Transcript Lecture 1 - Introduction to Geology

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Introduction to Geology
Phil Murphy
[email protected]
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“Civilisations are what they dig
from the Earth”
Gibbons
Decline 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)
Principle of Superposition
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Principle of Original Horizontality
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Two kinds of ages
• Relative - know order of events but not dates
•
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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
Superposition
Strata in the Grand Canyon
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Horizontality
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Cross-cutting Relationship
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Cross-cutting Relationship
Which
crater is
youngest?
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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
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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 Unconformity
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Angular Unconformity
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Angular Unconformity
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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 - Scarborough
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Fossil Ripple Marks
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Modern Mud Cracks
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Fossil Mud Cracks
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The makings of good Index Fossils
• Abundant
• Widely-distributed (Global Preferred)
• Short-lived or rapidly changing
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Correlation
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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
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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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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
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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
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Radioactive Decay Curve
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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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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
 C14 is produced in the upper atmosphere
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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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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.