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Dating The Earth

  Most societies have a creation myth which explains how and when the Earth was formed. For European societies, in the Christian era, the Book of Genesis was taken as a literal description of the creation until the 19th century. Not only did this state that the Earth had been formed in a week, it implied that the accounts of human events in the Old Testament covered the whole of the Earth\'s history. Archbishop Ussher of Armagh, writing in the 17th century, calculated that the genealogies given in the books of the Old Testament gave the date of the Creation as 4004  BCE.

This extremely short period gave little time for geological processes to have any effect, though Noah\'s Flood was taken literally and used to explain the existence of fossils of animals and plants now extinct. But as geologists studied rates of erosion and deposition, let alone contemplated the possibility of mountain building, it was clear that much longer timescales must be involved. This was described by James Hutton\'s principle of uniformitarianism and his dictum, ‘no vestige of a beginning, no prospect of an end’. The work of William Smith in assembling rocks from different areas into the stratigraphic column suggested that hundreds of thousands of feet of rocks had been laid down in the UK, in climatic conditions ranging from hot desert and tropical marshland to permafrost and ice sheet. From 1859, Darwin\'s work on evolution added another pointer to a long timescale. So the scientific community came to accept Genesis as allegorical, and that the age of the Earth had to be measured in hundreds or thousands of million years.

Towards the end of the 19th century, this consensus was rudely shattered, not by a backlash from the church, but by one of the most distinguished scientists of the day. Lord Kelvin used one of the most advanced sciences to construct what appeared to be an unimpeachable case for a rather short lifetime for the Earth. Measurements were made of the heat loss from the surface of the Earth—most obvious at volcanoes, but detectable elsewhere. It was assumed that this heat was the remainder of that present when the Earth formed as a molten ball of rock. Experiments in furnaces showed how much heat was required to raise rock to melting point, so the starting point could be calculated. Then the laws of thermodynamics could be used to calculate the time required to cool from the starting point to the observed temperatures. The calculation was done several times with slightly different assumptions, but Kelvin argued that the Earth could not be much more than 24 million years old. This was clearly insufficient to allow the theories of the geologists and biologists to be true, so there was a disabling lack of consensus.

The problem was solved shortly after Kelvin\'s death by the discovery of radioactivity. This both provided a means of generating heat within the Earth, so making possible a much longer lifespan, and a means of measuring the time. There are some 65 naturally occurring radioactive isotopes on Earth. These are unstable and spontaneously break down by emitting neutrons to form a daughter product, which may be another isotope of the same element or a lighter element. Each isotope breaks down with a constant half-life—a constant time to halve the amount present at the start of the period. If the half-life is known and mother and daughter products remain together it is simple to calculate the duration of the decay process. In practice it has proved very complex as the daughter product may itself be radioactive or may be more mobile, but the problems can be overcome by the use of more than one decay process. Even the first measurements, by Boltwood in 1906, using the decay of uranium to lead, were enough to refute Kelvin and vindicate the geologists. Boltwood\'s estimates for different rocks ranged from 400 million to 2.2 billion years, so he indicated that the Earth is a hundred times older than Kelvin\'s estimate. His measurements are now thought to be about 20% too long, but modern methods have found even greater ages for other materials.

The oldest rocks so far dated are calculated to be 3.5 billion years old, but the Earth itself is probably about a billion years older. This is indicated by measurements of the age of meteorites, 4.5 billion, and of Moon rocks, 3.7-4.6 billion years. Since the Earth originated simultaneously with the Moon, 4.6 billion years is now thought to be the most probable estimate. PS



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