Geoscience Reference
In-Depth Information
Chapter 6
Geochronology
6.1 Introduction
Radioactivity was discovered in 1896 by Henri Becquerel. The new understanding
of the atom that came from the work of Rutherford, Soddy, Boltwood and others
over the following decades had a major impact on geology. Before this work, the
age of the Earth was unknown. In the nineteenth century, Lord Kelvin (William
Thompson) attempted to calculate the age of the Earth by assuming that the
planet was a hot body cooling by conduction. He obtained a young age that
conflicted with the observations of geologists, who had concluded that the Earth
must be at least several hundred million years old. The geological reasoning
was based on rather qualitative evidence such as the observation of sedimentary
deposition rates, calculations about the amount of salt in the sea
1
and guesses
of evolutionary rates. Not unnaturally, Kelvin's apparently more rigorous and
quantitative physical calculation was regarded as much sounder by most scientists.
In 1904, while at McGill University in Montreal, Ernest Rutherford realized that
radioactive heat could account for some of the apparent discrepancy. Kelvin was
sceptical to the extent that he bet the younger Rayleigh (Hon. R. J. Strutt) five
shillings on the matter, but later he paid up. Rutherford gave a lecture at the Royal
Institution in London in 1904 about which he wrote the following:
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...Behold! the old boy beamed upon me.
In fact, Kelvin's calculation is now known to be too simple; not only does radioac-
tive decay provide an extra source of heat, but also, much more importantly, heat is
transferred within the mantle by convection, not by conduction (see Section 7.4).
Had Kelvin carried out a convection calculation, as he did for the Sun, he would
have obtained a more reasonable answer. However, even had he done this, the fact
that estimates for the age of the Sun (based on its energy output) were also only
1
Interestingly, the sea-salt puzzle was not solved until many years later when, with plate tectonics,
the significance of geochemical exchange in hydrothermal convection at ridges was understood.
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