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Following from online notes: Chapter 1Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA September 12, 2002
A.
Quantitative scientific methods for dating the Earth
In 1897, Lord Kelvin assumed that the Earth was originally
molten and calculated a date based on cooling through
conduction and radiation.
Age of Earth was calculated to be about 24-40 million years.
1.
In 1899 - 1901, John Joly (Irish) calculated the rate of
delivery of salt to the ocean. River water has only a small
concentration of salts. Rivers flow to the sea. Evaporative
concentration of salts.
2.
Age of Ocean = Total salt in oceans (in grams) divided
by rate of salt added (grams per year)
3.
Age of Earth was calculated to be 90-100 million years.
4.
Thickness of total sedimentary record divided by
average sedimentation rates (in mm/yr). In 1860, calculated
to be about 3 million years old. In 1910, calculated to be
about 1.6 billion years old.
5.
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.
6.
Sedimentation rates average about 0.3 m/1000 years.
Problem: Earth has an internal
heat source (radioactive decay)
Problems: no way to account for
recycled salt, salt incorporated into
clay minerals, salt deposits.
Problems: did not account for past
erosion or differences in
sedimentation rates; also ancient
sedimentary rocks are
metamorphosed or melted.
At this rate, the age of the first fossiliferous rocks is about 500
million years.
Charles Lyell 1800's compared amount of evolution shown
by marine mollusks in the various series of the Tertiary
System with the amount that had occurred since the beginning
of the Pleistocene. Estimated 80 million years for the Cenozoic
alone.
7.
Discovery of radioactivity by Henri Becquerel in
1896. In 1905, Rutherford and Boltwood used radioactive
decay to measure the age of rocks and minerals. Uranium
decay produces helium (He), leading to a date of 500 million
years.
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.
(our text p. 657: radioactive
decay)
There are three types of subatomic particles involved:
8.
Alpha particles
large, easily stopped by paper
charge = +2
mass = 4
9.
Beta particles
penetrate hundreds of times farther than alpha particles, but easily stopped
compared with neutrons and gamma rays.
charge = -1
mass = negligible
10.
Neutrons
highly penetrating
no charge
mass = 1