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Visualizing Earth Science

Visualizing Earth Science. Chapter 10 – How Old is Old? The Rock Record and Deep Time. By Z. Merali and B. F. Skinner. Relative Age The Geologic Column Numerical Age The Age of Earth. Chapter Overview. The Age Debate: A change of perspective. Catastrophism

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Visualizing Earth Science

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  1. Visualizing Earth Science Chapter 10 – How Old is Old? The Rock Record and Deep Time By Z. Merali and B. F. Skinner

  2. Relative Age The Geologic Column Numerical Age The Age of Earth Chapter Overview

  3. The Age Debate:A change of perspective • Catastrophism • Large scale geologic features formed in catastrophic events • The age of Earth measurable in human historical terms • Uniformitarianism • Past and present geological processes similar • Discarded catastrophic formation • Earth processes can be observed to understand past • Proposed the Earth was ancient much beyond the span of human history

  4. Relative Age • Determination of sequence of geological events • Use stratigraphy – Science of rock layers and strata forming processes • Law of original horizontality • Beds or strata laid in water are laid horizontal

  5. Relative Age • The principle of stratigraphic superposition • Undisturbed strata have younger layers above older ones

  6. Relative Age • Principles of stratigraphy • Principle of cross cutting relationships • Feature cutting through or disrupting a stratum – younger than the stratum

  7. Unconformities: Gaps in the record • Numerical age • Time elapsed since a feature formed • Unconformity • A chronological gap in sequence of rock layers

  8. Correlating Layers • Fossils • Signs of ancient living organisms • Usually consist of preserved hard parts • Paleontology • Study of fossils and ancient life • Fossils useful to correlate rock layers by age • Fossil succession – determine sequences in fossil records

  9. Correlating Layers

  10. Sequencing geologic time • The geologic column • Visual, suggestive representation of geologic time • Based on succession of rock strata • Major division -- Eon • Subdivisions – Eras, Periods and Epochs • Four eons • Current eon: Phanerozoic – visible life • Eras: Paleozoic, Mesozoic, Cenozoic • Past: Hadean, Archean, Protrozoic

  11. Sequencing geologic time

  12. Periods and Epochs Recent fossil record • Eras divided into periods • Based on fossil groups • Some period boundaries marked by extinctions • The Cambrian period • First of the Paleozoic era • Animals with hard shells • Period of “explosion” in bio-diversity • Precambrian – the whole of geologic time before the Cambrian period • Periods last for multi-million years • Subdivided into epochs

  13. Numerical Age • Attempts made at estimating the age of Earth • Rate of sedimentation and thickness of strata • Too much variability in estimates • Ocean salt deposit rate (Halley) • Flaw in assumption – salt content not varying • Darwin’s estimate based on erosion processes • Rejected on the basis of thermodynamic cooling of Earth • Discovery of radioactivity reveals internal heating of Earth • Modern estimates (> 4.5 billion years) • Based on radiometric dating

  14. Numerical Age

  15. Radioactivity and Numerical Ages • Rocks contain internal clock • Radioactive matter decays at determinable rate • Development of sensitive instruments measuring • Rates of radioactive decay • Concentrations of elemental substances • Enabled measurement of numerical ages of rocks

  16. Radioactivity and Numerical Ages

  17. Radiometric dating Half life • Radioactive atoms decay into daughter products • Half life: time taken for half the atoms in sample to decay • Known half life and known initial concentrations of • Parent atoms • Daughter atoms • Provide accurate estimates of age when final concentration are measured

  18. Radiometric dating • Radiometric dating difficulties • Need to find • Rock samples with initially discoverable parent/daughter ratios • No contaminations to externally change parent daughter ratio after rock formation • Different dating techniques found for • Rocks, fossils and biological material • Based on estimated age and composition

  19. Radiometric dating

  20. Radiometric dating and the geologic column • Finding sedimentary strata with igneous inclusions • Provides an application of the radiometric dating technique to the geologic column • Fossil correlated layers can be dated • using the principles of stratigraphy • Numerical dates from radiometric dating

  21. Magnetic Polarity Dating • Earth’s magnetic field periodically reverses • During rock formation • Orientation of magnetized mineral grains locked in • Allows determination of magnetic field polarity • Field polarity measurements along with radiometric dating • Provide a timescale for more recent geologic time • Used to date history and succession of hominid fossils

  22. Modern estimates of the age of Earth • Earth has volcanic, geological and biogenic activity • Pristine rock from early Earth difficult to find • Oldest sedimentary rock mineral dated to 4.4 billion years • However sedimentation requires prior rock formation and erosion • Earth older than oldest rock • Moon rock samples and meteorites provide clues to age of planetary formation • Oldest meteorite 4.56 billion years old

  23. Chapter Summary • Relative Age • The Age Debate • Stratigraphy: Principles and Application • The Fossil Record • The Geologic Column • Sequencing Geologic Time: Eons, Eras, Epochs and Periods • Numerical Age • Numerical Age: The Age of Earth Problem • Radioactivity and Radiometric dating • Magnetic Polarity Dating • The Age of Earth • Modern estimates of the Age of Earth

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