Geoscience Reference
In-Depth Information
Centigrade. We now consider the temperature at the centre of the
Earth to be about 7,200 degrees Centigrade. In 1862 Thomson stated
in the Transactions of the Royal Society of Edinburgh that the Earth
was somewhere between 20 and 400million years old, with 98million
years being the likely age. He argued, given the underground tempera-
tures known and the loss of heat due to conduction, that if the Earth
was as much as 20,000 million to 30,000 million years old the under-
ground temperatures observed should have been far lower. It was
possible however to generate the observed temperatures if the whole
of the Earth's surface had been heated up to 100 degrees Centigrade at
some time in the past 20,000 years. Naturally this could not have
happened as all traces of life would have been killed.
Six years later, in an address to the Geological Society of Glasgow
delivered on 27 February 1868, he concluded that the Earth was no
more than 100 million years old, and this timescale was questioned
the following year by none other than Thomas Henry Huxley
(1825-1895) who asked in his presidential address to the Geological
Society, 'Has it ever been denied that this periodmay be enough for the
purposes of geology?' In doing so Huxley laid the seeds of discontent
between the views of physicists in one corner and the geologists and
biologists in the other corner, a disagreement that continued for nearly
half a century. In 1895 Thomson penned a short paper for Nature in
which he discussed a recently published paper by Clarence King
(1842-1901), who had been the first director of the United States
Geological Survey between 1879 and 1881. King's paper re-examined
Thomson's work and using a new figure of 1,950 degrees Centigrade for
the fusion temperature of rocks, a figure which he had obtained from
the physicist Carl Barus (1856-1935), arrived at 24 million years as the
age of the Earth. This limit was accepted by Thomson who acknowl-
edged that Barus' data would have rendered his 100-million-year limit
too large, and that while the method would point to 10 million years,
the effects of pressure on the geological processes would push the age
determination to that of King. In a short letter to Nature in 1897, which
gave his final pronouncement on the cooling Earth method, Thomson
