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

5. 12_04c.jpg 5 Principle of Superposition

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

12. Cross-cutting Relationship 12

13. Cross-cutting Relationship 13 Which crater is youngest?

14. Cross-cutting Relationships 14

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

17. Angular Unconformity 17

18. Angular Unconformity 18

19. Angular Unconformity 19

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

22. Fossil Ripple Marks 22

23. Modern Mud Cracks 23

24. Fossil Mud Cracks 24

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 QuaternaryLatin, “fourth”1822 TertiaryLatin, “third”1760 CretaceousLatin creta, “chalk”1822 JurassicJura Mountains, Switzerland1795 TriassicLatin, “three-fold”1834 PermianPerm, Russia1841 CarboniferousCarbon-bearing1822 DevonianDevonshire, England1840 SilurianSilures, a pre-Roman tribe1835 OrdovicianOrdovices, a pre-Roman tribe1879 CambrianLatin 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 26 th 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

30. Radiometric Dating: Half-Life 30

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

33. Radioactive Decay Curve 33

34. Radioactive Decay Curve 34

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  C 14 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