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“Civilisations are what they dig from the Earth”
GibbonsDecline and fall of the Roman Empire, 1776
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“If Kuwait had of grown carrots no one would have given a damn!”
Senior Source - NSA
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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)
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5Principle of Superposition
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6Principle of Original Horizontality
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
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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
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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
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Superposition Strata in the Grand Canyon
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Horizontality11
Cross-cutting Relationship
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Cross-cutting Relationship13
Which crater is youngest?
Cross-cutting Relationships
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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
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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
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Angular Unconformity17
Angular Unconformity18
Angular Unconformity19
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
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Ripple Marks - Scarborough21
Fossil Ripple Marks22
Modern Mud Cracks23
Fossil Mud Cracks24
The makings of good Index Fossils
•Abundant •Widely-distributed (Global Preferred) •Short-lived or rapidly changing
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Correlation26
The Geologic Time ScaleQuaternary 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
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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!
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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
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Radiometric Dating: Half-Life
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•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
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Radiometric Decay
•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
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Radiometric Dating
Radioactive Decay Curve33
Radioactive Decay Curve
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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.
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•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
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Radiometric Dating
Some Geologic RatesCutting 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.
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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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