Archive: Plate Tectonics

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1:28and the racist, discredited pseudoscience of eugenics with Desiree Birch. We'll meet many medieval saints in their bone boxes with Rachel Parrott. So, if that sounds like your sort of thing, listen to You're Dead to Me wherever you get your podcasts. Thank you. Bye.

1:47This is In Our Time from BBC Radio 4, and this is one of more than a thousand episodes you can find in the In Our Time archive. A reading list for this edition can be found in the episode description wherever you're listening. I hope you enjoy the programme. I've chosen Plate Tectonics for the latest of our summer selection from Melvin Bragg's In Our Time because I was listening to the programme live when it first went out almost 20 years ago and it literally stopped me in my tracks. I was heading out the door in a rush but became so fascinated by the discussion I cancelled my meeting in order to finish listening to Melvin and his guests.

2:31Until this point, my only engagement with plate tectonics was as a rather bored 12-year-old in my school learning about how earthquakes are caused when two tectonic plates rub each other up the wrong way, as it were. But as the programme unfolded, the guests slowly revealed that these gigantic formations mediate between the vast fiery resources churning below the Earth's crust and the atmospherics of land and sea. How and why they push continents together and apart like pieces on a chessboard. For a long time, it was poorly understood how the conditions for life on Earth are absolutely dependent on this complex and delicate interplay.

3:15The programme reaches a crescendo as the academics have a vigorous exchange, well, a bit of an argy-bargy, really, about the validity of James Lovelock's Gaia theory. By then I was so engaged in listening, my coffee had gone stone cold. I guarantee you'll find this a classic in our time. Hello. America is getting further away from Europe. This is not a political statement, but a geological fact, just as the Pacific is getting smaller, the Red Sea bigger, the Himalayas are still going up, and one day the Horn of Africa will be a large island. These are findings from the theory of plate tectonics, a revolutionary idea in 20th century geology that saw the continents of the Earth to be dancing to the music of deep time, a dance of incredible slowness, yet powerful enough to throw up the mountains and pour away the oceans and dictate the character of the planet.

4:10Plate tectonics was a genuine scientific revolution. It made geologists, and many more besides, profoundly rethink what the Earth was, how it worked, and how it related to all the things in it. With me to discuss plate tectonics, a Joe Kahn, Senior Fellow in the School of Earth and Environment at the University of Leeds, Lynn Frostig, Director of the Hull Environment Research Institute and Professor of Physical Geography at the University of Hull, and Richard Caulfield, Visiting Senior Lecturer in Earth Sciences at the Open University. Richard Caulfield, plate tectonics is the mechanism by which the continents move around the surface of the Earth.

4:43It was developed in the 1960s, but let's go back. What sort of idea did people have of the sense of the movement of the Earth, if any? Well, the sense that the continents may have fitted together actually goes back to the 16th century when Abraham Ortelius, a Dutch mapmaker, realised that there was a great similarity in the way the bulge of West Africa looked as though it could have fitted into South America. and this was taken up and thought about over the years. Francis Bacon, for example, was interested in theory. Benjamin Franklin was also interested in it, this similarity in the way the continents looked, whether or not they might have once fitted together.

5:29But it wasn't until the early 20th century when a German called Alfred Wegener actually proposed a mechanism by which the continents might have once moved around and at one point been part of the same single supercontinent. And in fact, there are three components of the theory of plate tectonics, which is, as you say, a 1960s theory. You start with the idea of continental drift, which stems from this idea that the continents may once have fitted together. And then the second component is something which was discovered in the early years of the 20th century, which is the idea of seafloor spreading.

6:06And then this was all synthesized in the 1950s and 1960s into the theory of plate tectonics. And in fact, it's impossible to underestimate the importance of the theory of plate tectonics to the science of geology. It's really the royal oak of geology. It's the tree that dominates the forest and without which none of the science really makes sense. Can you tell us a bit more, though, before we come to the actual theory itself, about what people were saying about fossil distribution around the world and how puzzling that was with continents being so widely separated by oceans and so on. Right, well, on top of this idea that the continents look superficially as though they have, you know, shapes which would enable them to fit together like pieces of a jigsaw, you also have the distribution of rock types.

