted by fill patterns. • Divided into formations (mapable rock units). • Formations separated by contacts. Chapter 12
    43. 43. Stratigraphic Correlation• National Parks of Arizona & Utah. • Formations can be traced long distances. • Overlap in rock type sequences. • Overlapping rock columns are used to build a composite. Chapter 12
    44. 44. The Geologic Column• A composite global stratigraphic column exists. • Constructed from incomplete sections across the globe. • It brackets almost all Earth history. Chapter 12
    45. 45. Outline• Geologic time: perspective & a bit of history• Dating geologic materials -General: relative & absolute dating -Relative dating: -7 Principals & their application to a geologic history -Fossil successions• Gaps in the geologic record (unconformity) -3 types of unconformities -Stratigraphic correlation & the global geologic column• Numerical (absolute) dating -Radioactive decay -Meaning of a radiometric date -Other numerical dating methods -Dating the geologic column, geologic time scale, & age of Earth Chapter 12
    46. 46. Numerical (Absolute) Dating• Based on radioactive decay of atoms in minerals. • Radioactive decay proceeds at a known, fixed rate. • Radioactive elements act as internal clocks.• Numerical dating is called geochronology. Chapter 12
    47. 47. Radioactive DecayIsotopes • Atoms with same # of protons, different # of neutrons. • Have similar, but different mass numbers.Some are stable – never change (i.e., 13C).Some are unstable (radioactive) – Spontaneously change to something else (decay) at a fixed rate (i.e., 14C). Chapter 12
    48. 48. Radioactive Decay• Decay process has 2 main components: • Parent – isotope that decays. • Daughter – decay product isotope.• Decay process can: • Have 1 step (parent -> daughter) • Have many steps (parent -> daughter -> etc…) • Decay product is also unstable and hence also decays. • Eventually proceeds to a stable endpoint. Chapter 12
    49. 49. Radioactive Decay Time• Half-life (t½) – time for ½ unstable parent to decay. • t½ is unique for each isotope. • After one t½ - ½ original parent remains. • After three t½ - 1/8th original parent remains.• Parent disappears (nonlinear), daughter accumulates. Chapter 12
    50. 50. Radiometric Dating• Mineral age can be determined by: • Measuring parent/daughter isotope ratio. • Calculating time by using the known t½.• Must pick the right mineral & isotope.• Geochronology requires analytical precision. Chapter 12
    51. 51. Chapter 12
    52. 52. What Is a Radiometric Date?• Time since a mineral began to retain all parent & daughter isotopes. • Requires cooling below “closure (blocking) temperature.” • Daughter retained only below closure T. • Daughter leaks out above closure T. • Thus, if rock is reheated above closure T, the radiometric clock can be reset to zero. Chapter 12
    53. 53. Other Numerical Ages• Numerical ages are possible without isotopes. • Growth rings – Annual layers from trees or shells. • Rhythmic layering – Annual layers in seds or ice. Chapter 12
    54. 54. Other Numerical Ages• Magnetostratigraphy – Magnetic signatures in strata are compared to global reference column. Chapter 12
    55. 55. Other Numerical Ages• Decay process can cause scars (tracks) in minerals. • Decay by fission (explosion) produces scar (track). • Daughter isn’t another isotope, it’s a damage zone. • Fission Tracks! • Track density (daughter) is proportional to age. Chapter 12
    56. 56. Dating the Geologic Column• Use geochronology to: • Date specific strata OR • Bracket those that can’t be dated directly. Chapter 12
    57. 57. The Geologic Time Scale Chapter 12
    58. 58. Age of the Earth• Oldest rocks are 3.96 Ga.• Zircon minerals in some sandstones are 4.1-4.2 Ga.• Earth is ~4.57 Ga based on correlation with… • Meteorites, moon rocks. Chapter 12
    59. 59. End Chapter 12
    60. 60. Geologic Time• Deep time – The immense span of geologic time.• So vast, difficult to grasp. • We think of time in terms of our lives… • The lives of our parents and grandparents. • The lives of our children or grandchildren.• Human history is tiny compared to geologic time. Chapter 12
    61. 61. Geologic Time• James Hutton (1726-1797) – Scottish physician. • Called “the Father of Modern Geology.” • Stated the Principle of Uniformitarianism. • Of time, He wrote: “we find no vestige of a beginning; no prospect of an end.” Chapter 12
