Absolute Dating/Numerical Age Method and How It Is Used to Evaluate Geologic Time

The absolute dating or numerical age method approximates the number of years of a given rock, say 200 million years, as compared to relative dating which compares two rocks and merely determines if one is older or younger than the other (earthsci.org, 2008). The geologic time scale was originally based on relative dating methods but with the development of radioactive dating technology, numerical ages further enhanced the scale.

Absolute dating relies on the properties of various elements in rocks that over time decompose and may even transform into another element. Also called the “clocks in rocks”, the radioactive isotopes of elements or “parent isotopes” are atoms that decay into new isotopes or “daughter isotopes” where the “parent isotopes” may have varying numbers of neutrons but have the same number of electrons (earthsci.org, 2008).

However, the spontaneous and continuous process of decay will in time yield isotopes with very different chemical compositions. The absolute or numerical age of a rock material containing radioactive isotopes is established through “counting the number of its daughter isotopes and using the known decay rate of its parent isotopes, calculating the length of time required in generating the said number of daughters” (earthsci.org, 2008). In this manner, geologic time is evaluated with respect to the period it takes for elements in the rock formation to reach their current chemical state.

Parent isotopes and daughter isotopes have an inverse relationship. Over time, the number of parents decreases as they decompose into daughters and consequently, the latter increases. Time is expressed in half-lives which are the periods it takes for fifty percent of the radioactive atoms in isotope samples to decay into a form that is considered more stable. For instance, the half-life of the isotope U-235 (parent) in order to decay to the more stable Pb-207 (daughter) is 713 million years and is the numerical age of the rock sample from which it is derived if it is the only daughter isotope produced (earthsci.org, 2008).

List of References

Earthsci.org (2008). Radioactive Dating. Retrieved 3 June 2008 from             http://earthsci.org/fossils/geotime/radate/radate.html.