90 | Great Geologists
community with numerical age values for the whole of the
Phanerozoic.
During World War II Holmes was charged with the rapid
education of RAF cadets. Realising that no textbook existed
that captured the latest geological thinking he set about
creating one from his lecture notes. The result was the classic
Principles of Physical Geology first published in 1944. This
beautifully written and illustrated book became the standard
university textbook in its first and subsequent editions. Widely
read, it inspired future generations of geologists.
One aspect of the first edition is particularly remarkable – it
makes reference to continental drift, the argument promoted
by Alfred Wegener that the continents had not always been
in their current position and had moved through geological
time. Although now fully explained by plate tectonic theory,
the concept of moving continents was viewed by many
geologists in the mid-20th Century with extreme scepticism.
Holmes had been contemplating this problem since the
late 1920’s and considered that differential heating of the
Earth’s interior, generated by the decay of uranium and other
radioactive elements, caused convection in the “substratum”
(the mantle in today’s terminology), on which the continents
floated. Convectional cells would be generated that could drag
continents sidewards and apart allowing new crust to rise up
and form. First presented to the Edinburgh Geological Society
in 1927, this theory is a brilliant forerunner of the mechanisms
by which it is now known that plate tectonics work. Some
contemporary geologists were less impressed, as this quote
from William Bowie shows: “Holmes brings out a new thought
which is even more impossible than Wegener’s. That is that
the submerged ridge through the Atlantic Ocean is the place
at which North and South America separated from Europe and
Africa… I believe that we need to apply elementary physics
and mechanics to the continental drift problem in order to
show how impossible drifting would be”. Holmes always
considered his work in this sphere highly theoretical and was
never absolutely convinced of its correctness. Nonetheless it is
to his great credit that he included it in his teaching materials,
to be ultimately vindicated by the geophysical observations of
the 1960’s and onwards.
Holmes would continue to synthesise radiometric data and
evolve the geological timescale until his death in 1965. This
incorporated the age of the Earth determined as 4550 million
years +/- 70 million years as determined by Claire Patterson in
1953 integrating data from meteorites determined as forming
at the same time as the Earth. This age has not fundamentally
changed for over 60 years. Arthur Holmes was always the
impetus for such developments and the critical analysis of their
validity. In this respect he can be considered as the “father” of
geological timescales, that is to say the integration of geological
ages with absolute ages - geochronology.
The contributions of Holmes to geology are thus remarkable
– the dogged pursuit of the numerical dating of the geological
record, from which the rates of geological processes can be
measured. This interest in process is linked to his promotion,
against the conventional wisdom of the time, of the causes
by which continental drift (and subsequently plate tectonics)
might be explained. And ultimately he was able to convey the
excitement of the changing face of geological understanding
to generations of students through his masterly textbooks and
papers. For many he is the greatest British geologist of the 20th
century.
REFERENCES
This essay has drawn upon information from the following
sources:
Hallam, A. 1973. A Revolution in the Earth Sciences. Clarendon
Press, Oxford. 127pp.
Hallam, A. 1983. Great Geological Controversies. Oxford
University Press, 244pp.
Le Grand, H.E. 1988. Drifting Continents and Shifting Theories.
Cambridge University Press. 313pp.
Lewis, C.L.E. 2000. The Dating Game. Cambridge University
Press.
Lewis, C.L.E. 2001. Arthus Holmes’ vision of a geological time
scale. In: Lewis, C.L.E. & Knell, S.J. (eds.) The Age of the Earth:
from 4004 BC to AD 2002. Geological Society, London, Special
Publications, 190, 121-138.
Lewis, C.L.E. 2002. Arthur Holmes unifying theory: from
radioactivity to continental drift. In: Oldroyd, D.R. (ed.) The
Earth Inside and Out: Some Major Contributions to Geology
in the Twentieth Century. Geological Society, London, Special
Publications, 192, 167-184.
Oreskes, N. 1999. The Rejection of Continental Drift. Oxford
University Press. 420pp.
Powell, J.L. 2015. Four Revolutions in the Earth Sciences.
Columbia University Press, 367pp.