6:56Similarities, for example, in the nature of glacial sediments of permocarboniferous age, about 300 million years old broadly, which are found in South America and in Africa and in India. And the question then is, how come you get sediments of the same age found so widely separated? What explanations did people come up with and when and who? Well, there was an idea, because on top of this idea of similarity of rock types, there's this distribution of fossils as well, which are the Permian fossils, tetrapod fossils are very similar on a number of different continents. There's the so-called glossopterous flora, very similar across four continents.

7:42How could they possibly be so similar on four separate continents? Well, the idea was that there may have been land bridges, which sort of went up and down like a series of random elevators. Over the years, they came up and then they disappeared as completely as the lost continent of Atlantis. How long did the land bridges idea last and when did it come in? I can remember as an undergraduate people still trying to defend the idea of land bridges. They were an older generation of geologists, it has to be said. But basically land bridges seems to have been around, the idea of those kind of things seems to have been around sort of before, during and after the Second World War.

8:24Joe Kant, Richard's mentioned seafloor spreading. Can you explain how that developed through and after the Second World War? Yes, it started with Arthur Holmes, who was professor of geology in Durham and then in Edinburgh, who had the idea that the oceans were splitting apart in the middle and that the oceans were being carried on the back of giant deep earth currents to one side and the other. and that was taken up after the war by Harry Hess an American professor at Princeton who was also a sea captain all through the Second World War he'd sailed his ships to and fro across the Pacific and he had lots of time to make measurements and also lots of time for thinking and he refined this idea he was also the most amazingly persuasive speaker too he would stand there with his elbow on the rostrum chain smoking and talking away in a quiet voice but totally convincingly and that was one of the great routes from which a plate tectonic sprang.

9:23The other was the Second World War and during the Second World War all sorts of marvellous marine tools were developed. Magnetometers for searching for submarines, sonoboys for listening to submarines, explosives for destroying submarines and all of these were surplus to requirements at the end of the Second World War along with a lot of ship time and also, as well as that, there were scientists who'd been to sea during the Second World War. So they set about exploring the oceans, which had only been done in the most superficial way before the Second World War. And what primarily did they find that set us on the path towards the theory of plate tectonics?

10:04They first of all found that the mid-ocean ridges, that along the middle of the Atlantic Ocean and going all the way around the world wrapped in a sort of tennis ball-like seam, was a range of mountains with a rift valley in the middle of the mountains that might indicate things were pulling apart. It coincided with a chain of earthquakes. And then when you took ships across these mountain ranges, you found and towed a magnetometer across, you found these extraordinary big magnetic signals. Now, I remember when I was a student looking at these records and thinking, this is so weird, because it wasn't so much that the humps and bumps were there, it was the size of them.

10:45It indicated there was huge magnetic contrasts down on the bottom of the oceans, and that was very strange. So just after the Second World War, people are thinking that the old theories, whatever they were, which Richard began to outline, didn't obtain. Were they moving towards any new theory of what this told them about the planet as a whole? Well, this was where Harry Hiss was so influential because he really inspired all of us to start thinking about seafloor spreading and talking about seafloor spreading. And instead of being a sort of forbidden subject, as it had been certainly in America since 1928, in 1928 the Americans had a meeting where they declared continental drift as impossible.

11:25And so you couldn't teach continental drift in American universities at all after then. But in Europe and in the Southern Hemisphere particularly, There was a lot more sympathy to the ideas of continental drift and hence the ideas of seafloor spreading. So the ideas were there, but it took these magnetic anomalies that people found to really bring them from being a happy idea, which is how it was when I was a student, through to being a quantifiable fact where we could actually demonstrate what was truly happening. So, Lynne Frosty, can we go on from there? Let's say we've come to the end of the Second World War.

