Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 27 September Phone: x 5719
Home: 822-1116
e-mail: rap1

The History of the Earth and Deep Time

Time line of early Earth: from origin of solar system (4.57 Ga) to present (5 meter model).

Major occurrences of Precambrian (approx. 90% of Earth history):

Phanerozoic time ­ HSU NHM exhibits website

 

How do we know ages?

  1. Tree rings. Note that can go past oldest living tree if older wood which overlaps is available (e.g. in old buildings, preserved in bogs, at archeological sites). Limited to about 10,000 y so far.
  2. Rock layers. Layers in sedimentary rock generally will be seasonal, like tree rings. Again can extend range by looking for overlaps. Determined Earth must be 100's of millions of years old in nineteenth century. (Rock thicknesses for various eras tend to be km to miles thick.)
    1. Different biological organisms are characteristic of each time period, thus can determine relationships of geological deposits by comparing characteristic organisms.
  3. Radiometric dating. Depends on rate of decay of radioisotopes.
    1. All isotopes decay at constant rates characteristic of the isotope.
    2. These rates are independent of environment (not affected by submersion in water, heating or cooling, etc.)
    3. The decays are 'first order,' with characteristic half-lives (at any time 1/2 of whatever remains will disappear in the next half-life).
    4. k = 0.693/t1/2; t = ln(x/xo)/k, where x/xo is the remaining fraction at any given time.
    5. Example: 14C is continuously produced in the atmosphere by cosmic rays impacting nitrogen in the upper atmosphere: 10n + 147N -> 146C + 11H
      It is then incorporated into living organisms via carbon dioxide. All organisms will then have a 14C/12C the same as the atmosphere until they die, at which time the 14C will gradually decay. The half-life of 14C is 5670 y. In a piece of charcoal from an archeological site the fraction of 14C is 1/4 its initial concentration. How old is the charcoal? Solution ­ 1/4 = two half-lives, thus the charcoal is 2(5670y) = 11,340y.
    6. Some other important half-lives include: 238U = 4.5 Gy (decay series gives 206Pb as end product), 40K = 1.28 Gy, 232Th = 14.05 Gy


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© R A Paselk

Last modified 27 September 2004