PastCast: Plate tectonics – the unifying theory of Earth sciences

Kerri Smith

This is the Nature PastCast, each month raiding Nature’s archive and looking at key moments in science. In this show, it’s back to the Swinging Sixties – a time when continents too began to move.

Music: Neptune (From the Planets Suite) by Gustav Holst

Voice of Nature: John Howe

Nature, September 7 1963. Page 947: Magnetic anomalies over oceanic ridges.

Fred Vine

The paper when published went over like the proverbial lead balloon. I mean there’s all sorts of comments in the literature of people referring to it as a startling idea.

Melinda Baldwin

Now, it’s kind of taken as one of the first and most important steps on the way towards plate tectonics, but at the time it kind of fell like a stone in the water. My name is Melinda Baldwin. I’m a historian of science and I’m working on a book project about the history of Nature.

Fred Vine

My name is Fred Vine. I was a graduate student at Cambridge and I wrote this paper with Drum Matthews, my supervisor in mid-1963.

Melinda Baldwin

What they were proposing in this paper is that we might be able to support continental drift theory or really more accurately seafloor spreading theory by looking at magnetic patterns across mid-ocean ridges, which are ridges in the ocean floor.

Voice of Nature: John Howe

Figure 1 illustrates the essential features of magnetic anomalies over the oceanic ridges. This pattern has now been observed in the North Atlantic, the Antarctic and the Indian Oceans. In this article, we attempt to account for it.

Music: Neptune (From the Planets Suite) by Gustav Holst

Naomi Oreskes

I’m Naomi Oreskes. I’m a professor of the history of science at Harvard University. The idea of continental drift is generally credited to Alfred Wegner, a German geophysicist who first started developing the theory in the 1910s, around 1912-1913. He wasn’t the only person who was thinking about the idea of large-scale crustal mobility, but he probably developed the most fully articulated theory that was very, very widely discussed in the 1920s.

Melinda Baldwin

His sticking point and the reason that a lot of people didn’t accept his theory was that he couldn’t explain how continents moved. But he had this idea that the ocean floor was almost like a viscous fluid, but people just weren’t convinced.

Fred Vine

One of the main reasons why so many geologists didn’t believe in continental drift prior to the late 60s was the influence of physicists in stressing or being quite adamant that the mantle of the Earth’s interior couldn’t flow, that it couldn’t possibly accommodate the movement of continents because it was too rigid. So many geologists invoked that line – well, the physicists say it’s impossible and the interior of the Earth is too rigid so it can’t be.

Naomi Oreskes

And so most people thought the theory had been discredited or had been disproven or just not really accepted but then in the 1950s, a number of scientists began to look at the question again.

Fred Vine

I first came across the idea of continental drift when I was about fifteen, revising for an exam in geography. I was actually in the middle of a cycle ride in Kent. I was staying with my great aunt and uncle. I opened a textbook in a rather desultory way of doing some revision and on the first page of the first chapter was this diagram of South America fitted into Africa and this mention of continental drift, which people had speculated about but nobody knew whether it was right or wrong. I thought wait a minute, this is incredible! Laughs. That’s fantastic. I never thought I would end up proving continental drift.

Music: Fifties Rock and Roll Johnny Be Bad

Naomi Oreskes

The most important thing that made people rethink the question of continental drift and be willing to reopen the debate was new evidence. There was all this new evidence from the oceans that really invited a reconsideration of the question. But if we think about it in a broader political and cultural sense, it was very much tied up with the Cold War. The United States, the Soviet Union and to some extent the United Kingdom became very, very interested in the oceans as a potential theatre of warfare in a nuclear world. So, both the United States and the Soviet Union invested hundreds of millions or billions of dollars in exploration of the seafloor to facilitate submarine warfare, and oceanographers and geophysicists were the beneficiaries of that. So, the story of plate tectonics and the reopening of the debate is very much tied up with the Cold War and very much tied up with how the larger geopolitical environment helps to determine what scientific work we do and what scientific work we don’t do.