12:06Numerous observations have been made. As you have pointed out, scientists had been at sea. There was a lot of shipping around, a lot of time on people's hands, and they went on doing experiments. Where were we in terms of arriving at a theory? Where were you, in terms of arriving at a theory? Well, when I was an undergraduate, which I started in 1967, plate tectonic theory wasn't there. We did learn about seafloor spreading. But the problem with sea floor spreading is if you're spreading the sea and you're making the seas bigger, you've actually got to destroy something somewhere else if the whole Earth isn't expanding.

12:38And people knew that there was no evidence that actually there was an expanding Earth. There was a theory for a while, but it didn't last very long. So what you've got to do is find out where the ocean floor's going because we've not got an ever-expanding ocean floor, so we must be destroying it elsewhere. So they knew it was... The sea was spreading, but they didn't know... where it went to and how it went. Where it went to, and actually the mechanism by which it was being caused. So there were two aspects of it which weren't there. So what was the theory then before plate tectonics? What were you being taught?

13:10I was being taught about things called eugiosynclines and myogeosynclines, which sort of dropped down and came back up again for no explained reason. It was very much a jigsaw which didn't fit together, which had no underlying principles. And it was very confusing, actually, because that's what I was taught to start with. And when the theory of plate tectonics was published, which was during my time as an undergraduate, suddenly the pieces fell into place. It was like a jigsaw coming together. Let's stay with the sea floor for a moment or two before we come to this eureka moment, as you call it, actually, the plate tectonics.

13:42It's an extraordinary discovery quite recently. I mean, a lot of us were alive and aging in the 60s. What were they saying about sea? Can you just say more detail? What were they saying about sea floor spreading? It's a very nice phrase, but you imagine two hands together like that, and they spread out from one another. Was that what they were saying? What were they saying? More or less that, that there was tension at the mid-ocean ridge, which was opening up effectively large cracks, and magma was coming up in the middle, and then more tension, and magma came up in the middle and solidified. And it was that solidification.

14:18Magma under the crust. Magma under the crust of the Earth. Yes, volcanic material, volcanoes effectively. Which comes up and then hardens. And as it hardens, so little bits of iron, which there are in this very heavy, very dense rock, orient themselves in the direction of the polarity of the Earth, where the North Pole is and where the South Pole is. And, of course, there have been lots of magnetic reversals in the past. Do you have to tell people what you mean by that? Well, where the North Pole shifts to the South Pole and the South Pole shifts to the North Pole. So the whole thing turns round.

14:47And the Earth does this repeatedly over time, again and again and again. and we know that and we know that it happens at relatively frequent intervals. And can you work out when it happens? You can work out when it happens, yes. So we've got evidence of when it happens, the age of these reversals. But you look at the seafloor and what you can see is stripes of rock magnetised with the North Pole where it is now, actually at the Mid-Ocean Ridge. And then as you go out from it, reversals, you get the South Pole at the North, the North Pole at the South, and then another stripe with the North Pole at the North and the South Pole at the South.

15:19And these are repeated either side. This mid-ocean ridge you're talking about is down the middle of the Atlantic. The Atlantic, there's one in the Pacific, not complete. There's a whole string of them all around the Earth, and I think the description of it as a sort of tennis ball with these seams going all around the Earth and joined up is a very good way of thinking of it. So what we're saying is it was the exploration of the sea floor. The oceans. The ocean floor, more or less unknown until, let us say, the Second World War and afterwards, which led to the revolution of this. and then the theory of plate tectonics.

15:51Can you refine now, Richard Corville, how that came about in Cambridge, these two people? OK, actually, I wanted to just backtrack a little bit and say something that we'd missed, which is that the mid-ocean ridges, which is where new material is formed, comes out of the work of two American scientists called Bruce Hazen and Marie Tharp, and they were using another new technology, sonar, to produce remarkably detailed maps. And so it was known in the 1950s that new materials coming up at the mid-ocean ridges and that there soon became clear that there were earthquakes located there as well, so it was obviously seismically active.

