A. How old is the Earth?

4.6 billion years (4,600,000,000 years)

Early ideas of the age of the Earth:

1. 1654 Archbishop Usher (Ireland), genealogy in Bible
   Earth was created October 26, 4004 BC, 9:00 am
   Earth was 6000 years old.

   Led to the Doctrine of Catastrophism:
   Earth was shaped by series of giant disasters.
   Had to fit many processes into a short time scale.

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2. 1770's, 1780's "Revolution"
   James Hutton, Father of Geology (Scotland) 1726-1797.
   Published Theory of the Earth in 1785.

   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"

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   How old is Earth? Quantitative scientific methods.

   1. 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.

      Problem: Earth has an internal heat source (radioactive decay)

   2. 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.

      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.

   3. 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.

      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.

   4. 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.

   5. 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 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.

      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.

• Earth is geologically active.
• Has a hot, molten interior.
• Rocks are remelted and their internal clocks are reset.
  
• Also, rocks on Earth's surface are acted on by erosion and weathering. Rocks on Earth surface are not as old as the Earth, they are "recycled" rock materials

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.
Isostasy

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 created by: Pamela J. W. Gore
DeKalb College, Clarkston, GA

Document created Winter 1996
Last modified April 4, 1997