Historical Geology Notes

Radiometric Dating

Discovery of Radioactivity

In 1896 Henri Becquerel and Marie Curie discovered that certain isotopes undergo spontaneous radioactive decay, transforming into new isotopes. Atoms of a parent radioactive isotope randomly decay into a daughter isotope. Over time the number of parent atoms decreases and the number of daughter atoms increases. Rutherford and Soddy (1902) discovered that the rate of decay of a radioactive isotope depends on the amount of the parent isotope remaining. Later it was found that half of the parent atoms occurring in a sample at any time will decay into daughter atoms in a characteristic time called the half-life.

It was also learned that elements may have various numbers of neutrons in the nucleus, thereby changing the mass of each atom. These mass variants are called isotopes. Most carbon atoms have six protons and six neutrons for a mass of 12. A small percentage of carbon atoms have six protons and six neutrons for a mass of 13 (carbon 13). Others have six protons and eight neutrons for a mass of 14 (carbon 14). Carbon 12 and carbon 13 are stable isotopes of carbon while carbon 14 is unstable making it useful for dating organic materials.

Radiometric Dating

The duration of a half-life is unique for each radioactive isotope. Some examples: the half-life for the decay of potassium 40 atoms into argon 40 atoms is about 1.3 billion years, the half-life for the decay of uranium 238 into lead 206 is about 4.5 billion years, and the half-life for the decay of carbon 14 into Nitrogen 14 is 5730 years.

Many minerals are formed with small quantities of radioactive isotopes. For example, uranium is a common impurity in the mineral zircon. Most of the potassium atoms in potassium felspars are stable potassium 39, but a small percentage are unstable potassium 40.

One half-life after a radioactive isotope is incorporated into a rock there will be only half of the original radioactive parent atoms remaining and an equal number of daughter atoms will have been produced. The ratio of parent to daughter after one half-life will be 1:1. After two half-lives, half of the remaining half will decay, leaving one-quarter of the original radioactive parent atoms. Those transformed atoms bring the tally of daughter atoms to three-quarters of the crop of parent plus daughter atoms. The ratio of parent to daughter atoms after two half-lives is therefore 1:3 (one-quarter to three-quarters). Successive half-lives reduce the original parent to one-eighth, one-sixteenth, one-thirty-second, and so on. The ratios of parent to daughter isotopes for these are 1:7, 1:15, 1:31.

So assuming that when a rock forms it contains an unstable isotope and none of the daughter isotope (or a well-known amount), and assuming that over geologic time the rock remains a closed system (no parent or daughter enters or leaves the rock), then that rock can be accurately dated by determining the ratio of parent to daughter atoms. The first time this was done was by B.B. Boltwood in 1907, only eight years after the discovery of radioactivity.

Oldest Rocks: The oldest rocks known on the Earth are about four billion years old. The oldest samples from our solar system (moon rocks and meteorites) are 4.5 to 4.6 billion years old. Radiometric dating of igneous rocks contained in sedimentary sequences have enabled geologists to assign ages to the geologic timescale that was originally based entirely on relative geologic time.