Voice of Nature: John Howe

Page 947. In November 1962, H.M.S. Owen made a detailed magnetic survey over a central part of the Carlsberg Ridge as part of the international Indian Ocean Expedition.

Naomi Oreskes

The way they measured the magnetic field over the oceans at that time was with a magnetometer – a device that measures the magnetic field – and that was generally done by towing a magnetometer behind a ship.

Fred VineBasically, you tow the detector behind the ship to get it away from the magnetic effect of the ship itself. It was a bit inelegant but it just about did the job, though we had to lash one up on board the ship in the Indian Ocean because the sharks had cut through the cable and we’d lost all our other detectors. Well, my remit was to try and make sense of that magnetic data. The crucial starting point here that Drum Matthews, who was out in the Indian Ocean conducting gravity and magnetic surveys, when I joined the group he did some small surveys of some actual areas – one, the all-important one, over the crest of the oceanic ridge in that area, the Carlsberg Ridge, the ship going up and down, up and down many times, and the crew wondering what on Earth’s going on so that you can get a detailed, albeit relatively small, survey area, but that was absolutely crucial because you could then begin to see exactly how the magnetics correlate with the typography and that is crucial.

Voice of Nature: John Howe

Work on this survey led us to suggest that some 50 percent of the oceanic crust might be reversely magnetised and this in turn has suggested a new model to account for the pattern of magnetic anomalies over the ridges.

Melinda Baldwin

Sometime in the Earth’s past, possibly multiple times, the Earth’s magnetic field had switched and so sometimes you encountered oceanic crust that was magnetised the standard way, the way it is today, and sometimes you encountered crust that was reversibly magnetised. And so, what Vine and Matthews were saying is that if we look at magnetic patterns across oceanic ridges, if, in fact, the seafloor is spreading out from these mid-ocean ridges, we would expect to see kind of a zebra stripe pattern of normally and reversibly magnetised oceanic crust.

Voice of Nature: John Howe

If spreading of the ocean floor occurs, blocks of alternatively normal and reversely magnetised material would drift away from the centre of the ridge.

Melinda BaldwinIt seems to be a case of simultaneous discovery. Vine and Matthews were working on their paper independently. Vine, I think, was using some new computer modelling techniques and they were drawing on the data that the Cambridge department had collected on their own expeditions. But meanwhile in Canada, there was another geophysicist named Lawrence Morley who had also observed in published data that you see these interesting patterns of zebra stripes across oceanic ridges, and so Morley started to think, well, I wonder if we’re seeing these stripes because the seafloor is spreading out from the mid-ocean ridges, and he actually wrote up the idea and he sent it as a letter to the editor in Nature. He actually sent it before Vine and Matthews sent their letter, but his paper was rejected. But I think this goes back to the way that the editors of Nature at the time, the way they edited the journal. They had very strong connections to some prominent British scientists. I suspect what happened is that the Vine and Matthews paper came from Cambridge covered by a letter from someone they knew, and that immediately got it to the top of their stack, whereas Lawrence Morley was writing from Canada and didn’t really have anyone prestigious recommending his paper and so it just wasn’t a priority.

Naomi Oreskes

So, Lawrence Morley is a man who could be bitter that he had the same idea, it was rejected, and then Vine and Matthews got all the credit, but he’s not bitter. He feels that he had a good scientific career. He was part of an exciting time in science and he’s gotten some post-hoc recognition, although not as much as Vine and Matthews, and Alfred Wegner was the same. Alfred Wegner did not die unhappy. We have his notebooks. We know something about what he thought and he was confident that in the fullness of time, the scientific community would develop a theory of continental mobility along the lines of what he proposed, so that kind of faith in the collective enterprise I find really inspiring and I find that to be, well, just inspiring.

Music: Neptune (From the Planets Suite) by Gustav Holst

Voice of Nature: John Howe

Magnetic anomalies over oceanic ridges by F. Vine and Dr. D. Matthews.

Fred Vine

The paper, when published, went over like the proverbial lead balloon. I mean there’s all sorts of comments in the literature, people referring to it as a startling idea or this can’t possibly explain the data. It just seems extraordinary.

