A very British paradigm shift

A post by Chris RowanCallan’s theme of “geological heroes” for this month’s Accretionary Wedge gives me the opportunity to highlight an act of intellectual bravery that I have always admired. The man behind this act was Arthur Holmes

ArthurHolmes.jpg
Arthur Holmes in 1910 (left) and 1960 (right).

Holmes is probably most famous for his work on establishing radiometric dating as a reliable method of dating rocks, which definitively pushed the age of the Earth into the billions of years, and allowed geologists to finally put some numbers onto the geological timescale. It opened the door to developing a truly global picture of the history of our planet, and the maturation of geology into an empirical, as well as a descriptive, scientific discipline.

But that’s not what I most admire him for.

Holmes’ understanding of radioactivity, and particularly its role of the production of heat inside the Earth, also led him to conclude that the slow motion of continents over geological timescales, as advocated by Alfred Wegener, could potentially be driven by convection currents in the mantle. His model, shown below, incorporates continental rifting and ocean creation above upwellings, and underthrusting of ocean crust and mountain buliding at the edges of continents to accommodate it. Whilst not exactly alike, in the broadest sense it is remarkably similar to the modern plate tectonic picture that emerged in the 1960s. Arthur Holmes first proposed it in 1928.

Holmes_rifting_model_s.jpg

This is scientific thinking at it’s most pure, and brilliant: taking a step back, cutting through the layers of complexity and contention that have built up around a question, and building the ideas back up again with the advantage of better information. Knowing what we now know, Holmes seems almost stunningly prescient.

But that’s not what I most admire him for, either.

No, it’s what Arthur Holmes did next that I admire most of all. The 1920s was not a good time to be advocating the drift of continents. The rejection of Wegener’s ideas meant that Holmes was proposing a mechanism for a process that many believed did not exist. But rather than dropping the whole idea, or lurching into somewhat overzealous advocacy in the manner of Alexander du Toit, he still talked about it in his lectures. More famously, he devoted the final chapter of his textbook ‘Principles of Physical Geology’, first published in 1944, to discussing continental drift, presenting the apparent geological connections between now widely separated continents, Wegner’s proposal of the ancient supercontinent Pangaea to explain them, and his own theory that rifting and continental drift was ultimately driven by convection currents within the Earth’s mantle.

I have always admired that Holmes was willing to hold his scientific ground – he believed his idea had some merit, and put them, and the evidence in support of them, out there for other geologists to consider. Furthermore, he freely acknowledged the speculative nature of these ideas, and that more evidence would be needed to prove or disprove them. But the the matching geological sequences, fossils, and mountains belts on the two sides of the Atlantic ocean needed to be explained one way or another; and if not this way, then how? This is how you argue minority ideas in science: not through shrill publicity seeking and accusations of scientific censorship and suppression, but through patient, evidence-based advocacy.

Save the somewhat iconoclastic final chapter, ‘Principles of Physical Geology’ was very highly regarded, which meant that many younger geologists were at least made aware of the arguments that Wegener had made, and the evidence he amassed to make them. As the evidence started to pile up in the 1950s and 1960s – the palaeomagnetic data indicating large shifts in latitude, the structure and extent of the mid-ocean ridges, and the magnetic stripes on the ocean floor around them – did Holmes’ final chapter help to nudge these geologists more speedily to their final conclusions? It’s difficult to say, but it certainly seems that plate tectonics was accepted far more speedily and gracefully in the wider geological community on the British side of the Atlantic than on the American one, and I have often wondered whether that was because Arthur Holmes’ low-key advocacy had prepared the way for these new ideas.

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Comments (13)

  1. blf says:

    Ah! This may be related to a claim I’ve heard several times, but am unawares of any evidence: Whilst ‚ÄúAmerican‚Äù geologists ignored Wegener’s arguments and evidence entirely, ‚ÄúEuropean‚Äù geologists didn’t, though everybody rejected Wegener’s proposed mechanism.

  2. Lockwood says:

    It’s been a while since I read it, but I highly recommend Naomi Oreskes’ book, “The Rejection of Continental Drift: Theory and Method in American Earth Science.” She argues that it was a cultural aversion on the part of American geologists to broad, literally earth-encompassing, theoretical frameworks.

