|Coates, Donald R. Environmental Geology. New York: Wiley, 1981: 46.||"The Earth is calculated to be about 4.5 billion years old"||4.5 Gyr|
|Brown, Laurie M. Twentieth Century Physics. Philadelphia: Institute of Physics, 1995: 1961.||"In 1956 Patterson proposed the more accurate estimate: 4550 plus/minus 70 million years"||4.55 Gyr|
|Stassen, Chris. The Age of the Earth. TalkOrigins Archive. 1996.||"The generally accepted age for the Earth and the rest of the Solar System is about 4.55 billion years (plus or minus about 1 percent)."||4.55 Gyr|
|Holmes, Arthur. Holmes Principles of Physical Geology. New York: Halsted, 1978: 243.||"It can thus be fairly concluded that the earth has an age of about 4600 M.y."||4.6 Gyr|
|Asimov, Isaac. The Measure of the Universe. New York: Harper Row, 1983: 256.||"… a variety of lines of evidence point to the Earth, and, indeed, the entire Solar System, as having been formed about 4.6 eons ago"||4.6 Gyr|
The Earth is approximately 4.55 billion years old -- an inconceivable age when one considers that the human being we would recognize as modern man has existed for less than 50,000 years.
The oldest fossils of indisputable age found in Australia and South Africa suggest that life (blue-green algae) existed between 3.46 and 3.47 billion years ago. Radioactive dating methods give an age of up to 3.9 billion years for rocks found on Earth. It only stands to reason that the Earth must be as old as anything on it. It was from these numbers that estimates for the age of the Earth itself were formed.
In 1956, Clair Cameron Patterson of the California Institute of Technology, proposed the estimate for the Earth's age which today remains the accepted value. He calculated the Earth as having been formed some 4.55 billion years ago (plus or minus 0.07 billion years). Patterson worked under the assumption that the Earth and the entire solar system were the same age. He argued that some meteorites were formed 4.56 billion years ago and their debris constituted the Earth. The Earth continued to grow through the bombardment of small, solid planetary bodies until about 4.4 billion years ago when the Earth began to retain its atmosphere and create its core.
Patterson used the uranium-lead clock and samples of lead isotopes found on the Earth and in meteorites to come to this conclusion. The radioactive decay of uranium 238 into lead 206 and uranium 235 into lead 207 is frequently used to date rocks through the analysis of the daughter product, lead, of the parent, uranium. Similar methods dated samples of rock from the moon at a similar age of about 4.6 billion years.
The years that followed Patterson's work brought great change to the earth sciences with the beginnings of studies of plate tectonics. Knowledge of the Earth's past has changed and developed with time. Today it is less certain how the Earth was formed than when it was formed.
Molly Moran -- 1997