Monday 22nd April is Earth Day 2013. To celebrate in advance, here’s an extract from The Earth: A Very Short Introduction.
By Martin Redfern
Attempts to calculate the age of the Earth came originally out of theology. It is only comparatively recently that so-called creationists have interpreted the Bible literally and therefore believe that Creation took just seven 24-hour days. St Augustine had argued in his commentary on Genesis that God’s vision is outside time and therefore that each of the days of Creation referred to in the Bible could have lasted a lot longer than 24 hours. Even the much quoted estimate in the 17th century by Irish Archbishop Ussher that the Earth was created in 4004 BC was only intended as a minimum age and was based on carefully researched historical records, notably of the generations of patriarchs and prophets referred to in the Bible.
The first serious attempt to estimate the age of the Earth on geological grounds was made in 1860 by John Phillips. He estimated current rates of sedimentation and the cumulative thickness of all known strata and came up with an age of nearly 96 million years. William Thompson, later Lord Kelvin, followed this with an estimate based on the time it would have taken the Earth to cool from an originally hot molten sphere. Remarkably, the first age he came up with was also very similar at 98 million years, though he later refined it downwards to 40. But such dates were considered too recent by uniformitarianists and by Charles Darwin, whose theory of evolution by natural selection required more time for the origin of species.
By the dawn of the 20th century, it had been realized that additional heat might come from radioactivity inside the Earth and so geological history, based on Kelvin’s idea, could be extended. In the end, however, it was an understanding of radioactivity that led to the increasingly accurate estimates of the age of the Earth that we have today. Many elements exist in different forms, or isotopes, some of which are radioactive. Each radioactive isotope has a characteristic half-life, a time over which half of any given sample of the isotope will have decayed. By itself, that’s not much use unless you know the precise number of atoms you start with. But, by measuring the ratios of different isotopes and their products it is possible to get surprisingly accurate dates. Early in the 20th century, Ernest Rutherford caused a sensation by announcing that a particular sample of a radioactive mineral called pitchblende was 700 million years old, far older than many people thought the Earth to be at that time. Later, Cambridge physicist R. J. Strutt showed, from the accumulation of helium gas from the decay of thorium, that a mineral sample from Ceylon (now Sri Lanka) was more than 2,400 million years old.
Uranium is a useful element for radio dating. It occurs naturally as two isotopes – forms of the same element that differ only in their number of neutrons and hence atomic weight. Uranium-238 decays via various intermediaries into lead-206 with a half-life of 4,510 million years, whilst uranium-235 decays to lead-207 with a 713-million-year lifetime. Analysis of the ratios of all four in rocks, together with the accumulation of helium that comes from the decay process, can give quite accurate ages and was used in 1913 by Arthur Holmes to produce the first good estimate of the ages of the geological periods of the past 600 million years.
The success of radio-dating techniques is due in no small way to the power of the mass spectrometer, an instrument which can virtually sort individual atoms by weight and so give isotope ratios on trace constituents in very small samples. But it is only as good as the assumptions that are made about the half-life, the original abundances of isotopes, and the possible subsequent escape of decay products. The half-life of uranium isotopes makes them good for dating the earliest rocks on Earth. Carbon 14 has a half-life of a mere 5,730 years. In the atmosphere it is constantly replenished by the action of cosmic rays. Once the carbon is taken up by plants and the plants die, the isotope is no longer replenished and the clock starts ticking as the carbon 14 decays. So it is very good for dating wood from archaeological sites, for example. However, it turns out that the amount of carbon 14 in the atmosphere has varied along with cosmic ray activity. It is only because it has been possible to build up an independent chronology by counting the annual growth rings in trees that this came to light and corrections to carbon dating of up to 2,000 years could be made.
Martin Redfern is a former science producer at the BBC Science Radio Unit and author of The Earth: A Very Short Introduction. He is now a freelance science writer.
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Image credit: A composite image of the Western hemisphere of the Earth, by NASA/ GSFC/ NOAA/ USGS [Public domain], via Wikimedia Commons
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