PCB 4674C Lecture Notes - Lecture 16: Geologic Time Scale, Parent Material, Half-Life
Document Summary
Lyell concludes that the earth is at least 240 million years old. In fact, part of darwin"s tendencies to believe in slow gradual evolution are due to his belief in uniformitarianism, in contrast to catastrophists views. Other estimates of the age of the earth are generated based estimates of rates of sedimentation and on adding up the thicknesses of all sedimentary deposits, and from rate of salinification of sea. This is deeply troubling to darwin, but is later be overturned when it is discovered that the sun is powered by nuclear fusion, and that radioactivity is constantly adding heat to the earth. The important measure is therefore the ratio of the parent to daughter material: so after 1 half-life about 1/2 of the parent material remains, after 2 half-lives 1/4 of the parent material remains, etc. Of course, radiometric data makes some assumptions that, if not met, can result in error.
