The Age of Rock

How old is the Earth? The world’s great religions all have creation myths and some offer a date for when it all began.

The Date of Creation

The nineteen glyphs of the Mayan calendar split the year into eighteen periods of twenty days plus a remainder of five inauspicious days.

Many ancient cultures considered time to be cyclic. But the period they assigned between cosmic birth and death varied enormously.

The Mayans of Central America devised a sophisticated calendar with which they recorded their history. It enables us to precisely date their sacred monuments. According to one Mayan myth, in the beginning the sky was black as it lay on the primordial sea. The current cosmic cycle began when the Sky God created the world of humans. He arranged three stones as a pivot for the cosmos, then raised the sky and revealed the Sun. This was the first day of the Mayan calendar which is 11 August 3114 BC in the modern Gregorian calendar.

Archbishop James Ussher by Sir Peter Lely.

Ancient Hindus also believed in a cyclic cosmos. Their name for the period of a complete cycle of the universe is the kalpa and, according to calculations in the Vishnu Purana, this is equivalent to 4.32 billion years.

The Christian concern with the age of the universe came rather later. In 1650, Archbishop James Ussher (1581-1656), Primate of All Ireland published a chronology of the world based on a literal reading of the biblical Old Testament. Ussher argued, with uncanny precision, that the universe was created on the evening of Saturday 22 October 4004 BC. So just as astronomers were expanding the spatial extent of the universe, Ussher was firmly limiting its temporal bounds.

Digging and Delving

The canal-building boom of the industrial revolution stimulated an interest in geology in the later years of the eighteenth century. It was inevitable that the meagre timescale allowed by Ussher would come under pressure as soon as anyone delved within the Earth to consider the evidence. 

James Hutton painted by Sir Henry Raeburn in 1776.

The Scottish naturalist James Hutton (1726–1797) is known today as the father of modern geology. His analysis of minerals, strata and fossils indicated that the Earth’s rocks were the result of long ages of deposition and erosion. Hutton recognised that rocks could only have formed over the course of extremely long periods of time, but he had no way to calculate just how much time was required. His appraisal of what the rocks were telling him was that:

The result, therefore, of our present enquiry is that we find no vestige of a beginning, no prospect of an end.

In the year these words were published, 1785, the poet William Cowper (1731–1800) hit back in his Vanity of Human Pursuits complaining that:

Some drill and bore
The solid Earth, and from the strata there
Extract a register, by which we learn
That he who made it, and reveal’d its date
To Moses, was mistaken in its age.

On the Origin of Species

The opening salvoes of a century-long battle for the Earth had been fired. The conflict rumbled on until 1859 when Charles Darwin (1809-1882) published his theory of evolution by natural selection in On the Origin of Species, one of the most incendiary books in the history of publishing.

Charles Darwin

It was clear that Darwinian evolution would require a good deal longer than six thousand years. To illustrate how geology could accommodate the necessary time span, Darwin included a rough calculation of the time taken to erode the Weald—an area of chalkland in Sussex and Kent close to Darwin’s home in south-east England. Darwin estimated that something like three hundred million years must have elapsed since the formation of these rocks. This figure caused immediate protests. Others did their own calculations and arrived at much lower figures. Three years later one of the titans of Victorian physics was drawn into the debate.

In 1862, John Thomson (1824-1907), better known today as Lord Kelvin, published estimates of the ages of the Earth and the Sun based on his own thermodynamic arguments. He assumed that the Earth had formed as a molten mass of rock and calculated how long it would take for such a ball of magma to cool to its current temperature. He also estimated the Sun’s age on the assumption that its heat was derived from the release of gravitational binding energy as it gradually contracted. Kelvin concluded that the Earth and Sun are probably somewhere between ten and one hundred million years old, but certainly no more than four hundred million.

Lord Kelvin painted by Hubert von Herkomer.

