Essay on Earth: The planet Earth was not born instantaneously

The planet Earth was not born instantaneously. It evolved from some previously existing hot and gaseous state, which eventually condensed to its solid state, via a liquid state.

This change over was a period of very slow process of transition, which prolonged for a good length of time.

Thus, the age we seek, does never mean the time of its birth in gaseous or liquid state. Rather it means the time of the evolution of the planet as a solidified mass. As such, our planet’s birth or so to say, the age of the Earth, in fact relates to its time of solidification leading to the formation of its stable crust. Thus, the age of the Earth, we seek, is no precise time in terms of second / minute / hour / day / year or even centuries. It is an overwhelming number expressed in terms of millions of years. Since the 18th century, attempts have been made to estimate this age.

The estimates made by naturalists like Buffon and Lightfort in 18th century based on non-scientific notions were frantically low and untenable figures. The earliest astrological calculation relating to the date of birth of our planet was made by the ‘Archbishop Usher of Iceland’ in 1664 and accordingly, the Earth was said to have been born at 9AM on 26th October in 4004 BC – a figure to amuse only and never to rely upon.

Since the mid 19th century, various methods based on the teachings of astronomy, astrophysics, physics, biology, geology and nuclear chemistry have been meaningfully tried with. These methods have given multitude of esti pertaining to the age of the Earth, which vary within wide limits. Various methods of estimating the age of the Earth are classified and listed below; method may be otherwise described as a clock.

Extrinsic clocks which are based on external parameters com apparent and relative methods, which give widely variable age for these clocks, are briefly as follows:

Biological evolution clock:

This method is based on the Theo organic evolution of lives from simple unicellular to multicellular of organisms and then from a series of animals and plants to the evolved creature, the man. The beginning of this time from the appea of very first life on the Earth is traced back by radiocarbon analysis of most primitive fossil finds going back to the root of the evol reconstructed by the biologists. Age of the oldest fossil gives minimum age of the Earth. This method provides rather a very low figure which is of the order of 1000 million years (my) as deduced Darwin and Poulton during mid of late 19th century. This low age figuf due to the fact that it represents only a fraction of the actual age as the v first life (probably not fossilized) appeared on the planet much later the time of the Earth’s solidification.

Moon’s retreat clock:

It is broadly related to the tidal force of the E; moon. It is primarily based on the concept of lunar separation from Earth. Moon was very close to the Earth to start with and it has drifted a from the Earth to its present position. This distance divided by the mo retreat or shift from the Earth gives the age of the Earth. The rate off of the moon is calculated from the value of tidal energy in the shallow at different geologic times. At the beginning the rate of shift was f; with larger tides. Consequent upon the increase of distance, the ti became smaller in magnitude effecting slower rate of retreat. Based on premise and the present distance of the moon from the Earth as 0.38 lakh km, Jeffrey and Badwin estimated the age of the Earth to be about 4000 my.

Sedimentation clock:

Ever since the formation of hydrosphere, the surface rocks of the Earth were relentlessly eroded and deposited in the oceans during different geologic times. If the entire and aggregate thickness of the sedimentary column on the globe ranging from very inception to the present is geoscientifically deduced and the average rate of sedimentation per annum is considered, the time of beginning of the deposition could easily be traced back. According to Murray, the rivers of the world carry a sedimentary load of about 16 km³ to the sea annually.

The total thickness being known, the time of the start of the sedimentary deposition could be calculated. In this method the age of the Earth has been calculated to be 400 my. This method has a number of drawbacks as the global sedimentation through out the geologic periods was neither continuous nor of uniform rate. There have been major breaks in sedimentation due to large unconformities and nondeposition. These imperfections greatly affect the reconstruction of an ideal and optimum global sedimentary column.

Salinity clock:

This clock is based on the rate of salinity of oceanic water. The sea / oceanic water is assumed to have been fresh at the beginning and it gradually became saline by the addition salts (chlorides and sulphates etc) of Na, Ca and Mg supplied to the oceans through rivers annually. From the total mass of Na⁺ ions in the present day seas and oceans divided by the annual addition of Na⁺ per year the age of the oceans and hence that of the Earth calculated by Jolly, Clarke anu Solas varies between 99 to 250 my. This clock has a number of demerits and gives low age figures.

Rate of cooling of the Earth:

The Earth cooled to its present state from an initially hot ball of fire of initial surface temperature of about 3900° C. By rationally assuming an average rate of cooling, the age of the Earth could be estimated. It was first used by Kelvin in 1862 and then tried by Helmholtz in the late 19th century. The estimates have been variable from 20 to 400 my as advocated from time to time. The low estimate figure is due to the fact that these estimates did not consider the influence of radiogenic heat produced in Earth’s crust. Further this method has many other drawbacks arising out of assumptions considered in the estimates.

Intrinsic clocks:

The radiometric method is the only intrinsic clock which is considered the most modern and reliable method of estimating the age of the Earth in some absolute sense. This method is primarily based on the principle of radioactivity, a process of spontaneous disintegration of certain unstable natural isotope elements such as U, Th, K, Rb etc with emission of a, x and y rays.

This process is completely free from the influence of external parameters such as temperature, pressure and chemical reactions. Radioactivity was discovered by Becquerel in 1896 and application of this principle to the subject of age dating of rocks was first attempted by Boltwood in 1910. The basic principle is to find out the age of the oldest rocks and crust of the Earth by measuring the radioactive decay of the long-lived radioactive isotope series. Various long-lived radioactive series are characterized by their specific half-life times (T) and corresponding disintegration constants (X), which are related by the formula T = 0.693/A,. These two factors being known, the time (t) of the formation and hence the age of the parent mineral and rock can be computed with certainty. The factor ‘t\ which is the age can be found out from the following formula

t = 3.323Txlog₁₀ (l + ^_) where

t = time of formation of the mineral / rock in million years T = half-life period

N_d = amount of the stable daughter element (say Pb²⁰⁷) N_p = amount of parent element (say U²³⁵)

The values of N_d and N_p could be determined by sophisticated devices and t (age of the rock) could be calculated. The age of the oldest rock in the crust is the minimum age of the Earth.

The age of the oldest rock would indicate the age of the Earth at the minimum. The maximum age of the Earth i.e. time of formation of the first crust (solidified Earth) must be much earlier than the oldest dated rock of the crust so far dated. Because of the complex geological and erosional processes operating on the Earth’s surface ever since its solidification, it is indeed very difficult to discover and date the oldest rock. The oldest rock of the moon, which has not undergone erosion / denudation has been dated to be 4500 my. Earth’s daughter (moon) cannot be older than the Earth.

Thus in all logistic estimation, the age of the Earth may safely be put at 4500 my, which is also the figure reiterated by the age of the meteorite