Geologic Time and Earth History
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Transcript of 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 –
UniformitarianismThe 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 AgesRelative - 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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Superposition:Mindoro 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 PreservedSoft 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 Beringer’s “Fossils”
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Beringer’s Book
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Where Fossils OccurAlmost 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 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. (1584) • Too Short
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Absolute Ages: Early AttemptsSalt 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 RatesCutting of Grand Canyon• 2 km/3 m.y. = 1 cm/15 yrUplift 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 RatesMovement 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 yearsRecorded 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