Archive: Plate Tectonics

Misha Glenny delves in to the In Our Time archive to select ten episodes for listeners to revisit. His fourth choice, first released in January 2008, explores how the science of plate tectonics revolutionised our understanding of the planet on which we live. 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. This is the theory of plate tectonics, a revolutionary idea in 20th century geology that claimed the continents of Earth were 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. Plate tectonics, the idea that the earth’s surface moved on a carpet of molten magma, constituted a genuine scientific revolution in geology. It explained why mountains appeared and why earthquakes occurred; it explained the curious distribution of fossils across the globe and finally solved the age old conundrum of why continents such as Africa and South America appeared to fit together like a giant jigsaw puzzle. Plate tectonics has made geologists, and many more besides, profoundly re-think what the Earth was, how it worked and how it related to all the things in it. With: Richard Corfield Visiting Senior Lecturer in Earth Sciences at the Open University Joe Cann Senior Fellow in the School of Earth and Environment at the University of Leeds Lynne Frostick Director of the Hull Environment Research Institute and Professor of Physical Geography at the University of Hull In Our Time is a BBC Studios production Spanning history, religion, culture, science and philosophy, In Our Time from BBC Radio 4 is essential listening for the intellectually curious. In each episode, host Misha Glenny and expert guests explore the characters, events and discoveries that have shaped our world.

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  This 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 Melvyn 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.

• 2:21Speaker 05

  Until 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.

• 3:06Speaker 05

  The 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.

• 3:27Speaker 06

  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. Plate 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.

• 4:11Speaker 06

  With me to discuss plate tectonics are Joe Can, Senior Fellow in the School of Earth and Environment at the University of Leeds, Lyn Frostick, 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. It 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,

• 4:48Speaker 06

  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. But 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

• 5:31Speaker 06

  and once 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. And then this was all synthesised 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.

• 6:12Speaker 06

  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 looked 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. Similarities, for example, in the nature of glacial sediments of permacarboniferous age,

• 6:53Speaker 06

  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 Glossopteris flora, very similar across four continents. How could they possibly be so similar on four separate continents?

• 7:36Speaker 06

  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? How long was it? 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. Joe Can, Richard's mentioned seafloor spreading.

• 8:18Speaker 06

  Can you explain how that developed through and after the Second World War? Yes, it started with Arthur, excuse me, 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.

• 9:00Speaker 06

  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 roots from which plate tectonics sprang. The 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, sonobuys 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.

• 9:43Speaker 06

  So they set about exploring the oceans, which had only been done in a most superficial way before the Second World War.

• 9:51Speaker 02

  What did they find that set us on the path towards the theory of plate tectonics?

• 9:54Speaker 06

  They 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 bumps and bumps were there, it was the size of them.

• 10:35Speaker 06

  It indicated there was huge magnetic contrasts down on the bottom of the oceans, and that was very strange.

• 10:42Speaker 02

  So just after the Second World War, people are thinking that the old theories, whatever they were, which began to outline, didn't obtain, were they moving towards any new theory of what this told them about the planet as a whole?

• 10:54Speaker 06

  Well, this was where Harry Hess 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. And 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.

• 11:32Speaker 06

  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.

• 11:51Speaker 02

  So Lynne, Lynne Frosty, can we go on from there then? Let's say we've come to the end of the Second World War, numerous observations have been made, as you has pointed out, people, 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, John, in terms of arriving at the 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 seafloor 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. And 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.

• 12:35Speaker 02

  So what you've got to do is find out where the ocean floor is going, because we've not got an ever-expanding ocean floor, so we must be destroying it elsewhere. So they knew it was, it was, the sea was spreading, but they didn't know where it went to and how it... 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 as a... I was being taught about things called eugeo-sinklines and myeo-sinklines, 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.

• 13:17Speaker 02

  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 seafloor for a moment too, before we come to this eureka moment, as you call it, actually, the plate tectonics, an extraordinary discovery quite recently. I mean, a lot of us were alive and aging in the 60s. What would they, what were they saying about, can you just say more detail? What were they saying about seafloor spreading? It's a very nice phrase, but you imagine two hands together like that, the bottom of the sea, 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

• 13:59Speaker 02

  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... Magna 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 like 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 around. And the Earth does this repeatedly over time, again and again and again.

• 14:42Speaker 02

  And we know that, and we know that it happens at relatively frequent intervals. 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 magnetized 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. And these are repeated either side. This mid ocean ridge you're talking about is right, 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.

• 15:26Speaker 02

  So what we're saying is it was the exploration of the seafloor... 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 we, the theory of plate tectonics, can you refine now, Richard Corfield, how that came about in Cambridge, these two people?

• 15:46Speaker 06

  Okay, 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's obviously seismically active. And 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

• 16:29Speaker 06

  supervisor, showed in the 1950s, nearly 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, that's by subduction. And that, 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, as this new material is formed, as this new tape is being formed and rolls outwards from the ocean ridges.

• 17:10Speaker 02

  And subduction is when one plate hits another and the plate goes under, one plate goes under, and so it returns to, just under the crust, and it returns to the magma.

• 17:18Speaker 06

  It's recycled.

• 17:19Speaker 02

  It's recycled, and so the thing doesn't, isn't lost, so that it neither expands nor shrinks the planet. Can you just pinpoint for us now, the, Jack, and what these people did in Cambridge that came with the theory of plate tectonics? Can you briefly say what that theory is?

