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Time and Geology

Time and Geology. The Present is the Key to the Past. Catastrophism-October 23, 4004 BC @ 9:00AM James Ussher, Bishop of Ireland Uniformitarianism- James Hutton (1795) uniform rate of geologic processes Actualism gradualism and catastrophism shape geologic processes Relative Time

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Time and Geology

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  1. Time and Geology

  2. The Present is the Key to the Past • Catastrophism-October 23, 4004 BC @ 9:00AM • James Ussher, Bishop of Ireland • Uniformitarianism- James Hutton (1795) • uniform rate of geologic processes • Actualism • gradualism and catastrophism shape geologic processes • Relative Time • Sequence of Geologic Events • Absolute Age

  3. Principles Used to Determine Relative Age • Superposition- Nicolas Steno • Progressively younger from bottom => top • Lateral Continuity • Original Horizontality- Steno • Contacts • Formations • Cross-cutting Relationships- Lyell • Truncated units (dikes and faults)

  4. Correlation • Physical Continuity of Lithologic Units • Similarity of Rock Types • Superposition • Correlation by Fossils • Principle of Faunal Succession • Index Fossil • Fossil Assemblage

  5. Correlation • Correlation by Fossils • Principle of Faunal Succession • Wm. Smith & Georges Cuvier • Organism succeed one another in lithologic strata • One organism will NOT be found in rocks of widely different ages • Index Fossil • Wide geographic distribution • Of limited life span as a species • Easily identifiable • Fossil Assemblage

  6. Standard Geologic Time Scale • Established in the 19th century- Sedgwick &Murchinson • Based on Fossil Assemblages • Eons, Eras, Periods, and Epochs • PHANEROZOIC since 544 m.y.b.p. • Paleozoic Era 544-245 m.y.b.p. • Mesozoic Era 245-65 m.y.b.p. • Cenozoic Era 65 m.y.b.p.-present • Tertiary & Quaternary Periods • Recent (Holocene) Epoch last 10,000 years • Precambrian- All time before Paleozoic

  7. Unconformities • Unconformity - surface that represents a gap in the geologic record • Disconformity - contact representing missing parallel beds of sedimentary beds • Angular unconformity - younger strata overlie an erosion surface on tilted or folded layers • Nonconformity - erosion surface on plutonic or metamorphic rock

  8. Unconformities • Unconformity - surface that represents a gap in the geologic record • Disconformity - contact representing missing parallel beds of sedimentary beds • Angular unconformity - younger strata overlie an erosion surface on tilted or folded layers • Nonconformity - erosion surface on plutonic or metamorphic rock

  9. Unconformities • Unconformity - surface that represents a gap in the geologic record • Disconformity - contact representing missing parallel beds of sedimentary beds • Angular unconformity - younger strata overlie an erosion surface on tilted or folded layers • Nonconformity - erosion surface on plutonic or metamorphic rock

  10. Unconformities • Unconformity - surface that represents a gap in the geologic record • Disconformity - contact representing missing parallel beds of sedimentary beds • Angular unconformity - younger strata overlie an erosion surface on tilted or folded layers • Nonconformity - erosion surface on plutonic or metamorphic rock

  11. Absolute Age • Isotopic Dating • Isotopes and Radioactive Decay • Isotopes - Differing number of neutrons, but same number of protons • Radioactive Decay - Spontaneous nuclear disintegration of unstable isotopes • Daughter Product • Half-life • Various techniques (e.g. U-Pb, K-Ar, radiocarbon dating)

  12. Absolute Age • Uses of Isotopic Dating • Igneous Rocks • Reliability of Isotopic Dating • Accuracy and precision of analytical instrumentation

  13. Isotopic Decay • Unstable Nucleus • spontaneously change into another type of atom • Decay through Beta emission (Rb>Sr), Positron emission(N>C), Electron capture (K>Ar), Alpha emission (U>Th), Spontaneous fission • Decay Rate is Constant, it depends on the particular isotope

  14. Radioactive Decay Series

  15. Age of the Earth • Early speculation • Dating of meteorites and moon rocks • Between 4.5 & 4.6 billion years old • Comprehending Geologic Time

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