Dating  the Past:

Today, that  a particular  rock  is a million ycars old, or a fossil is 50 thousand ycars old. For  instance, how do we estimate the duration of various geological periods on a time scale of  billions of years?  Initially, such estimates were based on the rate at which  geological processes such as depositions of stones and rocks occur. Many layers of  rock  originated as deposits in  the sca~and  at the mouths of rivers. Therefore. one way  to get an idea of  the length of  time required  to produce a deposit of  a given  thickness,  is to measure the rate at which  rivers arc depositing sediments in  thc sca today. With the dcvcloping knowlcdgc of radioactivity. more accurate dating of fossils and certain  types of  rocks has become possible. Radioactive substances can be easily detected using certain instruments. They have built-in "clocks"  in  the form of 'radioactive isotopes'  that change or decay at a constant rate into non-radioactive form. If this rate is known, the length of time since the fossil or the rock was formed can be estimated by measuring  the quantities in  the rock, of  the radioactive  isotopes arid  the non-radioactive ones into which  they have changed. For  example, Uranium  is transformed  into certain isotopes of  lead which are not radioactive. So, the age of uranium-containing rocks can be determined  by  comparison of  the proportions of undecayed Uranium and that of  the corresponding lead isotopes present  in  the rock. see Fig. The method most commonly used, now, for estimating the age of  fossils is radio-carbon dating. Radioactive isotope of  carbon is commonly known as carbon- 14. Since carbon- 14  is chemically the same as ordinary carbon, both arc absorbed by  plant and animal tissues in  the same proportion  as they arc present  in  the atmosphere as carbor. dioxide. Plants use this carbon dioxide  in  making  their food. Animals eat the plants. Hence, the proportion  of carbon- 14 in  the tissues of plants and animals is the same as in  the atmosphere, as long as the plant or the animal  is  living. But as soon as it dies, no  

more carbon can enter its body as photosynthesis or food intake stops. Following death, the carbon-14, already present in the body, decays steadily into ordinary carbon. So  the smaller the number of  carbon-14 atoms remaining, the older is the fossil. Thus, if  we take a piece of  ancient wood or bone and measure the amount of  carbon-14 present  in it, we can estimate the age of the material. This technique has been applied to materials of known age, and thus its accuracy was tested giving confidence in  the age determinations of  unknown samples. The carbon-14 method  is applicable only to organic materials which still contain carbon. It cannot be used for fossils in which all organic matter has decayed. In that case the age of the fossil can be estimated by determining the presence of other radioactive elements like flourine, or phosphorus.