How Scientists Calculate the Age of Earth

Like the complex and mysterious problems of the origin of the solar system and the earth there is a lot of difference of opinions about the age of the earth and its past geological history. Various scientists have attempted to calculate the age of the earth on the basis of different scientific basis, experimental researches and logical arguments but their findings, results and conclusions are so varied and contrasting that it be­comes very difficult to arrive at convincing conclusion which may be acceptable to the majority of scientific community.

In fact, it is quite difficult task to find out the exact time of the origin of the earth and periods of its evolutionary stages. It may be argued that geologi­cal processes work so slowly that no one can observe them fully during one’s lifetime.

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This is why Scottish geologist James Hutton opined in 1775 that ‘the earth’s surface undergoes frequent changes but these changes consume such a long time that it becomes difficult for the man to find out the dates of such changes.’

In fact, the dating of the past geological events is ‘wild catting’ as there is ‘no vestige of a beginning, no prospect of an end’ (James Hutton). Inspite of the fact that no univer­sally acceptable method could be evolved till now to calculate the age of the earth and its geological events, several methods and concepts have been propounded from time to time and are still being proposed to unravel the mystery of the earth’s history.

It is, there­fore, desirable to discuss some of the prevalent con­cepts and methods to have some generalized under­standing of the chronological sequences of the earth.

Religious Concept:

Serious efforts have been made since time im­memorial by the religious priests, philosophers and astrologers to calculate the age of the earth. Old Iranian religious priests calculated the age of the earth as 1200 years while Archbishop Usher of Iceland in 1664 maintained that the earth was created at 9.00 A.M. October 26, 4004 B.C. (presumably Greenwich mean time) but these concepts cannot be accepted because these only make a fun of the age of the earth. According to Indian religious records the age of the earth has been determined as 2 billion (2000 million) years.

The following version of Karmakand gives some idea about the age of earth:

Indian religious experts have calculated the age of the earth on the basis of the interpretation of the above statement as 1,972,949,032 years (about 2.0 billion years). This calculation is based on guess work and deduction but it nearly matches with the calculations based on scientific and mathematical methods but the result (2 billion years) is not closer to commonly accepted age of the earth (4 to 5 billion years).

On the Basis of Oceanic Salinity:

Present-day oceanic waters contain salt content but it is believed that the oceans at the time of their creation would have contained pure water that is water without salt content. Later on rainwater after passing through continental surfaces removed salt contents from the land areas due to subaerial erosion and thus terrestrial salt used to reach the oceans through the rivers and thus oceanic water began to become saline.

With the passage of time oceanic salinity continued to increase. It has been generally established on the basis of experiments and observations that about 60 per cent of sodium of the oceanic salt is contributed by the rivers. It has also been demonstrated that there is more or less similarity between the oceanic salt and the salt brought down by the rivers. It has been believed on this basis that rivers are the major source of oceanic salin­ity.

Thus, there is gradual increase in the oceanic salinity because of deposition of terrestrial salt by the rivers in the oceans every year. If the total amount of oceanic salt is known and if the annual rate of increase of oceanic salinity is determined, the age of the oceans may be calculated and determined.

Thus, the age of the oceans = total oceanic salt/annual rate of oceanic salinity

(1) Joly has calculated, on the basis of a series of experiments, the total amount of salt of all the marine waters to be 1.26. x 1022 grams. He has further main­tained that about 1.56 x 1014 grams of salt are derived from the land areas and are deposited in the oceans every year.

Thus, on the basis of data of oceanic salinity as provided by Joly the age of the oceans can be calculated as follows:

Age of the oceans = 1.26 x 1022/1.56 x 1014= 80,000,000 yrs

It appears, on the basis of the aforesaid calcula­tion, that world oceans were created about 80 million years ago, so the earth might have been originated much earlier to the origin of the oceans but question arises, how many years ago? There is no unanimity about the probable answer to this question. Some scientists believe that the oceans were created at least 40 million years after the origin of the earth.

