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 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
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 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
Fossil Tree in Lava Flow, Hawaii
Good Index Fossils
• Abundant
• Widely-distributed (Global Preferred)
• Short-lived or Rapidly Changing
Correlation
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
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
How to calculate radiometric
dates.
• Ever heard of Plutonium? It's the stuff we use
in our nuclear things -- weapons, submarines,
etc. Plutonium-239 has a half-life of 24,110
years.
• "Half-life means that, if you have 100 pounds
of Plutonium-239...
• In 24,110 years, you'd still have 50 pounds
left...
Half life continued.
• In another 24,110 years, you'd still have 25 pounds
left.
• This stuff just won't go away! This is why it is such a
big concern when a nuclear submarine sinks...
Eventually, the salt water will eat through the steel
and release the Plutonium (which, as you know, is
quite lethal.) They usually talk about either trying to
raise the sub or encase it in concrete where it rests.
The last figure I heard was that there are currently
eight nuclear subs on our ocean floors. Now that I've
completely depressed you... back to the math!
Half Life (con’t).
• Hey, did you know that YOU are radioactive? You've got this
stuff in you called Carbon-14... It comes from cosmic rays that
rain down on the earth (and us) from outer space. (By the
way, you are mostly Carbon-12, which is not radioactive.
That's why we are called "Carbon-based life forms
• Scientists use Carbon-14 to make a guess at how old some
things are -- things that used to be alive like people, animals,
wood and natural cloths. It doesn't work for sea creatures
and other things that are under water. Think about it...
Cosmic rays can't get through the water.
Half Life (Con’t).
• Anyway, they make an estimate of how much Carbon-14
would have been in the thing when it died... Then they
measure how much is left in the specimen when they find it.
This is where the half-life comes in... the half-life of Carbon14 is about 5730 years.
• Here's one of the formulas they use:
Half life (Con’t).
• If we mess with this a bit, we can make
it simpler:
Half Life (Con’t).
•
Let's walk through a problem without the formula first just to make sure that we get the
concept of half-life:
•
In 2000, you buried 15 kg of Carbon-14 in your backyard. Someone digs it up in the year
13,460. How much Carbon-14 did they find?
•
OK, That's 11,460 years (which is two half-lives...)
•
After 5730 years, there'd be 7.5 kg.
•
After 5730 more years, there'd be 3.75 kg.
•
Now, let's do one with the formula:
•
You discovered a new radioactive isotope and named it boogonium (don't ask). It's half life is
1.23 years. If you start with a sample of 45 grams, how much will be left in 6.7 years?
Half Life (con’t).
Plug this stuff in!
Grab a calculator!
Half Life (con’t).
• YOUR TURN:
• An alien radioactive isotope has a halflife of 238years. If you start with a
sample of 8 kg, how much will be left
in 100 years?
•
Half Life (con’t).
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 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.
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.
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
Miscellaneous Photos
• Niagara
escarpment
Miscellaneous photos
• Dinosaur extinction.
Miscellaneous photos
• Ice age in Wisconsin
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