When these estimates were published, they were regarded as offering support for the time frames required by the geologists. As the decades passed, however, Kelvin’s age for the Earth would stir up its own controversy. Geologists had a habit of finding ever older rock formations that continually pushed back the date of Creation. Whilst Kelvin’s ongoing efforts to refine his estimates moved in the opposite direction. The crunch came in 1897 when Kelvin declared that the Earth

was more than twenty and less than forty million years old, and probably much nearer twenty than forty.

The dating systems employed by the geologists were based on rates of sediment deposition, and although these methods were imprecise it was clear that Kelvin’s estimates were not supported by the geological evidence. By the beginning of the twentieth century the discrepancy between the physicists and the geologists cried out for a resolution.

Half-Life

In 1899, Ernest Rutherford (1871-1937) left Cambridge to take up a professorship at McGill University in Montreal, Quebec. In Canada, Rutherford continued his research into radioactivity, and soon established that two types of ray are emitted by radioactive atoms. He named them alpha and beta rays. The existence of a third type—gamma rays—was demonstrated soon afterwards.

In January 1900 Rutherford published his paper A Radioactive Substance emitted from Thorium Compounds in which he set out the principles of radioactive half-life. Rutherford described the properties of a new radioactive gas produced by the element thorium. He named it thorium emanation. It is known today as radon. 

Ernest Rutherford

Rutherford’s investigations showed that radioactivity has a statistical nature. The radioactive decay of each type of atom takes place on a time scale that is characterized by its half-life. This is the time taken for half the atoms in a given sample to decay. Rutherford found, for instance, that the amount of radiation emitted by his radon sample halved every minute. (We now know Rutherford was studying the isotope radon-220. It has a half-life of 55 seconds.)

Radioactive Clocks

Other radioactive elements such as thorium and uranium have very long half-lives and Rutherford could see how they might resolve the controversy about the age of the Earth. When a uranium atom undergoes radioactive decay it releases an alpha particle, and further alpha particles are emitted by its decay products. Rutherford suspected that these alpha particles were positively charged helium atoms. (He would later show that an alpha particle is identical to a helium nucleus.) So as time passes and uranium decays, helium accumulates within the rock. William Ramsay had first identified helium in 1895 in a uranium-containing mineral known as cleveite.

Rutherford suggested that the amount of helium trapped within a rock would reveal how much uranium has decayed since the rock formed. And by comparing the amount that has decayed to the amount that remains the age of the rock could be determined. For instance, if half the uranium has decayed then the rock’s age equals the half-life of uranium, which can be measured in the laboratory. 

When Rutherford put his idea into practice he was delighted to find that his rock sample was five hundred million years old. On 20 May 1904 he announced his findings at the Royal Institution in London. He later recalled:

I came into the room which was half-dark and presently spotted Lord Kelvin in the audience, and realised that I was in for trouble at the last part of my speech dealing with the age of the Earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me.

Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the Earth, provided no new source [of heat] was discovered. That prophetic utterance referred to what we are now considering tonight, radium! Behold! The old boy beamed upon me.

The Future of Dating the Past

But Kelvin never, in fact, accepted the implications of Rutherford’s breakthrough. In 1906, the Age of the Earth controversy erupted in the Times newspaper, with the diehard Kelvin taking a stand against the new generation of radiochemists Rutherford, Ramsay, Soddy and Strutt. The battle raged for months and a schoolboy in Gateshead in the North-East of England eagerly kept abreast of the fight.

Rutherford once said that all science is either physics or stamp collecting.

Rutherford had demonstrated the potential of radioactive dating and made the first tentative step towards a new science of geochronology. But unfortunately his method had a serious flaw. Helium atoms are much smaller than other atoms and they do not form chemical bonds, so with the passage of time they may seep slowly out of a rock crystal. Rutherford’s technique is, therefore, only reliable for providing a minimum age of a rock.

It was not long before more sophisticated dating techniques were devised, and the schoolboy from Gateshead would play a key role in their development. He would become a pioneer in radioactive dating and use it to bring order to the science of geology. His name was Arthur Holmes and I will tell his story in a future article.

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