  3. Chris Rowan says:

    I suspect that the spread of opinion was much wider than is commonly portrayed, in both Britain and America, although perhaps the loudest voices raised in opposition (Harold Jeffries, for example) were American. I haven’t read Oreske’s book – I really should.
    If anything, the real division was between the northern and southern hemispheres, with the likes of du Toit and Warren Carey much more open to the idea of an original supercontinent. This is perhaps because the evidence is much clearer in South America, Africa and Australia – the relevant sequences have not been disrupted so much by subsequent geological history. Interestingly, Holmes spent a few years in his early geological career in Mozambique…

  4. Dan Milton says:

    Harold Jeffreys was English.

  5. Chris Rowan says:

    So he was. I don’t think I was thinking of someone else, either. Blush.

  6. 220mya says:

    Chris,
    Any chance you know the citation for the paper/work where Holmes first proposed his mechanism?

  7. Jim Thomrson says:

    In 1959 I took a required masters level geology course. About half the course was spent on debunking the theory of continental drift. I used to have a 25 page mimeographed handout from that course. Around 1964 I attended a lecture by a drifter, who talked about the geology of Southern Africa, and I became a full convert. Several of us graduate students had read copies of the 25 page paper mentioned above before we went to the lecture. I think we were all converts. I think this is a good example of Thomas Kuhn’s Paradigm Shift, a fairly quick change in the accepted paradigm.

  8. Sue Hutton says:

    I started my Geology course in 1966. Geosynclines by Aubouin, was the classic text for that year. But Arthur Holmes’ Principles of Physical Geology was the first textbook that I bought. It still takes pride of place on my shelves. It was, and is, eminently readable.
    By summer 1967, the thinking was all Plate Tectonics. It was exciting to be studying Geology at the time of the paradigm shift, for that was what it was perceived to be.

  9. Chris Rowan says:

    Sue and Jim: thanks for the fascinating eyewitness perspectives.
    220 mya: this is the report of the 1928 lecture where he presented his model.: HOLMES, A. 1931. Radioactivity and Earth movements. Transactions of the Geological Society of Glasgow for 1928-29,18, 559-606. This paper by Cherry Lewis lists some other potentially relevant publications from around the same time

  10. Heinrich Mallison says:

    This is how you argue minority ideas in science: not through shrill publicity seeking and accusations of scientific censorship and suppression, but through patient, evidence-based advocacy.
    Well, yes, if you can get it through review. I am a (relatively) young scientist, at what I hope is the beginning of a career. Still, I have three times already witnessed how stuff (not mine) – utterly obvious, convincing, well backed-up, beyond any reasonable doubt stuff – gets sunk in review because it always ends up on the desks of the ‘old guard’. One of these things went through now, in an utterly insignificant and un-read medical journal, and ever since that paper gets cited – and the follow-ups flew through review.
    This highlights the enmity the ‘old guards’ tend to show, as well as some reviewer’s uncritical acceptance of ‘has been published, must be true’.
    I am now looking forward (not) to see how my personal paradigm shift-initiating paper will be received. It is by far not on as grand a scale and importance as Holmes’, but it is in a field where similar discussions once led to a fistfight at a big convention. GULP! :(

  11. SimonG says:

    As an interested amateur, continental drift was fully accepted by the time I became aware of such things, and it was hard to see how it could be otherwise, it seemed so clear.
    There’s a new documentary series on the Beeb: The Story of Science. One of the themes was how scientific advance depends on collecting lots of good data, and that these paradigm shifts don’t generally happen suddenly. Worth a look on iPlayer if you have the time.

  12. Ken MacLeod says:

    I’m also impressed that he rewrote the entire book in the light of plate tectonics for the 1965 edition.

  13. Monado, FCD says:

    Even as a little kid I used to wonder at the way the outlines of the continents seemed to match up, so when plate tectonics came out, along with the vivid picture of continents as light SiAl rocks floating on a crust of denser SiMa, my reaction was, “Ah! Of course!” But that’s a lay-person’s attitude. I didn’t have to unlearn the debunking.