88 | Great Geologists
(Ma)
Holmes was born in 1890 in Gateshead in north-eastern
England. Inspired by teaching at school, he gained a
scholarship to study physics at the Royal College of Science
(now part of Imperial College) in London. He became
increasingly interested in geology whilst at university and the
link between physics and geology that research into radioactive
elements represented - in particular the radioactivity studies
carried out in the physics department by Professor Robert
Strutt. Holmes took on a research project with Strutt to
use radiometric dating to date a set of rocks from different
geological periods, the oldest being a speciment from the
Precambrian from Sri Lanka. Rutherford and others were
using the measurement of helium, which was understood
to be released in the transformation from parent to daughter
element, as the basis for their radiometric dating. Because
some helium is lost during the radioactive process, its results
were likely to only provide minimum ages. Therefore, inspired
by the pioneering studies of the American chemist Bertram
Boltwood, Holmes decided to measure the quantity of uranium
versus the amount of lead (the ultimate stable daughter
element derived from a parent unstable element). This was
painstaking, difficult work but ultimately he was able to derive
a set of ages. This included the observation that the oldest
Precambrian sample was 1640 million years old. These results
were presented to the Royal Society (by Strutt on behalf
of his student) and subsequently published in the society’s
Proceedings. It seems very commendable that Strutt took no
direct credit or co-authorship for the work of his student!
The reason why Holmes did not present the results of his
work was that at the time he was engaged in an expedition
to Mozambique on behalf of a mineral mining company.
His principal motivation for participation was financial, his
university scholarship being barely enough to keep pace with
the cost of living, but nonetheless, he enjoyed the opportunity
to carry some genuine geological exploration. It almost cost
him his life. Shortly before the expedition ended he contracted
0
0
50000
changing the understanding of physics and ultimately geology
at a rapid pace. The energy provided by the radioactive decay
of uranium and other elements indicated that the earth was
not simply cooling as Lord Kelvin had suggested. Moreover,
the time for the transformation of radioactive elements from
unstable parent elements to stable daughter elements meant
that the proportion of one to another could be used to measure
numerical age (radiometric dating). As early as 1906 Ernest
Rutherford had suggested that a rock could be 500 million
years old based on such an analysis.
100
200
300
400
500
600
Pliocene
Nier’s radiometric control points
Miocene
Oligocene
Eocene
Interpolation line
100000
Cretaceous
Jurassic
Triassic
Permian
150000
Carboniferous
Devonian
Silurian
200000
Ordovician
250000
Cambrian
300000
12 26 38
58
127 152
182
203
255
313
350
430
510
Holmes’ 1947 interpolation of numerical ages (derived from the work
of Alfred Nier) with sedimentary thickness to generate a Phanerozoic
timescale.
the complication of malaria known as blackwater fever. The nuns
in charge of the local hospital where he was admitted did not
expect him to live and a premature notice of his death was sent
back to London.
But survive he did and he returned to London to continue his
studies into the radiometric dating of rocks. By 1913 he had
published a short book on the subject, “The Age of the Earth”,
subsequently revised several times in his lifetime. His motivation
for doing so was the increasing and perhaps surprising
resistance in the geological community to the age of the Earth
being suggested by Holmes and others using radiometric
techniques. Whereas they had previously considered Lord
Kelvin’s calculation of the age of the Earth of 20 million years
to be too short, they now considered radiometric ages of 1600
million years to be too long! Put simply, many geologists felt
that there was not enough rock on the planet to satisfy such
a long Earth history. Plate tectonics and the true nature of our
dynamic Earth would ultimately provide the explanation of this
conundrum, but in the meantime Holmes felt the need to explain
that geologists need not doubt the physics and chemistry that
radiometric dating is based upon.
His physical condition after the malarial infection contracted in
Mozambique excused him from front line service in the Great
War. Instead he was engaged in mapping and mineral studies
to aid the war effort whilst trying to continue his research. In
particular, he needed to adapt the developing knowledge of