Geologic Time and Earth History
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green
Bay
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
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.
Pseudo-Fossils: Look Like Fossils But Aren't
- Dendrites
- Concretions
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
These Are Difficult Requirements! Probably Only One Organism in Millions Ever Gets Fossilized
Good Index Fossils
- Abundant
- Widely-distributed (Global Preferred)
- Short-lived or Rapidly Changing
Correlation
If we have index fossils at widely separated locations, we can correlate rock units. Short-lived but widespread fossils like D, E, and F are very useful. So are first occurrences like B, last occurrences like C, and overlaps like C-D and B-D.
However, it takes organisms time to expand their ranges, and the absence of a fossil doesn't mean it the organism was never there. We find fossil D on both sides of the gap, but E only on the left and F only on the right. E must correlate somewhere below D and F somewhere above, but we don't know exactly where.
Mass Extinctions
Permian
- Trilobites
- Many Brachiopods
- Many Tree Ferns, Scale Trees
- Worst in Earth History - 90% of species
Cretaceous
- Dinosaurs
- Ammonites
- Moderately Bad - only 60% of species
- Three others as bad or worse
Causes ?
- Climate Change
- Disease
- Mountain-building
- Sea Level Change
- Competing Organisms
- Over-specialization
- Catastrophe
- Volcanism
- Meteor Impact
Were the Dinosaurs Failures?
We often use the term "Dinosaur" to denote something clunky or obsolete, but consider:
| Written History Lasted 5000 Years | The Dinosaurs Lasted 150,000,000 Years |
| For Every Year of written history: | The Dinosaurs had 30,000 Years |
| For Every Day of written history: | The Dinosaurs had 82 Years - a long human lifetime |
| For Every Minute of written history: | The Dinosaurs had 3 weeks |
| For Every Second of written history: | The Dinosaurs had 8.3 hours |
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 1861-1965
- World War II 1939-1945
- Glaciers Left Wisconsin About 11,000 Years Ago
Relative Ages
Superposition - Young Events Leave Traces Behind of Older Rocks
- Young Rocks Laid Down on Older Rocks
- Intrusions Are Younger Than The Rocks They Intrude
- Folds And Faults Are Younger Than The Rocks They Occur in
- When Rocks Are Tilted, It is Possible to Determine Which Way Was Originally Up
Absolute Ages
Early Attempts
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
- Today, it's fashionable to make fun of Lightfoot and Ussher, but they were being the best scientists they could be given the state of knowledge at the time (note this is before most people believed the Earth goes around the Sun and 50 years before Galileo ran into trouble).
Salt in Ocean
- Rivers bring dissolved solids to ocean
- If we know rate salt is added, and how much salt is in ocean, can find age of oceans.
- Gave age of about 100 million years.
- Problems
- Is rate at which salt is added constant?
- How much salt leaves ocean?
Sediment Thickness
- Add up Thickest sediments for each period
- Estimate rate of deposition to find age
- Problem: rates of deposition very variable!
- Indicated ages of at least 100 million years
Age of The Sun
One of the Great Scientific Controversies of the 19th Century
- If sun gets its heat from burning, could only last 10,000 years or so.
- Best That 19th century astronomers could guess was that 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...
- We now believe sun gets its heat from nuclear reactions. These release enough energy to keep sun going for billions of years
- The Geologists were right after all.
- Score One For My Team
Absolute Ages
Use Radioactive Decay to Date Rocks.
Radioactive atoms give off particles and change into different types of atoms.
Radioactive atoms decay at a certain rate. They have half lives. After each half life, half of the atoms will have decayed.
Half-life
Example I
- You have $20 in pennies (2000)
- Once a minute, you flip one, and if it comes up heads you put it aside
- Someone can figure out how long it's been since you started, if they know how many pennies there were originally, how often you flip, and how many heads and tails there are.
Example II
- You have a large bowl full of jelly beans. You like the green ones but not the red ones.
- Every time you dip into the bowl and get a green bean, you eat it.
- Every time you dip into the bowl and get a red bean, you put it back.
- The number of green beans you eat is large at first but tapers off. However, it may take a very long time to get that last green jelly bean.
Why Radiometric Dating is Different from Earlier Methods
Physical Processes
Vary in Kind, Rate, Intensity
- Erosion, Weathering
- Salt Addition to Oceans
- Tectonic Activity
- Continental Drift
- Growth of Organisms
Physical Laws
Underlie Physical Processes
Do Not Vary
- Speed of Light
- Laws of Thermodynamics
- Gravity
- Electromagnetism
- Quantum Mechanics
- Nuclear Physics (Radioactive Decay)
Dating Methods
C-14
- Half-life 5500 Years
- Organic Material Only
- Range: Younger Than 50 - 100,000 Yr.
K-40
- Half-life 1.3 Billion Yr.
- Range: Older Than a Few 100,000 Yr.
U-Th-Pb Many Methods
- U-235 Half-life 700 M.Y.
- U-238 Half-life 4.5 B.Y.
- Th-232 Half-life 14 B.Y.
- Fission: 1 U Decay/1,000,000
- Fission track dating range: 0 - billions of years
Rb-87 - Half-life 50 B.Y.
Present Radiometric Dating Methods
Cosmogenic
- C-14 5700 Yr.
- Be-10 2.5 M.Y.
Primordial
- K-Ar (K-40) 1.25 B.Y.
- Rb-Sr (Rb-87) 48.8 by
- U-235 704 M.Y.
- Th-232 14 B.Y.
- U-238 4.5 B.Y.
- Nd-Sm (Sm-147-Nd-143) 106 B.Y.
- Re-187 43 B.Y.
- Fission
- Lu-Hf (Lu-176) 36 B.Y.
The Geologic Time Scale
Cenozoic Era
Quaternary 0-2 MY
- Holocene (10,000 years ago - present)
- Pleistocene (Ice ages, humans)
Tertiary 2-67 MY
- Pliocene
- Miocene
- Oligocene
- Eocene
- Paleocene
Mesozoic Era 67-220 MY
- Cretaceous
- Jurassic (First birds, mammals)
- Triassic
Paleozoic Era 220-570 MY
- Permian (First reptiles)
- Carboniferous (Coal forests)
In North America, subdivided into:- Pennsylvanian (mostly terrestrial rocks)
- Mississippian (mostly marine rocks)
- Devonian (First amphibians)
- Silurian (First fish, land animals)
- Ordovician
- Cambrian (First abundant fossils)
Precambrian 570-4600 MY
- 3000 (Earliest Life?)
- 3900 (Oldest Known Rocks)
- 4600 (Formation of Earth)
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Last Update February 28, 1997