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Determining fossil's age

QUESTION: How is the age of fossils determined?

G.R. Seshadri, Ariyalur, T.N.

ANSWER : Dating of fossils is known as finding out of the age of fossils. The age of fossils indicates the age of the strata and the rock from which it is collected by using radioactive isotopes. This is called Radiometry or Radioactive clockmethod. The main principle underlying this method is that many rocks have natural clocks in the form of radioactive isotopes and these radioactive isotopes changes into stable elements slowly, and at a steady rate. Eg. one gram of radio active element changes into half gram in a particular duration of time. This duration is called half life of the element.

Half life is the time taken for the activity of the radioactive element to decay to one half of its original value. This duration of half life varies from element to element. The most common radioactive elements used for dating the fossils are Uranium, Carbon14, Pottassium, Rubidium.

In Lead method Uranium238, slowly disintegrates in to Lead206 and Helium. Here 1 gram of Uranium disintegrates into half gram of Uranium and another half gram of Lead206 and Helium. Here the half life period of Uranium is 4.51 billion years. The age of rock or fossil is determined by estimating the ratio of Uranium and Lead. This has three limitations.

- Uranium is not a common element.

- it can be used only for old rocks.

- it is not known that the Lead produced is lost or retained in the aquatic rocks.

Carbon method

The living organism contain a definite amount of carbon in bones and tissues as C14. When the animal dies the C14 disintegrates into C12 at a constant rate. The dating method cannot be used for the fossils which are more than 70,000 years old. Hence the age of recent fossils can be calculated accurately and sensitively with 30 years plus or minus by this method. Another limitaiton in this method is the fossil should contain at least some of the organic materials.

Fission track method:

This method is used when rocks contains glass or crystal. When Uranium 238 is present in the rock, it undergoes spontanious fission. This radiation causes the appearence of tracks on the rocks. When such a rock is etched with Hydrofluoric acid, the tracks are revealed and the tracks can also be seen with a microscope and counted. From the count the age of the glass or rock can be computed. This method may be used for glass as young as 2000 years.

Sangeetha Manoj, Thiruvananthapuram

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