Geoscience Reference
In-Depth Information
Holmes' 1911 paper can be regarded as one of the greatest pieces
of geological literature ever published. The paper had appeared only
four years after the death of Kelvin, and while the enormity of the date
and his work did not please everyone in the geological community, it
was not long before the general consensus was that Holmes was right.
The paper was the first in a line that eventually produced an absolute
timescale for the geological column. Throughout his life Holmes
continued to refine his geological timescale as his own research or
In 1915 he calculated the ages of more minerals taken from a
wider stratigraphical range than he had attempted in 1911, but there
were still gaps in the dating scheme: he had no Mesozoic data and a
date for the Silurian was absent. By 1935 the coverage was better still
and he noted that the Earth was no less than 1,900 million years old,
and that it seemed likely that the Earth and the Solar System could be
the same age, which was approximately 2,000 to 3,000 million years.
By his 1947 paper he had produced a timescale that for all intents and
purposes is very similar to the 2004 version. In 1947 he produced two
scales: the A scale defined the maximum age limit of the geological
unit and the B scale the lower age limit of the unit (Figure
13.6
). The
B scale became more widely quoted subsequently. The precision of
dates in the 1947 scheme had been improved since the discovery
that uranium formed isotopes one of which decayed to
207
Pb. Prior to
this,
207
Pb had been thought not to have been formed by radioactive
decay, but if this was so, was there any lead isotope not formed by
radioactive decay? If there was then all the previous results using the
uranium-lead method would be inaccurate, and one could only deter-
mine an accurate age if one knew the original proportions of the
uranium isotopes. In stepped Alfred Otto Carl Nier (1911-1994),
known to all as Al, who was working at Harvard at the time. He
modified a mass spectrometer, a machine used to measure the mass
of atoms, improving its precision, and this allowed him in 1939 to
determine the ratio of the two uranium isotopes. He said in 1939 that
if one examined a lead ore, one found that a proportion of it was lead
