Lecture_1.ppt
- 1. Introduction to Geology Phil Murphy p.murphy@see.leeds.ac.uk 1
- 2. “Civilisations are what they dig from the Earth” Gibbons Decline and fall of the Roman Empire, 1776 2
- 3. “If Kuwait had of grown carrots no one would have given a damn!” Senior Source - NSA 3
- 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) 4
- 6. 12_04e.jpg 6 Principle of Original Horizontality
- 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 7
- 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 8
- 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 9
- 10. Superposition Strata in the Grand Canyon 10
- 11. Horizontality 11
- 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 15
- 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 16
- 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 20
- 21. Ripple Marks - Scarborough 21
- 25. The makings of good Index Fossils • Abundant • Widely-distributed (Global Preferred) • Short-lived or rapidly changing 25
- 26. Correlation 26
- 27. The Geologic Time Scale Quaternary Latin, “fourth” 1822 Tertiary Latin, “third” 1760 Cretaceous Latin creta, “chalk” 1822 Jurassic Jura Mountains, Switzerland 1795 Triassic Latin, “three-fold” 1834 Permian Perm, Russia 1841 Carboniferous Carbon-bearing 1822 Devonian Devonshire, England 1840 Silurian Silures, a pre-Roman tribe 1835 Ordovician Ordovices, a pre-Roman tribe 1879 Cambrian Latin Cambria, “Wales” 1835 27
- 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! 28
- 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 29
- 31. • 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 31 Radiometric Decay
- 32. • 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 32 Radiometric Dating
- 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. 35
- 36. • 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 36 Radiometric Dating
- 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. 37
- 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. 38