If we accept this corollary, then the age of the earth becomes 120 million years but this calculation is totally false because the earth may not be so young. It may be pointed out that 200 million years old rocks of the oceanic crust have already been dated on the basis of the study of palaeomagnetism.

(2) A few scientists have tried to demonstrate correlation between the deposition of salt in the oceans and periods of mountain building at global scale. According to them the amount of salt brought to the oceans by the rivers would have not always been the same. The amount of salt brought by the rivers through erosion to the oceans would have gradually decreased due to continuous decrease in the rate of denudation of the mountains because of continuous lowering of their height consequent upon continued sub-aerial erosion.

The rate of deposition of salt in the oceans would have again increased after the creation of new mountains during the next period of global mountain building. Based on this premise, the advocates of the above concept have determined the age of the oceans to be 1500 million years. They have further assumed that the earth was originated before about one quarter of the age of the oceans (i.e. 375 million years). Thus, the age of the earth may be calculated as 1500+375 =1875 million years.

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It may be pointed out that this method of the determination of the age of the earth is not without faults. It is very difficult rather impossible task to measure total amount of salt in the world oceans and to determine the annual rate of salt deposition in the oceans. The rate and amount of sub-aerial erosion is neither equal everywhere nor is the same every year, rather it varies both spatially and temporally.

Thus, the rate of deposition of salt in the oceans may not be same every year. It is erroneous to believe that the land areas are the only source of oceanic salinity. It has been established that thermal convective currents bring salt to the oceanic crust which, thus, also contributes to the oceanic salinity.

On the Basis of Sedimentation:

There are several methods for the calculation of the age of the earth on the basis of the formation of sedimentary rocks and their period of formation. The first igneous rocks were formed due to cooling and solidification of hot and liquid magma and lava after the origin of the earth.

These igneous rocks were disintegrated and decomposed and thus the resultant sediments of various sorts were transported and depos­ited by denudational agents (geological agents e.g., rivers, wind etc.) into water bodies to form first sedi­mentary rocks on the earth’s surface.

Since then the processes of sedimentation and the formation of sedi­mentary rocks continued throughout geological peri­ods and are still operative. Continuous sedimentation resulted into thickening of sedimentary rocks and thus their thickness continued to increase. If we can find out the total thickness and annual rate of deposition of sedimentary rocks, then we can calculate the age of the formation of the first sedimentary rocks on the earth’s surface and by applying common sense the age of the earth may be roughly estimated.

Age of first sedimentary rocks = total thickness of sedimentary rocks/annual rate of deposition  

Various scientists have attempted to calculate the age of the earth on the basis of above method but their results are not compatible because of variations in the thickness of sedimentary rocks at different places.

A few calculations are presented below:

(1) The stone statue of Ramses II was found buried under 9-foot thickness of sediments in Egypt (now U.A.R.) in the year 1854. The statue of Ramses II was installed about 3000 years before it could be discovered in 1854. Based on this fact it may be safely argued that 9-foot thick sediments were deposited in 3000 years. Thus, the annual rate of sedimentation may be calculated. It is believed that the known depth of sedimentary rocks on the earth’s surface is about 100 miles (160 km).

The following calculation may be made to compute the age of the first sedimentary rocks:

9-foot deposition = in 3000 years

3-foot deposition = in 1000 years

total depth of sedimentary rocks = 100 miles or 528,000 feet

3-foot deposition = 1000 years

528,000-foot deposition = in 176 million years.

Thus, the age of the first sedimentary rock is determined to be 176 million years. It may be pointed out that the sedimentary rocks were formed at much later date from the time of the origin of the earth. If the age of the earth may be taken 3 times to that of the age of the first sedimentary rock then the age of the earth may be estimated at roughly 500 million years but this age is much lower than the expected age of the earth and hence this method is not acceptable.

(2) Attempt has been made to calculate the age of the earth on the basis of sedimentary deposits in the valleys of Colarado and Wyoming of the USA. It has been estimated that half mile-thick sediments were deposited in about 6.5 million years.

