The age of the earth as determined by current methods of radiometric dating is about 4600 million years. Before the discovery of radiometric dating several attempts were made to determine the age of the earth. The important one’s are as follows.
- From history of organic evolution.
- From rate of sedimentation.
- From salinity of sea water.
- From rate of Cooling.
From History of Organic Evolution:
As soon as the earth cooled and solidified, life speared on it. Initially the organisms had a simple body structure. The complexity increased with the passage of time. Man which has evolved quite recently, possesses the most complex structure. From the history of organic evolution, the biologists have determined the age of the earth to be about 1000 million years.
From Rate of Sedimentation:
The formation of sedimentary rocks started with the solidification of the earth. Hence efforts were made to calculate the age of the earth from the rate of sedimentation in the ocean. The method involved was to determine the total thickness of sedimentary rocks that had been deposited during the earth’s history and then to divide it by the rate of sedimentation. This method gave an age of around 500 million years. This method had many difficulties some of which are as follows.
- Different sediments accumulate at different rates under varying conditions.
- A rate of sedimentation determined for recent sediments can not necessarily be applied to the past.
From Salinity of Sea Water:
It is assumed that the oceans had originally had fresh water. If the amount of salt carried to the ocean each year by rivers is known, it can be calculated how much time it took to accumulate the salt now present in the ocean. This method gave an age of earth about 120 million years. this estimate was not accurate as the rate of accumulation of salts could never been uniform all the the history of the earth.
From rate of Cooling:
In 1897, the British physicist Lord Kelvin calculated the age of the earth from the temperature difference between the initial molten planet and its present state. He assumed that the rate of heat loss was constant throughout the history. This method gave an age of 20 to 40 million years which was more than 100 times lower than current estimates.
Kelvin also made estimate based on assumptions concerning the origin of the sun’s heat. The age of the earth calculated by this method gave similar results, that is the figures ranged from 20-40 million years. Kelvin’s estimates for the sun and the earth were wrong because he did not take into account the fierce heat generated by the decay of radioactive elements.