Factors for calculating the actual Age of the Earth

Even though there is a divergence of opinion among the profounder of the various theories on ‘the origin of earth’; it is almost unanimously believed that earth took its birth in a hot, gaseous, molten state. Different criteria have been used and various factors have been used to determine the age of the earth. The two distinct processes of estimation are as follows:

1. Dating of Geological-formations and geological-events by indirect methods (relative age).

2. Radio-active methods for determining the actual age.

1. Indirect methods:

It includes various processes of determining the age, like varve-clocks, sedimentary-clocks, salinity-clock, etc. Besides, attempts were made to determine the age of the earth from the rate of cooling of the earth, from the evolutionary changes of animals etc.

(a) Varve-clock method:

It uses the fine sedimentary deposits of glacial origin, which represents the annual accumulation of the layers marked by variation in colour and gradation in the size of materials constituting the layers.

Geologic time ranging from ‘0’ to 10,000 years only could be counted on this varve clock, where varve occurs as in Kashmir Himalayas.

(b) Sedimentation-clock:

In this case the average annual rate of deposition of sediments and the thickness of all strata deposited during the whole geological history are taken into account. Although this method is full of imperfections and variations from place to place, the age of the Cambrian as determined by this and other accurate methods closely approach each other.

Here it is accepted that there is an average rate of deposition of about 1 ft. per 755 years; thus the beginning of the Cambrian- sedimentation comes to about 510 million years.

(c) Salinity-clock:

All the salts of the seas of today have been acquired from the land by its weathering and erosion through ages. By determining the yearly rate of increase of salinity the age of the earth was calculated to be 100 million years by Joly.

(d) Rate of cooling of the earth:

Assuming the initial temper­ature of the earth to be 3900°C, Lord Kelvin determined the age of the earth to be between 20 to 400 million years. But, it was full of imperfections, since the generation of radio-active heat and other allied factors were not taken into account.

(e) Evolutionary changes of animals:

As we know, the first- formed animals were unicellular which underwent various phases of the evolutionary processes and multi-cellular organisms with more complexities came into being gradually and man is considered to be the most evolved one. Taking into account the evolutionary developments, Biologists determined the age of earth to be 1000 million years.

2. Radioactive methods:

The basic principle underlying all the radioactive methods is that “a radioactive parent element decays into a stable daughter element at a constant rate”. For geological purposes the unit of time is one year. Usually the “Half-life” period is determined and accordingly it is equated to find-out the age of the earth. The following’s are the common methods which are used for the purpose of determining the age of earth:

(a) Uranium leads method:

Here two isotopes of uranium can be used, U238 and U235; the half-lives of which are 4498 million years and 713 million years respectively.

(b) Thorium-lead method:

Thorium (232) yields lead (208) through radio-active decay and the half-life of thorium is 13,900 million years.

(c) Potassium-argon method:

Potassium is having three iso­topes like 39 K, 40 K and 41 K. But only 40 K is radio-active. The half-life of 40 K to convert itself into 40-Ar is 11,850 million years.

(d) Rubidium strontium method:

Rubidium (Rb87) yields strontium (Sr87) and its half-life period is about 50,000 million years.

It is a particularly valuable method for metamorphic rocks and for pre-Cambrian rocks.

(e) Radio-carbon method:

The carbon (C14) is radio-active, with a half-life of 5570 years. All organisms take in C14 and a constant level of this isotope is maintained by all living organisms.. At death the C14 intake ceases and its proportion decreases at constant rate. This method is especially useful for dating relatively recent materials up to 70,000 years.

The application of radio-active methods for the determination of the age of earth has been successful in the estimation and the age of the earth, accordingly, comes to about 4500 million years, i.e., 4.5 billion years.