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How old is the Earth
AT FIRST, man must have wondered, simply out of curiosity, as to how old the Earth was. By and by, as he became more inquisitive about the natural phenomena, such questions became more than mere idle enquiries. Ideas about the evolution of the world and the life therein then started germinating, although there were no precise theories.
Geologists started observing the natural phenomena, which presumably had shaped the structure of the globe, more closely. Such inquisitive ideas invariably led to the question: How old is the Earth?
The first estimates
Archbishop James Ussher (1581-1656) of Armagh, Ireland, “calculated that creation began on Oct 23, 4004 BC”. (http://en.wikipedia.org/wiki/Age_of_the_Earth). He presented this information in a book in 1654, two years before his death. According to him, the world began some 6,000 years back. Many people, including some notable scholars and scientists, such as Martin Luther, Johannes Kepler and Isaac Newton, believed in this or a similar date.
The Han Chinese believed in the cyclic creation of the world and they thought Earth was recreated after every 23 million or so years.
The traditional Jewish calendar begins from 3,760 BC, which is taken as the date for the creation of Earth. Aristotle believed in the eternity of the world.
Sign readers and sceptics
Edmund Halley (1656-1742), a prominent astronomer, said: “Rivers are continually washing small amounts of dissolved salts into the oceans so, over time, the oceans should get more salty. He then asked how much time would it take for an initially freshwater ocean to achieve its current level of salinity.
“He then argued that Earth couldn’t be very young because otherwise the oceans would still be fairly fresh. Similarly, the Earth could not be infinitely old, otherwise the ocean would be thoroughly saturated with salt.” (http: astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit5/deeptime.html). Mr Halley, however, did not formulate any estimate of his own.
William Smith (1769-1839) was a British civil engineer and a surveyor. He came from a lowly social background, which proved a great handicap as far as his professional life was concerned. He worked in the coalmines and on canals.
After painstaking work in the mineshafts, he argued that the Earth’s strata were formed due to the deposition of eroded material with the help of wind and water. The top layers were recent, while the deeper ones were old.
He also noted that the structure of the layers was similar, irrespective of their geographical location. He had seen similar stratification in excavated formations in canals. On close scrutiny, he detected the remains of old life — the fossils — in the layers.
Wonder of wonders, the fossils in the upper layers were similar to the existing life, while those in the bottom layers were entirely different. He argued that the species whose fossils were embedded in the lower strata must have become extinct. From this, he concluded that life on Earth was not created in one single act.
Secondly, the Earth must be much, much older than the current estimates, because the process of stratification was very slow. He used to remain so preoccupied with these thoughts that he was actually nicknamed “Strata.” So he actually became known as William “Strata” Smith. Many give him credit for founding the field of fossils, which is called palaeontology.
Mr Smith prepared the first national geological map of England, which was plagiarized by his rival and more influential peer, George Bellas Greenough, first president of the Geological Society of London.
Charles Lyell (1797-1875), known for writing an authoritative account of geology in his Principles of Geology, which ran through 12 volumes, used the strata of different rocks to separate the ages of previous geological history, noted changes in fossil strata as ways to fix relative ages, and noted that the old rocks were like the young ones but life showed great variation with time. He introduced the idea of Stratigraphic Ages for geological formations, which are still used in modified forms.
He also introduced the principal of “uniformatarianism,” which says: “The development of the surface of the Earth has been going on all through the ages without interruption, and that the process of very slow change that we observe today has been responsible for the present surface features of the earth.” (Norman Smith in Millions and Billions of Years Ago: Dating Our Earth and Its Life). He argued that the features of Earth are constantly changing by erosion and reformation and the rate of this change was nearly constant. He proposed that this principle could be used to estimate the Earth’s age, although he did not give any estimate himself.
Charles Darwin (1809-1882) formulated his theory of natural selection based on his observations of fauna and flora in the Galapagos Islands. His theory required an extremely long time for life to evolve from lower forms to the present complex forms. Earth, in his view, could not be as young as tens of thousands of years. It had to be much older.