    62. 62. Geologic Time• Principle of Uniformitarianism: • “The present is the key to the past.” • Processes seen today are same as in the past. e.g. Old mudcracks formed as mudcracks do today.• Geologic change = slow; large changes = long time. Chapter 12
    63. 63. Chapter 12
    64. 64. Stratigraphic Correlation• Lithologic correlation is based on rock type. • Sequence – The relative order in which the rocks occur. • Limited to correlation between nearby regions.• Fossil correlation – Based on fossils within rocks. • Applicable to much broader areas. Chapter 12
    65. 65. Correlation among rock strata in 3 national parks. Chapter 12
    66. 66. Angular Unconformity• James Hutton - 1st to realize the vast time-significance of angular unconformities. • Mountains created, then completely erased. • Then new sed. deposition. Chapter 12
    67. 67. Angular Unconformity• “Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists. Chapter 12
    68. 68. Geologic Time• The composite column is divided into time blocks.• This is the geologic time scale, Earth’s “calendar.” • The structure of the geologic time scale. • Eons – The largest subdivision of time (100s to 1000s Ma). • Eras – Subdivisions of an eon (65 to 100s Ma). • Periods – Subdivisions of an era (2 to 70 Ma). • Epochs – Subdivisions of a period (0.011 to 22 Ma). Chapter 12
    69. 69. Geologic Time• Time scale subdivisions are variously named. • The nature of life (“zoic” means life); i.e., Proterozoic. • A characteristic of the time period; i.e., Carboniferous. • A specific locality; i.e., Devonian. Chapter 12
    70. 70. Geologic Time and Life• Life first appears on Earth ~ 3.8 Ga.• Early life consisted of anaerobic single-celled organisms.• Oxygen from cyanobacteria built up by 2 Ga.• ~ 700 Ma, multicellular life evolved.• ~ 542 Ma marks the 1st appearance of hard shells.  Shells increased fossil preservation.  Life diversified rapidly – the “Cambrian Explosion.” Chapter 12
    71. 71. The Geologic Time Scale• Names of the eons. • Phanerozoic – “Visible life” (542 Ma to the present). • Started 542 Ma at the Precambrian – Cambrian boundary. • Marks the 1st appearance of hard shells. • Life diversified rapidly afterward. • Proterozoic – “Before life” (2.5 to 0.542 Ga). • Development of tectonic plates like those of today. • Buildup of atmospheric O2; multicellular life appears. • Archean – “Ancient” (3.8 to 2.5 Ga). • Birth of continents. • Appearance of the earliest life forms. • Hadean – “Hell” (4.6 to 3.8 Ga). Chapter 12
    72. 72. The Geologic Time Scale• Names of the eras. • Cenozoic – “Recent life.” • 65.5 Ma to present. • The “Age of Mammals.” • Mesozoic – “Middle life.” • 251 to 65.5 Ma. • The “Age of Dinosaurs.” • Paleozoic – “Ancient life.” • 542 to 251 Ma. • Life diversified rapidly. Chapter 12
    73. 73. The Age of the Earth• Before radioactivity-based dating methods… • 20 Ma – From Earth cooling. • 90 Ma –Ocean salinization. • Assumed oceans were initially freshwater. • Measured the mass of dissolved material in rivers. • Uniformitarianism and evolution indicated an Earth older than ~100 Ma. Chapter 12
    74. 74. Geologic Time• The immensity of time is beyond comprehension.• Metaphors illustrate the scale of time. • The age of Earth (4.6 Ga) can be compared to pennies. • Lined up, 4.6 billion pennies would be 87,400 km long. • More than twice around Earth. Chapter 12
    75. 75. Geologic Time Chapter 12
    76. 76. What a Geologist Sees: UnconformityThe photo shows the Siccar Point unconformity in Scotland,on the coast about 60 km east of Edinburgh; the sketchshows a geologist’s interpretation of the unconformity. Formore information, see Section 12.5 Unconformities: Gaps inthe Record starting on p.423 and Figure 12.8 in yourtextbook.
    77. 77. Zoomable Art: The Record in Rocks: Reconstructing Geologic HistoryWhen geologists examine a sequence of rocks exposed ona cliff, they see a record of Earth history that can beinterpreted by applying the basic principles of geology,searching for fossils, and using radiometric dating. For moreinformation, see the Featured Painting on pp.426-427 inyour textbook.
    78. 78. What a Geologist Sees: Stratigraphic ColumnThe succession of rocks in the Grand Canyon can bedivided into formations based on notable changes in rocktype and changes in fossil assemblages. For moreinformation, see Section 12.6 Stratigraphic Formations andTheir Correlation starting on p.424 and Figure 12.11 on p.429 in your textbook.

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