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
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
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)
- You flip all of them, and any that come up heads you put aside
- An hour later you flip the pennies that came up tails and put the heads aside
- Each hour, you repeat the process. Each time, the number in the tails' pile is cut in half
- Someone can figure out how long it's been since you started, if they know how many pennies there were originally 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