1 Introduction to Geology Phil Murphy p murphy see

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)

Principle of Superposition 12_04 c. jpg 5

Principle of Original Horizontality 12_04 e. jpg 6

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

Superposition Strata in the Grand Canyon 10

11 Horizontality

12 Cross-cutting Relationship

13 Cross-cutting Relationship Which crater is youngest?

14 Cross-cutting Relationships

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

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 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 Silurian Devonshire, England Silures, a pre-Roman tribe 1840 1835 Ordovician Ordovices, a pre-Roman tribe 1879 Cambrian Latin Cambria, “Wales” 1835

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!

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 C 14 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.