The age of the Earth was a matter of speculation till very recent times and as such there was divergence of opinions about the antiquity of the Earth. Until recently geology relied extensively on the concept of the relative age of rocks.
It was only with the discovery of radioactivity that a new method giving an approximate age, with comparatively less chances of errors, was found.
The determination of the age of the earth was attempted through two distinct processes?
1. Indirect methods for ascertaining the earth’s age, and
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2. Radioactive methods for determining the actual age (Direct- method).
1. Indirect Methods
(a) Sedimentation-Clock
This takes into account the average annual rate of sedimentation and the thickness of all strata deposited during the whole geological history.
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Although this method is full of imperfections and variations from place to place, the age of the Cambrian as determined by this and other sophisticated methods closely approach each other.
According to this method, there is an average rate of deposition of 30 cm of sediments in 755 years. As such the beginning of Cambrian sedimentation come to about 510 million years.
(b) Salinity Clock
Joly and Clarke took into account the rate of accumulation of sodium in ocean-water for determining the age of the Earth. The yearly rate of increase in the sodium content (salinity) of sea water per unit volume can be determined from direct observation and the content of sodium per unit volume of ocean water can also be calculated.
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Accordingly, the total amount of sodium in the ocean water can also be determined from the total volume of ocean water.
This method has established that the Earth was formed about 100 million years ago. But this method also suffered from a lot of imperfections, for e.g. the rate of supply of sodium to ocean is not uniform throughout the geological history.
(c) Evolutionary Changes of Animals
The evolution of life has proceeded since the first forms of life appeared on earth. As we know, the first formed animals were, unicellular which underwent various phases of the evolutionary processes and multicellular organisms with more complexities came into being. Man is, accordingly, considered to be the most evolved one.
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Evidence of the process of evolution, is found in the form of fossils, (preserved parts of animals and impressions of plants and animals). Taking into account the evolutionary developments biologists have estimated the age of the earth to be about 1,000 million years.
(d) Rate of Cooling of the Earth
Kelvin estimated the age of the earth on the basis of his study of the history of codling of the earth. Since temperature increases with depth, Kelvin assumed that the earth began as a molten mass and has been cooling ever since i.e. the earth is progressively losing heat.
Assuming the initial temperature of the earth to be 3900°C, Lord Kelvin estimated the age of the earth to be 100 million years and later on revised it to be between 20 to 400 million years.
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This method of estimation is full of imperfections and one of the most important consideration regarding the generation of radio-active heat was not taken into account in this method.
(e) Varved Sediments
The term varve is applied to glacial-lake deposits containting rhythmically laminated sediments of clay and silt. The silt and clay laminae occur alternate with each other in a regular pattern. The couplet of beds is recognized as representing a year, like the annual rings of a tree, and is called a varve.
It is presumed that these so called varved clays accumulated in lakes as a result of thawing of glacier-ice; the fine bands having been deposited in winter and the coarser ones, in summer.
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A varve is usually about 10 mm in thickness and occasionally it ranges up to 300 mm. By counting the number of pairs of bands in a given section, the time represented by the section can be ascertained in years. Geologic time ranging from ‘0’ to 10,000 years could be counted on this varve clock.
Apart from the above indirect methods, other attempts for estimating the age of the earth were made on the basis of contraction of the earth’s surface due to cooling, by the rate of limestone deposition etc.
Even, on the basis of the concept of separation of the moon from the body of the earth, Charles Darwin estimated the age of the earth to be 57 million years.
2. Radioactive Method
The discovery of radioactivity brought a new and apparently precise concept of measuring the age of the earth. Radioactivity, as we know, consists in the spontaneous disintegration of the nucleus of unstable elements and their transformation into stable isotopes or new elements.
The disintegration is accompanied with the emission of alpha particles (i.e. helium nuclei), beta particles (electrons) and energy in the form of gamma radiation. The basic principle underlying all the radioactive methods is that “a radio-active parent element decays into a stable daughter element at a constant rate”.