16:27And then, as Joe says, the other half of this equation is the work of Hess, who, based on gravitational work, showed that, which is work he'd done in the 1930s with his thesis supervisor, showed in the 1950s and early 1960s that the other half of the plate tectonic equation was where this material which is produced must go. which is at the edges of the oceans, by subduction. And a consequence of that is that the oceans are very young. The oldest ocean crust is only 180 million years old. And these stripes, which my colleagues have been talking about, are effectively these magnetic reversals, which happen, if you imagine vertically in time, are being transposed laterally as a sort of tape recording of the Earth's changing magnetic field.

17:13As this new material is formed, as this new tape is being formed and rolls outwards from the ocean ridges. And subduction is when one plate hits another and the plate goes under, one plate goes under, and so it returns to this under the crust, and it returns to the magnet. It's recycled. It's recycled, and so the thing isn't lost, so it neither expands nor shrinks the planet. Can you just pinpoint for us now, Jack Ann, what these people did in Cambridge that came with the theory of plate tectonics? Can you briefly say what that theory is? It was a very exciting time. I was there as a research student and watched it all happen.

17:52Fundamentally, the people who drove it forward were a young student called Fred Vine and his supervisor, Drummond Matthews. And what Vine and Matthews did was to say, look, let's suppose, let's just suppose that seafloor is being created by seafloor spreading. Let's suppose, and this was not established at the time, that there's a regular reversals of the Earth's magnetic field. then this should produce a regular stripy pattern as the sea floor spreads apart and the Earth's magnetic field switches to and fro it should produce a regular stripy pattern on the sea floor and that should be symmetrical about the mid-ocean ridge and you should be able to measure the rate at which the ocean is spreading apart and that's what they found, that's what they demonstrated and so we moved from a happy idea to a really quantitative foundation of sea floors spreading.

18:45And that theory which you've called the Eureka Moment, I'm repeating myself but it's worth repeating, is something that was resisted in America in particular in Russia at Lynn Frosty. Can you tell us briefly why it was so heavily resisted? I think because there was an enormous amount invested in the previous ideas of what was going on. There have been a lot of observations, a lot of ideas about how things worked and I don't think anyone was ready to give up those ideas. It was only people who really thought beyond the accepted that would take this theory on. But I think it was accepted relatively quickly if you compare it with the ideas of Copernicus and how long they took to permeate through astronomy.

19:26Seafloss breading and plate tectonics really did speak to a lot of people because it explained so. Prep for a busy week with Whole Foods Market. Start your day with fully cooked breakfast sausages from Amy Liu, 365 brand frozen waffles with no bleached flours, And of course, Whole Foods Market eggs, which are all cage-free or better. In the evening, bring home a build-your-own family meal that feeds four for just$35. Choose one entree and two sides. Shop smarter, not harder, at Whole Foods Market. Hello, Greg Jenner here. I'm the host of You're Dead to Me, the BBC comedy show that takes history seriously.

20:07Every episode, I pair up a top historian with a fantastic comedian. and we have a lovely, funny, fascinating chat about a different subject from world history. And in this new series, we're beginning with an epic voyage through the story of Homer, the Iliad and the Odyssey. And that's with Kyle Smith-Bino joining us. And then we'll be learning about Francis Galton and the racist, discredited pseudoscience of eugenics with Desiree Birch. We'll meet many medieval saints in their bone boxes with Rachel Parrott. So if that sounds like your sort of thing, listen to You're Dead to Me wherever you get your podcasts.

20:40Thank you. Bye.

21:10the atmosphere together. It makes you look at everything as one. In fact, it's the prelude to what really are the ideas at the moment of Earth system science where the Earth is one big system and it works together. And the driving force of this really is plate tectonics. Can you just, Richard Caulfield, before we move on, can you just tell listeners how it works straightforwardly? We've used the word plate, we've used the word tectonics in the introduction that might have been in the trailer, I can't remember. And we said there were seven or nine large plates and innumerable smaller plates. Now what goes on specifically?