Based on this estimate the following calculation may be made to find out the age of the first sedimentary rocks:

1/2 mile thick deposits = in 6.5 million years

100-mile thick deposits = 1300 million years

If the age of the earth is taken to be 3 times to that of the age of the first sedimentary rocks then the age of the earth becomes 3900 million years or 3.9 billion years. This calculation more or less gives fairly good idea about the age of the earth which is generally accepted to be in the range of 4 to 5 billion years.

Some scientists argue that the age of the earth may not be more than double the age of the first sedimentary rocks. If we accept this connotation, then the age of the earth is calculated as 2600 million or 2.6 billion years. This calculation nearly matches with the result of ‘radioactive mineral method’ according to which the age of the earth has been estimated at 2000 million years.

On the Basis of Erosion:

Some scientists have attempted to calculate the age of the earth on the basis of the rate of erosion of the land areas. This method is based on this belief that the continental areas are regularly eroded by the exog­enous or denudatonal processes every year. If we can find out the total amount of denudation of the surficial materials till now and the annual rate of denudation, then the age of the earth can be estimated.

It has been generally believed that one-foot thick surface of the earth is generally eroded down in about 10,000 years. It is also true that the eroded sediments are deposited by the fluvial processes as sedimentary rocks. The known thickness of sedimentary rocks is about 100 miles (528,000 feet).

Thus, based on above facts the following calculation can be made:

1-foot erosion = in 10,000 years

528,000 feet erosion = in 5280 million years

(100 miles) (5.28 billion years)

If we take the age of the earth to be double of the age of the thickness of deposited sediments derived through continuous denudation of land areas, then the age of the earth may be estimated at 10,560 million (or 10.5 billion) years. This estimation is not acceptable because the age of the earth may not be so much in any case. This method also suffers from numerous inherent defects. The rate of erosion varies both spatially and temporally.

In fact, the rate of erosion of surficial materials is affected by such a host of environmental factors (e.g., lithology, structure, relief, vegetation, gradient, effi­ciency of denudational processes, climate and so on) which also vary from one place to another and from one climatic region to another region. Thus, it is very difficult, rather impossible task to determine the rate of erosion.

On the Basis of Tidal Force of the Moon:

The age of the earth, based on tidal force of the moon, is calculated in a variety of ways. First method- It is commonly believed that the moon was originated from the earth because it is her satellite as the moon revolves around the earth (now this old concept has been refuted as many scientists claim that the moon is not the satellite of the earth).

If this is so, the moon might have been very close to the earth at the time of its birth from the earth and the tidal friction of the moon might have been maximum. With the passage of time the moon gradually moved away from the earth and hence the tidal friction of the moon also gradually decreased.

The age of the moon and ultimately the age of the earth is calculated on the basis of the rate of decrease of the tidal friction of the moon. The scientists have calculated the age of the earth, on this basis, as 4000 million (4 billion) years. Second method-It is believed that the rotational force of the earth is reduced due to tidal friction of the moon. In other words, the time of the rotation of the earth increases (due to decrease in the speed of earth’s rotation) due to tidal friction of the moon.

Thus, on the basis of the calcula­tion of the tidal friction of the moon and the change in the speed of the rotation of the earth it has been concluded that the moon moves away from the earth at the rate of about 13 cm per year. The present distance of the moon from the earth is 3,84,000 km. Thus, the moon would have taken 2,953,846,000 years to move 3,84,000 km away from the earth. On this basis the age of the earth has been estimated as 4000 million (4 billion) years.

The Concept of Lord Kelvin:

Though Lord Kelvin proposed scientific base for the calculation of the age of the earth but now his concept is not tenable. He has suggested to calculate the age of the earth on the basis of the rate of the cooling of the earth. On an average the temperature increases with increasing depth of the earth at the rate of 1° C per 32m.

