Radiogeology is the science that deals with the relation of radioactivity to geology. Geologists can determine the age of rocks, fossils, and other objects by measuring the radioactive elements in them.
The earth, the waters of the oceans, the air we breathe, and all living things contain small amounts of radioactivity. This radioactivity is caused by (1) the radioactive elements uranium and thorium, and their decay products; (2) radioactive potassium; (3) small amounts of less abundant radioactive elements, such as samarium and rubidium; and (4) radiocarbon, which forms when high-energy particles called cosmic rays strike nitrogen in the earth’s atmosphere.
The rocks of the earth’s surface contain an average of five parts of uranium per million parts of rock. Uranium has been in the earth since the earth was formed. It decays over time to become lead and helium. The rate at which a radioactive isotope decays is measured by its half-life–that is, the time required for half the atoms in the isotope to decay into another isotope. Lead isotope 206 is formed from uranium isotope 238, which has a half-life of about 41/2 billion years. Lead-207 is formed from uranium-235, which has a half-life of 700 million years. Scientists can measure the amounts of these isotopes in a rock sample and then calculate the rock’s age from the ratio of lead-206 to uranium-238, the ratio of lead-207 to uranium-235, and the ratio of lead-206 to lead-207. Similar calculations can be made using other radioactive isotopes.
On the basis of lead-to-uranium ratios, scientists estimate that the age of the solar system is about 41/2 billion years. This figure agrees with the ages of meteorites and the oldest moon rocks as measured by the decay of other radioactive elements.
The decay of long-lived radioactive isotopes is like a great clock that measures time in millions or billions of years. The clock also has a “second hand” that measures time in thousands of years. This is radiocarbon, which has a half-life of 5,700 years. Analysis of radiocarbon content makes it possible to determine the age of wood, bone, and other once-living materials.
