Led to the Doctrine of Catastrophism:
Earth was shaped by series of giant disasters.
Had to fit many processes into a short time scale.
Hadrian's Wall built by Romans, after 1500 years no change. Suspected that Earth was much older.
Slow processes shape earth.
Mountains arise continuously as a balance against erosion and weathering
Doctrine of Uniformitarianism: "Present is key to the past".
The physical and chemical laws that govern nature are uniform
Unconformity at Siccar Point, Scotland
"No vestige of a beginning, no prospect of an end"
Problem: Earth has an internal heat source (radioactive decay)
Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)
Age of Earth was calculated to be 90-100 million years.
Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.
Early measurements of maximum thickness of sediment ranged from 25,000 m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks is at least 150,000 m.
Sedimentation rates average about 0.3 m/1000 years.
At this rate, the age of the first fossiliferous rocks is about 500 million years.
Problems: did not account for past erosion or differences in sedimentation rates; also ancient sedimentary rocks are metamorphosed or melted.
In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion years.
So far, oldest dated Earth rocks are 3.96 billion years.
Older rocks include meteorites and moon rocks with dates on the order of 4.6 billion years.
Moon rocks, highland ~ 4.5 by, mare basalt ~ 3.2 - 3.8 by
Meteorites - older than 4.5 by
Mass spectrograph was used after WWI (1918). Led to the discovery of over 200 isotopes.
Many radioactive elements can be used as geologic clocks. Each element decays at its
own nearly constant rate. Once this decay rate is known, geologists can estimate the
length of time over which decay has been occurring by measuring the amount of
radioactive parent and the amount of stable daughter elements.
Example: Potassium-Argon dating.
Rocks broken down into sediment (gravel, sand, silt, clay).
Sediment will turn into sedimentary rock over time. Older rocks are buried deeply under younger rocks.
Before this, oldest rocks known were from Isukasia region of Greenland (3.8 by)
Glaciers 2 miles thick scraped off young recycled rocks.
Land rose 250 ft since ice was removed => more erosion.
Very old rocks are at the surface in the Canadian Shield area.
Up to about 3.8 or 3.96 billion years old.
Multicellular life did not appear until about 1 billion years ago.
Before this, 3 billion years ago single celled life only.
Hard parts like shells don't appear until 600 million years ago. (Trilobites)
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Document copyright: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
Document created Winter 1996
Modified April 4, 1997
Modified February 3, 1999
Modified December 12, 2003
Links updated August 15, 2009