Scientific estimates
The earliest estimate of the Earth’s age, using its cooling process, was proposed by the French naturalist Georges Buffon (1707-1788). He constructed a small globe that resembled Earth in composition and then measured its rate of cooling. According to his calculations, Earth took approximately 75,000 years to cool down to its present condition. This was only a rough estimate and many didn’t pay much attention to it.
Later, the physicist William Thomson (Lord Kelvin) of Glasgow (1824-1907) worked on the idea more elaborately and published his calculations in 1862, which showed Earth was between 20 million and 400 million years old.
He had assumed that initially the Earth was a molten ball of rock and then computed the time needed for it to cool down. Although 400 million years was a much longer period, as compared to the other estimates, it still did not fit Darwin’s theory of natural selection, which required a much more ancient Earth.
Darwin’s great advocate Thomas Huxley rejected Thomson’s estimate, arguing that though there was no fault in his calculations, his inherent assumptions were wrong. However, some other estimates prepared independently were nearly in the same range as Thomson’s.
German physicist Hermann von Helmholtz and the American astronomer Simon Newcomb calculated the time that the Sun would take to condense down to its present diameter and brightness from the nebula of gas and dust from which it was born. Their estimate was 100 million years. John Joly of the University of Dublin came up with an age of 90 million years, using the accumulation of salts in the oceans.
The estimates continued refining with the passage of time but a true revolution was heralded by the discovery of radioactivity in 1896 by the French chemist A. Henry Becquerel.
Radioactivity
Estimates based on the Earth’s cooling process were not accurate because a vital parameter, that of radioactivity, was neglected simply because nobody was aware of it at that time. The radioactive material, which were discovered in the air, soil, rainwater, snow, groundwater and rocks, emitted radiation which would act at odds with the cooling process. The rate of emission of radiation or the radioactive decay was constant for a given material and the parent material was transmuted to different radioactive lighter elements, in the process releasing alpha, beta, or gamma radiation.
The decay rate is given in terms of a “half life”, which is the time taken for the material to transmute half of its mass through radioactive decay. The true phenomenon of radioactive decay was finally comprehended from the pioneering works of Ernst Rutherford, Bertram Boltwood, Frederick Soddy, Williams Ramsay, among many others.
The radioactive material decayed through a series of lighter radioactive material into a base stable metal, lead. This was, so to speak, alchemy in reverse. The alchemist had sought to convert base metals into gold.
Starting with uranium (238), for example, which is an isotope of uranium with an atomic weight of 238, it decays into thorium (234), protactinium (234), uranium (234), thorium (230), radium (226), radon (222), polonium (218), lead (214), bismuth (214), polonium (214), lead (210), bismuth (210), polonium (210), and finally into the stable element, lead (206).
Each of these isotopic elements has a different half life. In a given sample of rock, the proportion of the decayed elements are determined and used with their respective half life to compute the age of a rock sample.
However, one of the problems encountered in computing Earth’s age using radiometry was the existence of naturally occurring lead in the sample. Fortunately, this issue was resolved because the decayed isotope of lead was different from the naturally occurring lead, so that they could be distinguished from each other.
The transient stage of radon, a gas which might leak out of rock crystals, also posed a problem. But fortunately, this is a brief stage. All these factors were taken into consideration and were duly accounted for in computing the age of a given rock sample.
Subsequently, using radiometric methods it was found that the Earth’s age was in billions, not millions. So, the oldest rocks in Greenland were dated by four independent radiometric methods and the age ranged between 3.7 and 3.8 billion years. Samples of the Moon rocks, collected during the Apollo missions, gave an age of between 4.4 and 4.5 billion years. A majority of the 70 well-dated meteorite samples gave an age of between 4.4 and 4.6 billion years. The presently accepted age of Earth is 4.55 billion years.
The writer, based in the US, has a PhD in civil engineering from University of London. Email address: akramgill@yahoo.com
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