Radiometric dating is one of the most accurate methods of absolute age determination of rocks and the fossils preserved in them.
The radio dating method was invented after radioactivity was discovered by Henry Becquerel, a French Physicist, in 1896. He found that certain natural ores can spontaneously generate a mysterious form of energy that affects photographic plates, even though the materials are opaque to light. It was later found that these ores are radioactive minerals and the mysterious form of energy they release is due to the phenomenon of radioactivity.
Atoms, Isotopes and Radioisotopes
The atomic structure of an atom consists of a central nucleus with an electron rotating around it. The nucleus is made up of protons and neutrons. The number of protons determines its atomic number and the sum of protons and neutrons is its atomic weight.
In nature, some elements occur as isotopes, ie, they have the same atomic number but different atomic weights. As their atomic numbers are the same, their chemical properties are also the same. For example, the normal carbon atom C12 has 6 protons and 6 neutrons whereas the isotopic carbons C14 and C13 have 6 protons and the number of neutrons is 8 and 7 respectively.
In addition to isotopes, radioisotopes also occur for certain elements. A radioisotope is one whose atoms tend to emit one or the other of the three particles such as alpha, beta, or gamma as radioactive waves. Radioisotopes are sometimes referred to as geologic clocks.
A radioisotope is always unstable and hence may transform into another stable isotope. This transformation from an unstable radioisotope to a stable isotope is termed radioactivity, which provides the basis for radioactivity. Radioactivity is therefore a phenomenon in which a radioisotope transforms into a stable isotope by emitting particles such as alpha, beta, or gamma, during which, energy is released in the form of heat energy.
The fundamental basis of radioactivity is the constant transformation rate of radioisotopes into stable isotopes. The rate of disintegration or transformation of radioisotopes is commonly known as half-life. The half-life of radioisotopes is called constant and is defined as the time required for any given quantity of radioactive material to be reduced to one-half.
Radiometric Dating of Rocks
Different kinds of radioisotopes are incorporated in igneous rocks at the time of their formation. Soon after the formation of these rocks, the radioisotopes start to disintegrate due to the phenomenon of radioactivity. It is possible to ascertain the absolute age of such rocks.
In other words, the time interval of the years since the radioactive material was incorporated into them can be determined. It is calculated by determining the ratio of daughter material to parent material. The age of the rocks can be determined by comparing the ratio with the half-life of the radioisotope.
The Method of Radiometric Dating
The most commonly found radioisotope in nature is Uranium, U238, which disintegrates and gives rise to a stable isotope of lead, Pb206. The half-life of U238 is 4510 million years.
It has been experimentally determined that 1 gm of U238 produces 1/7600000000 gm of Pb206, annually.
In that case, U grams of U238 will produce U/7,600,000,000 gm of Pb206 annually.
U grams of U238 will be produced in “t” years will be
U x t/ 7600000000 of Pb206
t = Pb/U x 7,600,000,000, where
t = absolute age or time
t = Daughter material x 7,600,000,000 / Parent material.
The above formula was used for determining the absolute age of uraninite crystals from Branchville, Connecticut in the USA, in which the ratio of Pb/U was equal to 0.050 ( Daughter material: Parent material = 1:2).
Hence, the geological age of the crystals or t = 0.050 x 7,600,000,000
= 380,000,000 or 380 million years
Thus using the radiometric dating method, it was found that the uraninite crystals were incorporated into the rocks 380 million years ago.
The following list shows the various rocks and minerals, which contains radioisotopes (parent material) with their half-life in million years and their disintegration product (daughter material).
| Parent material | Half-life in a million years | Daughter material | Minerals and rocks commonly dated |
| Uranium 238 | 4510 | Lead 206 | Zircon, Uraninite, Pitchblende |
| Uranium 235 | 7100 | Lead 207 | Zircon, uraninite, Pitchblende |
| Thorium 232 | 13900 | Lead 208 | – |
| Rubidium 87 | 47,000 | Strontium 87 | Muscovite, Biotite, Lepidolite, Gluconite |
| Potassium 40 | 1300 | Argon 40 | Sanidine, Whole volcanic rocks, Muscovite, Biotite, Homblende. |
The Potassium-Argon Transformation
Potassium minerals are common in both sedimentary and igneous rocks. Potassium-40 (K-40) can decay by emitting or capturing an electron. If it picks up an electron it becomes Argon 40 (Ar 40). If it sheds a beta particle it becomes Calcium 40 (Ca 40). The half-life of the transformation of Potassium 40 to Argon 40 is 1300 million years.
Carbon 14 (C14) Method for Radiometric Dating
The radiometric dating of fossils, using C14, was developed by W F Libby, a physicist from Chicago, USA. He earned the Nobel Prize for this achievement.
In this method, the age of the given fossil is determined directly using the fossil itself. It is based on the fact that all past organisms originally contained a constant amount of C14. It is known that two carbons exist in nature. One is ordinary carbon (C12) which is stable and the other is radioactive carbon (C14).
Formation of C14
This radioactive isotope, C14, is created continuously in the atmosphere by the bombardment of nitrogen-14 with neutrons derived from primary cosmic radiation. Neutrons are captured by nitrogen according to the equation,
14N + n …………….> 14C + H
The carbon (C14) thus created joins with oxygen to form CO2 enriched with C14. This carbon dioxide is incorporated into plant materials through the photosynthesis process. Animals in turn incorporate radiocarbon into their tissues by eating these plants.
As long as a plant or animal is alive a balance is maintained between the radioactive C14 and normal C12. The proportion of these carbons in the tissues will be the same as in the atmosphere. For every atom of C14, there are 1,000,000 million atoms of C12 in all living beings that absorb both forms of carbon. New C14 is added as fast as the old disappears.
How is the C14 method used for radiometric dating?
When an organism dies, however, there is no further addition of C14. The C14 ‘clock’ gradually runs down by giving off beta particles and reverts to nitrogen.
As a result, the proportion of C14 to C12 in the tissues is disturbed. The half-life of C14 is 5568 years. The amount of C14 remaining after 4000 years in a fossil is so small that it cannot be measured accurately. Hence the C14 method is used for radiometric dating of fossils that are less than 4000 years old.
The age is determined by comparing the ratio of C14 remaining in the sample to the amount of C14 present in modern wood or tissue. Only Holocene events can be dated by this method.
Discovery of the C14 Method
The most effective dating range of the fossils to be used with the carbon-14 method is 100 to 50,000 years.
The radiometric dating through carbon 14 was first tested with dated Egyptian mummies and on tree rings from the wood of known age formed within the last 5000 years or within the time of recorded human history. Most of the results derived were amazingly accurate.
