Great Geologists | 87
Arthur Holmes
One of the distinguishing traits of geologists is the ability to
discuss Earth’s history in millions (indeed billions) of years in
a very matter of fact way. Laypersons often find this amazing
– “those limestones are 400 million years old?” they will ask
in slightly disbelieving tones when during a car journey you
have casually remarked on an outcrop you are driving past and
commented on the age of the rocks present. Moreover, “how
do you know?” will be their next question. A more complex
question than they might imagine of course, but part of the
answer lies in the pioneering work of English geologist Arthur
Holmes with whom the numerical dating (geochronology) of
the standard geological time scale as we know it today began.
But Holmes achieved much more in his career than providing
chronological ages to the subdivisions of geological time. He
suggested a mechanism for explaining plate tectonics at a
time when this process was viewed as heretical by most of
the geological establishment of the mid-20th century. He also
authored what was the standard university geology textbook of
much of the late 20th century: “Principles of Physical Geology”.
Every British geology undergraduate of the 1950s through to
the 1980s can recall swatting up on the knowledge and theory
presented in their copy of “Holmes” in preparation for their
exams. It was widely used internationally and translated into
several different languages.
The Scottish Enlightenment geologist James Hutton had, by the
latter part of the 18th century, recognised ‘the abyss of time’.
Geological processes and the rock record implied a much, much
longer duration of Earth history than estimates based purely on
interpretations of religious texts. Building on the work of William
Smith and Georges Cuvier, stratigraphers of the 19th century
(Murchison, Sedgwick, d’Orbigny and others) had set about
creating the subdivisions of geological time as expressed by
intervals of the rock record characterised by particular fossils
placed in order by the Law of Superposition (Silurian, Devonian,
etc). But what was the numerical age and duration of these
subdivisions and of Earth history in general? As the 20th century
dawned these questions could not be answered with certainty.
The situation was akin to knowing history but without any dates.
Lord Kelvin, the Scottish mathematician, and physicist, William
Thompson had provided a typically quoted view of the age of
the Earth in the late 18th century based on thermodynamics. His
view that the Earth was 20 million years old (although his earlier
estimates allowed for it to be as much as 400 million years old)
was based on the time it would take for the Earth to cool from
its assumed molten state at formation to its present day state.
For many geologists working as Kelvin’s contemporaries, the
notion of a 20 million year old Earth seemed too short to account
for all the physical processes that the rock record indicated had
occurred in the geological past. Using estimates of the time to
accumulate sedimentary thickness, many geologists preferred
to estimate the age of the Earth as around 100 million years,
although this was a matter of some dispute.
Arthur Holmes aged 22, the time at which he was first publishing
on the numerical age of the Earth.
Arthur Holmes entered into the debate on the age of the Earth
in the pre-war years of the 20th century. These were years
when discoveries concerning radioactivity by Henri Becquerel,
Marie and Pierre Curie, Ernest Rutherford and others were