Earth, Page 24: [Previous Page] [Next Page] [Class Home Page] Textbook pages 114 - 116

Modern Determinations of the Age of the Earth

Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay. Radioactive parent isotopes decay to stable daughter isotopes. Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

Half Lives for Radioactive Elements

Radioactive Parent	Stable Daughter	Half life
Potassium 40	Argon 40	1.25 billion yrs
Rubidium 87	Strontium 87	48.8 billion yrs
Thorium 232	Lead 208	14 billion years
Uranium 235	Lead 207	704 million years
Uranium 238	Lead 206	4.47 billion years
Carbon 14	Nitrogen 14	5730 years

Radioactive decay occurs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present.

The proportion of parent to daughter tells us the number of half-lives, which we can use to find the age in years. For example, if there are equal amounts of parent and daughter, then one half-life has passed. If there is three times as much daughter as parent, then two half-lives have passed. (see graph, above)

So, What's the Answer?
Depends where you look! Earth is geologically active. If you were to test rocks in Hawaii, you'd find specimens which are just a few million years old (or less) on the Big Island of Hawaii. The Rocky Mountains are just 60 million years old. However, there exist stable regions on Earth that have been untouched for almost 4 Billion years, including areas in Canada and Minnesota. This and other indicators suggest the Earth formed more than 4.3 billion years ago.