21:42Well, what's happening is... Right around the globe these plates are. What's happening is that the continents, which are granite, are carried around on oceanic crust which is basalt and the crust is continually moving. Imagine a conveyor belt in Sainsbury's, moving moving, moving. Imagine two conveyor belts in Sainsbury's converging on each other. And that is the situation that you have with plate tectonics. Because the plates are continuing moving towards each other and being subducted underneath each other, the continents are moving closer and closer together. For example, even as we speak, Africa is closing in on southern Europe.

22:23The Mediterranean that we know and love for our family holidays is in fact the remnant of a world-girdling super-ocean which went right around the world, around the equator, called Tethys. So Mediterranean is in fact a sad remnant of a once great ocean. And in 150 million years, it won't be there. There'll be a range of mountains, which will be already called the Mediterranean Mountains. And in 250 million years, the Mediterranean Mountains will be even higher. The Atlantic will be only a sad little pond locked up in a new supercontinent called Ultima Pangaea. So this is what's happening, is that the continents are always moving.

23:05as a function of plate tectonics and crashing into each other, continuously reshaping the face of the Earth. Joe, what's going on underneath the plates? They're not only crashing into each other, they're pulling away from each other, converging and diverging. And what's the deeper structure of the planet? The fascinating thing is that the sliding of the plates as they move along is almost silent. They are definitely moving, absolutely. But as they move, you see earthquakes at the edges of the plates, but you don't see a sheet of earthquakes underneath the plates. So the earthquakes are sliding on a soft sole about 100 kilometers down, and that soft sole is the Earth's mantle with tiny little bits of melt in it.

23:53It's lubricated by these little bits of melt, little drops of melt in the mantle at that depth. Deeper down it becomes solid. Above that it's solid, but there's just a narrow zone where it's soft. And it's the existence of that soft zone that allows the plates to slide so comfortably over the surface of the Earth. Of course, as they slide, oceanic floor goes down. It's about three square kilometres of new seafloor are created every year. Three square kilometres are destroyed. The plates move about as fast as your fingernails grow, about as fast as your hair grows. But when the continents arrive, the continents are a great thick object.

24:36They don't sink. They can't be recycled. They smash into each other. They collide. They're complicated because the continents have got all sorts of ancient fissures. They've got wrinkles and creases in them, just as us somewhat senior citizens have wrinkles and creases. and this means that when they're squashed instead of behaving simply if the ocean floor does they break along old fracture lines they're more complicated so the Himalayas, the mountains where two continents are colliding get quite complicated things Lynne, what happens when a plate's driven back into the earth? I just want to listen to me and I know doubt as to what is actually going on now and the things that are moving Joe's told us how they can move so apparently easily but they do collide and these collisions do result in the Himalayas, great ridge across the Atlantic.

25:28They do diverge. These diverges result in the Atlantic getting wider, just as the Pacific getting smaller and so on. But when it's driven back into the Earth, can you just tell us how this goes? Yes, what happens is that the lighter plate, which is normally the one with the continent on it, because that's much lighter material than the ocean floor, when it meets an ocean floor plate, and it depends what's on the plates as to what happens, the ocean floor, because it's denser, heavier, it actually dips down beneath the continental part of the plate. And as it dips down, so there's friction, so there's heat, and so there's melting, and this creates volcanic activity and creates mountain ranges.

26:06But this dipping down is a very strong feature, and it occurs all around the edges, for example, of the Pacific, around the Rim of Fire. There are what are called ocean trenches. They're the deepest places in the oceans. And also the Rim of Fire, that's where the earthquakes and volcanoes are. And that's where the earthquakes are and the volcanoes are. Because of the plate boundary activity. Because of the plate boundary and huge amounts of activity. You think about how much energy is expended by pushing or pulling a slab of oceanic crust beneath a plate. It's an enormous amount, and that's released as earthquakes, and it also, in the end, causes, for a whole range of different reasons, the melting, which forms the volcanoes.