Introduction%20to%20Geology

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Introduction to Geology

• Phil Murphy
• p.murphy_at_see.leeds.ac.uk

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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)

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12_04c.jpg
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
• 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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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
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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 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 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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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.