2. “Civilisations are what they dig
from the Earth”
Gibbons
Decline and fall of the Roman Empire, 1776
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3. “If Kuwait had of grown carrots
no one would have given a damn!”
Senior Source - NSA
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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)
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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
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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
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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
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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
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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
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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
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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
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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!
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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
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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
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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
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Radiometric Dating
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
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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.
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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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