Melinda Baldwin

Now it’s kind of taken as one of the first and most important steps on the way towards plate tectonics, which is sort of the dominant approach in geophysics, but at the time, and Vine and Matthews have both said this, they felt like it kind of fell like a stone in the water. They didn’t think that anyone was paying attention to it. They were kind of disappointed by its lacklustre reception.

Fred Vine

What surprised me even more was that there were several other people – okay, not many people in the world and no more than half a dozen – who were working on the same problem, that is the interpretation of marine magnetic anomalies, and I’m surprised that one or two of those in particular didn’t come up with this idea. In fact, I know certainly one of them kicked themselves for not doing that. I was even more surprised, really, that those same people who had been wrestling with the same problem as I had as a PhD student pretty well rejected the idea when it was published. I can understand if other people who were not familiar with the problem thought it was a wild idea and very speculative, but I was quite shocked that people who’d struggled with the same problem didn’t think it was a promising idea. And of course, it was two or three years before we were able to prove it. I mean it was speculative in that sense at the time.

Melinda Baldwin

So, it wasn’t until later in the 1960s when Vine started working with a Canadian geologist, J. Tuzo Wilson, and started collecting more and more magnetic data about these oceanic ridges that the geophysics community became converted to the Vine and Matthews model. It must have seemed slow at the time – to Vine and Matthews especially – but in retrospect, that’s a fast conversion to an entirely new view of the Earth.

Music: Neptune (From the Planets Suite) by Gustav Holst

Naomi Oreskes

People began to look more closely again at this question of mobility and in the next five years or so, scientists began to recognise that the older idea of continental drift, which focused mostly on the motion of the continents, could be integrated into a more global way of thinking that connected the mobility of the continents to the mobility of the oceans as well, and it’s that integrated, connected vision that leads to this notion of plates and so the name or the idea of plate tectonics emerges as a way of emphasising this is not just about the mobility of the continents but it’s about the mobility of the entire crust of the Earth.

Fred Vine

It seems so obvious now – not least because we have a very accurate picture of seismicity or where earthquakes occur – but in the early 60s we didn’t have a very clear picture of the actual distribution of earthquakes. It was 1967 when we had a really decent first picture of earthquake activity, which picks out the plate boundaries perfectly, and the only reason we had it then – I always like this story because it goes back to the defence of the collections – is because of the 1959 Nuclear Test Ban Treaty which banned nuclear testing in the atmosphere, which literally drove all the tests underground and in driving it underground, the Americans and the Russians needed to know what the others were doing so they set up very accurate, state-of-the-art, worldwide seismograph networks. It was only because of that by 1967 we had a decent map.

Naomi Oreskes

Once you begin to interpret the Earth in terms of plate tectonics, you realise that virtually all of the major regions of seismicity, of earthquake activity, of volcanism, they all correlate with plate boundaries. I mean it is our theory of the Earth so if you teach geology – which I did in the early stages of my career – everything we teach is in the context of plate tectonics because we understand the phenomena that we observe, we understand in terms of this large comprehensive framework. So, plate tectonics is often referred to as the unifying theory of modern Earth science and I think that’s correct.

Music: Fifties Rock and Roll Johnny Be Bad

Fred Vine

So many people, cameramen I’ve worked with and everything else, they say, well, I did this in primary school in the early 70s – I didn’t realise it had only been discovered a few years before. Laughs.

Voice of Nature: John Howe

Nature, September 7 1963.

Music: Fifties Rock and Roll Johnny Be Bad

Fred Vine

Science in general does move forward in quantum leaps. Every now and again there’s a breakthrough and suddenly it opens up whole new vistas.

Music: Fifties Rock and Roll Johnny Be Bad

Voice of Nature: John Howe

Editorial and Publishing Offices of Nature, St. Martin’s Street, London. Telephone number: Whitehall 8831.

Kerri Smith

You’ve been listening to the Nature PastCast, produced this month by Charlotte Stoddart with contributions from Fred Vine, Melinda Baldwin and Naomi Oreskes. Next time: a proposal from the 1990s for how to test for life on other planets.