Added: Yentl Douglass - Date: 03.07.2021 14:10 - Views: 23098 - Clicks: 7775
R J Pankhurst. Get permission to re-use this article. Buy this article in print. Although in principle this term could be applied to estimation of relative ages according to traditional geological observation, it is nowadays usually restricted to the quantitative measurement of geological time using the constant-rate natural process of radioactive decay. The halflife of this decay is only years. Even using pre-concentration techniques and highly sensitive detectors, the practical range of the dating method does not extend back beyond about years-a period utterly inificant in terms of the geological evolution of the Earth, which extends over the past million years.
For geological dating one requires naturally occurring elements with much longer halflives. Most of the handful of appropriate decay schemes are listed. Most of the parent elements are rare metal constituents in the bulk chemical composition of the Earth. For such 'trace' elements it is generally convenient to express their concentration in natural materials in parts per million by weight ppm and even in the one case of a fairly common element potassium only a very small proportion occurs as the radioactive 40 K.
email: [email protected] - phone:(787) 271-8581 x 7489
How Old is Earth, and How Do We Know?