4. Putting Numbers on Geologic Ages

While geologists knew simply that the Earth was very old, physicist Lord Kelvin calculated its age to be about 100 million years. On the basis of the rate of delivery of salt to the ocean by rivers, the Earth’s age was calculated to be 90 million years. Many geologists felt that these ages were too short to account for everything that had happened. A major breakthrough came at the end of the nineteenth century with the discovery of radioactive decay of heavy elements. Also, at the end of the century Svante Arrhenius noted that the carbon dioxide being introduced into the atmosphere from burning fossil fuels would warm the planet. It was soon realized that the rates of decay of radioactive elements could be used to date rocks. The Earth’s age turned out to be measured in billions, not millions, of years. By 1920, a numerical age framework similar to the one we have today had been established.