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Introduction to Geology

Introduction to Geology. Phil Murphy p.murphy@see.leeds.ac.uk. “ Civilisations are what they dig from the Earth” Gibbons Decline and fall of the Roman Empire, 1776. “If Kuwait had of grown carrots no one would have given a damn!” Senior Source - NSA.

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Introduction to Geology

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  1. Introduction to Geology Phil Murphy p.murphy@see.leeds.ac.uk

  2. “Civilisations are what they dig from the Earth” Gibbons Decline and fall of the Roman Empire, 1776

  3. “If Kuwait had of grown carrots no one would have given a damn!” Senior Source - NSA

  4. Why is geology different from other sciences? • Often lacks experimental control • Incompleteness of data • Methodologies and procedures used to test problems rather than the generation and testing of universal laws • GEOLOGY WORKS • (everyone wants to drive to Sainsburys)

  5. Principle of Superposition 12_04c.jpg

  6. Principle of Original Horizontality 12_04e.jpg

  7. Two kinds of ages • Relative - know order of events but not dates • Napoleonic wars happened before W.W.II • Bedrock in Scotland formed before the glaciers came • Absolute - know dates • Civil War 1803-1815 • World War II 1939-1945 • Glaciers finally left Scotland About 11,000 Years Ago

  8. Two conceptions of Earth history • Catastrophism • Assumption: great effects require great causes • Earth history dominated by violent events • Uniformitarianism • Assumption: we can use cause and effect to determine causes of past events • Finding: Earth history dominated by small-scale events typical of the present. • Catastrophes do happen but are uncommon

  9. Principles of Relative Dating • Law of superposition • Undeformed section of sedimentary or layered igneous rocks • Oldest rocks are on the bottom • Principle of original horizontality • Layers of sediment are generally deposited in a horizontal position • Rock layers that are flat have not been disturbed (deformed) • Principle of cross-cutting relationships • Younger features cut across older features

  10. Superposition Strata in the Grand Canyon

  11. Horizontality

  12. Cross-cutting Relationship

  13. Cross-cutting Relationship Which crater is youngest?

  14. Cross-cutting Relationships

  15. Principles of Relative Dating • Inclusions • A piece of rock that is enclosed within another rock • Rock containing the inclusion is younger • Unconformity • Break in rock record produced by erosion and/or non-deposition of rock • Represents period of geologic time

  16. Principles of Relative Dating • Types of unconformities • Angular unconformity • tilted rocks (disturbed) are overlain by flat-lying rocks • Disconformity • strata on either side of the unconformity are parallel • Nonconformity • metamorphic or igneous rocks in contact with sedimentary strata

  17. Angular Unconformity

  18. Angular Unconformity

  19. Angular Unconformity

  20. Uniformitarianism • Continuity of Cause and Effect • Apply Cause and Effect to Future - Prediction • Apply Cause and Effect to Present - Technology • Apply Cause and Effect to Past – Uniformitarianism The present is the key to the past

  21. Ripple Marks - Scarborough

  22. Fossil Ripple Marks

  23. Modern Mud Cracks

  24. Fossil Mud Cracks

  25. The makings of good Index Fossils • Abundant • Widely-distributed (Global Preferred) • Short-lived or rapidly changing

  26. Correlation

  27. The Geologic Time Scale

  28. Absolute ages: early attempts • The Bible • Add up dates in Bible • Get an age of 4000-6000 B.C. for Earth • John Lightfoot and Bishop Ussher - 4004 B.C., October 26th 9 a.m (1584) • Too short!

  29. Absolute ages: early attempts • Salt in Ocean • If we know the rate salt is added, and how much salt is in ocean, we can find the age of oceans. • Sediment thickness • Add up thickest sediments for each period and estimate rate. • Both methods gave age of about 100 million years • Problem: rates variable

  30. Radiometric Dating: Half-Life

  31. Radiometric Decay • Parent • an unstable radioactive isotope • Daughter product • the isotopes resulting from the decay of a parent • Half-life • the time required for one-half of the radioactive nuclei in a sample to decay

  32. Radiometric Dating • Principle of radioactive dating • The percentage of radioactive toms that decay during one half-life is always the same (50%) • However, the actual number of atoms that decay continually decreases • Comparing the ratio of parent to daughter yields the age of the sample

  33. Radioactive Decay Curve

  34. Radioactive Decay Curve

  35. Present Radiometric Dating Methods Cosmogenic • C-14  5700 Yr. Primordial • K-Ar (K-40) 1.25 B.Y. • Rb-Sr (Rb-87)  48.8 B.Y • U-235 704 M.Y.

  36. Radiometric Dating • Sources of error • A closed system is required • To avoid potential problems only fresh, unweathered rock samples should be used • Carbon-14 (radiocarbon) dating • Half-life of only 5730 years • Used to date very recent events • C14 is produced in the upper atmosphere

  37. Some Geologic Rates Cutting of Grand Canyon • 2 km/3 m.y. = 1 cm/15 yr Uplift of Alps • 5 km/10 m.y. = 1 cm/20 yr. Opening of Atlantic • 5000 km/180 m.y. = 2.8 cm/yr. Uplift of White Mtns. (N.H.) Granites • 8 km/150 m.y. = 1 cm/190 yr.

  38. Some Geologic Rates Movement of San Andreas Fault • 5 cm/yr = 7 m/140 yr. Growth of Mt. St. Helens • 3 km/30,000 yr = 10 cm/yr. Deposition of Niagara Dolomite • 100 m/ 1 m.y.? = 1 cm/100 yr.

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