The radioactive decay is usually expressed in terms of “half life period’. A half-life period is the time required for one half of an original amount of a radioactive element 10 disintegrate.
For geological purposes the unit of time is one year. The relationship between the half life period and the rate of decay has been found out to be as:
T= 0 .693/ where T = half- life period ,X = rate of decay
Usually the ‘half life period’ is determined and accordingly it is equated to find out the age of the earth.
The following are some of the common methods used for the purpose of determining the age of earth:
(a) Uranium-lead method
Here two isotopes of uranium are used, U238 and U235. The chemical element uranium, a heavy metal of atomic weight 238 spontaneously gives away X-rays and atoms of the helium gas and is ultimately converted into the chemical element-radium of weight 236. This element too continues emitting helium atoms till it is reduced to lead of atomic weight 206.
U238_____> Pb206+ 8He4
The half life of Uranium 238 is 4500 million years. One gram of uranium-238 will produce 1/7600,000,000 gram of stable lead.
Similarly the second isotope U235 undergoes spontaneous disintegration and ultimately gives rise to lead of atomic weight207.
U235——— > Pb207 + 7He4
The half life of U-235 is 713 million years.
Potassium argon method Potassium, an element present in many minerals and rocks, have three isotopes K3‘, K40 and K41. Only K40 is radioactive.
The radioactive transformation of K-40 consists in absorbing the electron by the nucleus with the nearest to its electronic shell. Thus, a radiogenic stable isotope of argon with exactly the same atomic weight (Ar40) is
(b) Thorium lead method
Thorium-232 through radioactive disintegration gives rise to lead 208. The half life period, in this case is 13,900 million years.
Th232——-_>Pb208 + 6He4
(c) potassium argon method
Potassium, an element present in many minerals and rocks, have three isotopes K39, K40 and K41. Only K40 is radioactive. The radioactive transformation of k40 consists in absorbing the electron by the nucleus with the nearest to its electronic shell.
Thus, a radiogenic stable isotope of argon with exactly the same atomic weight (Ar 40) is formed. Only 12.4% of K40 through this process is converted to Ar-40. The half life period, here is 11900 million years.
K40 ……..>electron capture………> Ar*40
About 87.6% of K40 through emission of beta particles get converted into Ca40 (calcium-40) with a half life period of 1470 million years. But this method does not find much application in the determination of the age of earth.
It is mainly due to the fact that most common rock forming minerals already contain so much of primary calcium (Ca40) that the comparatively minute amounts of radiogenic Ca-40 cannot be determined.
(d) Rubidium Strontium method
This method is based on the radioactive decay of Rb87 and its transformation in to Sr87. In minerals Rb-atoms are associated with its decay product as well as the common Sr86. Natural rubidium is having two isotopes Rb85 and Rb87. Rb87 is radioactive.
The ratio of Sr87 to Sr86 can be determined with great accuracy so also the ratio of Rb87 to Sr87. It is therefore, this method is more reliable for metamorphic rocks. The half life period is 50,000 million years.
Rb 87……….> Beta particles ………>>> Sr87
(e) Radio Carbon Method
Cosmic rays, at the upper atmosphere, change nitrogen (N-14) to Carbon (C-14), an isotope of carbon ‘C12‘. Once formed carbon-14 is quickly disseminated through the atmosphere and reacts with oxygen to form C02 which is absorbed by all living matter.
It has been observed that a constant level of this isotope (i.e C-14) is maintained by all living organisms. At death, the organism stops absorbing the carbon-14 and the carbon-14 present in the organism starts decreasing at a constant rate.
The half life period of C-14 is about 5,730 years. Since the half life of radioactive carbon and its present content are known, it is possible to establish the time when the organism died and, therefore, the age of the rock. This method is especially useful for dating relatively recent materials up to 70,000 years.
CONCLUSION
The best means of estimating the age is provided by the Uranium- lead, Rubidium-strontium and Potassium-argon methods.
On the basis of the data provided by the radioactive methods described above, the age of the earth has been estimated to be of 4500 million years i,e 4.5 billion years.