The
age of the earth has been scientifically determined to be 4.55 billion
years by using multiple radiometric dating methods on various samples.
Radiometric dating is based on the fact that radioactive
isotopes naturally decay to stable isotopes at a measurable rate. Mathematics
has then given us a simple equation to describe this decay:
(Amount of sample now) = (Amount of sample originally) * 2^(-age/halflife).
From this decay equation, the age of a given sample can be determined. However, this generic dating assumes that the amount of daughter isotope is initially zero or a known value, an assumption which is unlikely to be accurate in a real world scenario. To solve for this problem, the isochron dating method was created, which measures an additional quantity—the amount of another isotope of the stable daughter element. Then, by graphing ratios of the three measured quantities, a linear plot is created such that the slope of the best-fit line is the age of the sample. The beauty of this approach is that the uncertainty in the age is also known by the variance in the slope, or how well the best-fit line fits the data points. Common isochron techniques involve Rb-Sr, Sm-Nd, Ar-Ar, and Pb-Pb elements, each with their advantages and disadvantages. For a more detailed explanation of isochron dating, look at the website—www.talkorigins.org/faqs/isochron-dating.html.
In order to scientifically establish a date of 4.55 billion years, radiometric dating has been done on a variety of sources, including old Earth rocks, moon rocks, and meteorities. Earth rocks have been found with ages as old as 4.1 and 4.2 billion years. While this does not establish the specific age of the Earth, it does yield a lower limit for its age. The Earth must be as old or older than its native rocks. Moon rocks have also been dated, to values as old as 4.5 billion years, with the majority falling between 3.2 and 4.0 billion years. Since the moon is assumed to have been formed at about the same time as the Earth, these values for the age of moon rocks, and thus the moon, correlate to approximately the age of the Earth. Meteorites, which give us a glimse of the age of the solar system and the planets, have been dated, with a substantial amount falling between 4.4 and 4.6 billion years old.
Thus, by sampling the Earth, the moon, and meteorites,
and using the isochron method of radiometric dating, the age of the Earth
is scientifically determined to be 4.55 billion years old.
REFERENCES
Dalrymple, G. Brent. The Age of the Earth. Stanford,
CA.: Stanford University Press.
1991.