Geologic Time and Earth History
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Transcript of Geologic Time and Earth History
Geologic Time and
Earth History
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
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
Ripple Marks, Bay Beach
Fossil Ripple Marks, Baraboo Range
Modern Mud Cracks
Fossil Mud Cracks, Virginia
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
Superposition:Mindoro Cut,
Wisconsin
Geologic Map
Fossils
Remains of Ancient Plants And Animals, Evidence of Life
Commonly Preserved:
Hard Parts of Organisms:
• Bones
• Shells
• Hard Parts of Insects
• Woody Material
Rarely Preserved
Soft or Easily Decayed Parts of Organisms:
• Internal Organs
• Skin
• Hair
• Feathers
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
Dinosaur Tracks, Texas
Rubbing Rock?
Wisconsin
Rubbing Rock?
California
Pseudofossils
Look Like Fossils But Aren't
• Dendrites
• Concretions
Pseudofossils
Natural or Sculpture?
Johannes Beringer’s “Fossils”
Beringer’s Book
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
Fossil Tree in Lava Flow, Hawaii
Good Index Fossils
• Abundant
• Widely-distributed (Global Preferred)
• Short-lived or Rapidly Changing
CorrelationCorrelation
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
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
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
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.
The Fundamental Rule of Absolute Ages
The Earth is older than everything on or in it
-Except its atoms
-All ages are minimum ages
Radiometric Dating: Half-Life
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.
The Geologic
Time Scale
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.
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.
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
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