Geologic Time and Earth History

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Geologic Time and Earth History
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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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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, Bay Beach
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Fossil Ripple Marks, Baraboo Range
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Modern Mud Cracks
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Fossil Mud Cracks, Virginia
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Two Kinds of Ages

• Relative - Know Order of Events But Not Dates
• Civil War Happened Before W.W.II
• Bedrock in Wisconsin Formed Before The Glaciers
  Came
• Absolute - Know Dates
• Civil War 1861-1865
• World War II 1939-1945
• Glaciers Left Wisconsin About 11,000 Years Ago

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SuperpositionMindoro Cut, Wisconsin
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Geologic Map
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Fossils

• Remains of Ancient Plants And Animals, Evidence
  of Life

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Commonly Preserved

• Hard Parts of Organisms
• Bones
• Shells
• Hard Parts of Insects
• Woody Material

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Rarely Preserved

• Soft or Easily Decayed Parts of Organisms
• Internal Organs
• Skin
• Hair
• Feathers

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Types of Fossils

• Original Material
• Casts Molds
• Replacement (Petrified Wood)
• Carbonized Films (Leaves)
• Footprints, Tracks, Etc.
• Trace Fossils Our only preserved record of
  behavior of fossil organisms

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Dinosaur Tracks, Texas
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Rubbing Rock? Wisconsin
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Rubbing Rock? California
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Pseudofossils

• Look Like Fossils But Aren't
• Dendrites
• Concretions

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Pseudofossils
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Natural or Sculpture?
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Johannes Beringers Fossils
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Beringers Book
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Where Fossils Occur

• Almost Exclusively in Sedimentary Rocks
• Heat of Melting or Metamorphism Would Destroy
  Almost Every Type of Fossil
• Rare Exceptions
• Some Fossils in Low-grade Metamorphic Rocks
• Trees Buried by Lava Flow
• To Be Preserved, Organisms Have to Be
• Buried Rapidly After Death
• Preserved From Decay

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Fossil Tree in Lava Flow, Hawaii
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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.
  (1584)
• Too Short

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Absolute Ages Early Attempts

• Salt in Ocean
• If we know rate salt is added, and how much salt
  is in ocean, can find age of oceans.
• Sediment Thickness
• Add up thickest sediments for each period,
  estimate rate.
• Both methods gave age of about 100 million years
• Problem Rates Variable

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Age of The Sun

• If sun gets its heat from burning or other
  chemical reactions, could only last 10,000 years
  or so.
• Best 19th century guess sun was slowly
  contracting.
• Problem only 30 million years ago, sun would
  have extended out to earth's orbit!
• Geologists wanted more time, but you can't fight
  the laws of physics...
• Sun actually gets its energy from nuclear
  reactions and can keep going for billions of
  years
• The Geologists were right after all. Go Team.

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The Fundamental Rule of Absolute Ages

• The Earth is older than everything on or in it
• -Except its atoms
• -All ages are minimum ages

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Radiometric Dating Half-Life
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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 by
• U-235 704 M.Y.

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The Geologic Time Scale
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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.

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1 Second 1 Year

• 35 minutes to birth of Christ
• 1 hour to pyramids
• 3 hours to retreat of glaciers from Wisconsin
• 12 days 1 million years
• 2 years to extinction of dinosaurs
• 14 years to age of Niagara Escarpment
• 31 years 1 billion years

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Were The Dinosaurs Failures?

• Dinosaurs 150,000,000 years
• Recorded History 5000 years
• For every year of recorded history, the dinosaurs
  had 30,000 years
• For every day of recorded history, the dinosaurs
  had 82 years
• For every minute of recorded history, the
  dinosaurs had three weeks