Thus, there is very high temperature in the core of the earth. Consequently, there is continuous transfer of heat from the core of the earth to its outer cells from where the heat is lost to the atmosphere through the mechanism of radiation. The amount of lost heat from the earth through radiation can be determined on the basis of underground temperature gradient, thermal conductivity of the crustal rocks and the surface area of the earth.

The time of the solidification of the earth’s crust can be found out on the basis of the rate of the loss of heat from the earth’s crust. Based on above considerations Lord Kelvin assumed that the whole earth was solidified at the temperature of 7,000° F.

Thus, the outer part of the earth was solidified on cooling about 40,000,000 years ago. Lord Kelvin, thus, estimated the age of the earth as 40 million years but this little age of the earth cannot be accepted. Moreover, this method also suffers from certain defects.

It may be pointed out that radioactive elements were not discovered at the time of the postulation of Kelvin’s .concept of the determination of the age of the earth. Heat is generated due to disintegration of radioactive miner­als and thus the heat of the core of earth is increased.

Lord Kelvin did not consider this in his concept and thus his concept is not acceptable.

On the Basis of Radioactive Elements:

The method of the calculation of the age of the earth on the basis of radioactive elements has gained maximum success in the modern scientific world because this method is more convincing than other methods. Radioactive elements generate heat after their disintegration.

This fact was first discovered by Pierre Curie in the year 1903 while renowned scientist Rutherford presented his scheme of the calculation of the age of the rocks on the basis of the radioactive elements in the year 1904. Uranium, thorium etc. contain maximum amount of radioactive elements in them.

Uranium and thorium are found in all types of rocks though their amount varies significantly from one type of rock to the other type. When the rocks are disintegrated, the radioactive elements are also disintegrated and de­cayed and in the process emit different types of rays which generate heat. Thus, the radioactive minerals play a major role in supplementing the heat of the earth’s interior. When uranium is disintegrated, it is changed into lead because of excessive heat generated during its disintegra­tion. If we can know the time of the transformation of uranium to lead, we can easily find out the time of the formation of uranium and radioactive elements.

It may be pointed out that uranium after being disintegrated gener­ates alpha particles. Thus, the rate of metamorphism of uranium into lead can be determined by counting the alpha particles coming out from the rocks. It is estimated that 1/67th part of uranium is changed into lead in 100 million years. It may be pointed out that the quantity of lead in different rocks varies significantly and hence the leads of different rocks were transformed in different times.

Inspite of this problem the exact quantity of lead present in each rock is determined on the basis of available scientific techniques. Based on above considerations it has been concluded that the radioactive minerals were present at least 1500 million years B.P. (before present). Thus, the age of any rock may not be older than 2000 million years. Based on above considerations, thus, the age of the earth is estimated to be between 2000 to 3000 million years.

Besides aforesaid methods, some scientists have developed their own methods to calculate the age of the earth. For example, Joly has attempted to estimate the age of the earth on the basis of the age of the minerals. According to him there are several concentric rings around mica minerals. These concentric rings have been called by Joly as ‘halo’.

According to Joly the age of the minerals can be calculated on the basis of the diameter of these concentric rings and the rate of disintegration of their atoms. The oldest mineral may be used to estimate the age of the earth. H.N. Russell, has calculated the age of the earth on the basis of uranium-lead and thorium-led as 2250 million and 4600 million years respectively.

According to Russell, thus, the age of the earth ranges between 2.0 to 5.0 billion years. The biologists have calculated the age of the earth on the basis of biological evolution as 1000 million years.

It appears from the aforesaid discussions that sev­eral concepts and methods have been proposed to calcu­late and estimate the age of the earth but none of them could be accepted by the majority of scientific commu­nity. Some concepts and methods are so confusing and erroneous that these present wrong notion about the age of the earth. Most of the concepts and methods are based on deductions and estimations.

It may be pointed out on the basis of available information that the age of the earth may not be, in any case, less than 2000 million years. Most likely, the age of the earth may be put between 3000 to 8000 million years. Recently the age of the earth has been estimated between 4 and 4.5 billion years.

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