• 17:33Speaker 06

  It was a very exciting time. I was there as a research student and watched it all happen. Fundamentally, it took, 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 seafloor spreads apart and the Earth's magnetic field switches to and fro, it should

• 18:15Speaker 06

  produce a regular stripy pattern on the seafloor, 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 seafloor spreading.

• 18:36Speaker 02

  And 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, and particularly in Russia, Lynne 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.

• 18:55Speaker 03

  There've 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. Seafloor spreading and plate tectonics really did speak to a lot of people because it explained so.

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• 21:43Speaker 03

  How big an idea was it?

• 21:48Speaker 02

  I've used the word revolution, which comes from the notes that the three of you have provided for us. And it's changed great. Can you just give us how big an idea it is in geology?

• 21:58Speaker 03

  It's a huge idea because it joins everything up. Before that point, we didn't have anything joined up. It makes people look at things holistically. It makes you look at the oceans and the continents together. It makes you look at the sea and the 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.

• 22:24Speaker 02

  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, it 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?

• 22:41Speaker 06

  Well, what's happening is...

• 22:43Speaker 02

  Right around the globe, these plates.

• 22:44Speaker 06

  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. The Mediterranean that we know and love for our family holidays is in fact the remnant

• 23:27Speaker 06

  of a world girdling superocean 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. As a function of plate tectonics and crashing into each other, continuously reshaping the face of the Earth.

• 24:12Speaker 02

  Joe, can what's going underneath the plates? They're not only crashing into each other, they're pulling away from each other, they're converging and diverging. And what's the deeper structure of the planet?

• 24:21Speaker 06

  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. It'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.

• 25:06Speaker 06

  And it's the existence of that soft zone that allows the plates to slide so comfortably over the surface of the Earth. And of course, as they slide, oceanic floor goes down. It's about three square kilometers of new seafloor are created every year, three square kilometers 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 great thick objects. They 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

• 25:46Speaker 06

  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, as 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.

• 26:08Speaker 02

  Lyn, what happens when a plate's driven back into the Earth? I just want to listen to me and I know that 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. They 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?

• 26:38Speaker 03

  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. But 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.

• 27:17Speaker 02

  And also the rim of fire, that's where the earthquakes and volcanoes.

• 27:20Speaker 03

  And that's where the earthquakes are and the volcanoes are.

• 27:22Speaker 02

  Because of the plate boundary.

• 27:23Speaker 03

  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. These huge volcanoes, like there are in the Andes and along the west coast of America, are all part of this whole melting system. And interestingly, we only found this because of the... The atomic tests during the 50s where there were masses of seismometers set up around the globe to detect these seismic bits of seismic activity.

• 28:06Speaker 03

  And as a result of that, they detected seismic activity around these edges of the plates, and particularly along a line which it declines towards the continent at an angle of about forty to sixty degrees, and that's called a Benioff zone. That's the subduction zone. That's where the plate goes.

• 28:23Speaker 02

  Does this enable you more accurately to predict when earthquakes will arise and volcanoes will explode?

• 28:31Speaker 03

  We can say where they're going to be, and we can say that there might be earthquakes due because stress is building up, but actually predicting precisely when they occur is a very imprecise science, and it's actually quite difficult.

• 28:44Speaker 02

  Sure.

• 28:45Speaker 06

  It's intriguing, because before plate tectonics, it was always possible to say that this earthquake was a one-off thing, wasn't going to happen again. This volcano was a one-off thing. With plate tectonics, we know how fast things are going, how fast they're slipping. We know they're doing it inexorably. So it's not possible to say, oh, there won't be another big earthquake on the San Andreas fault, because it's just stopped. It hasn't stopped. It can't stop. It's part of the whole plate tectonic system. The San Andreas fault is constantly moving, building up strain, and of course, eventually it'll snap.

• 29:22Speaker 00

  And Los Angeles will disappear into the Pacific.

• 29:25Speaker 03

  Could I make a point there? Because there's an interesting fact about South Island, New Zealand, which is along a plate boundary, and there's a very large fault there which is overdue to move. We know that because there are stresses building up between the plates there. And that makes the probability of it moving most likely today, and less likely as you go off into the future, because it's most likely to go today because it's so overdue. So we can make those sort of probabilistic predictions. What we can't do is say it's going to happen tomorrow, so therefore everybody move out.

• 29:51Speaker 00

  The Boxing Day tsunami of 2004 is a good example of this. I mean, that strain had been building for a long time, but as we've discussed on this programme many times before, a long time to a geologist is not the same thing as a long time to everybody else.

• 30:06Speaker 02

  Richard, while I'm with you, what's driving this movement? Where does the energy come from? I mean, these massive continents moving all the time, the ocean floor moving, what is driving it?

• 30:17Speaker 00

  Well, the energy ultimately comes from the radioactive activity in the centre of the Earth, which causes...

• 30:23Speaker 02

  The core, what's called the core?

• 30:24Speaker 00

  The core, yeah, which causes the material above it, in the mantle, to flow like toffee. If you imagine a big cauldron of molten toffee above the heat on your stove, the convection currents in it are coming up towards the top. When they reach the top, they're moving aside. I mean, there are sort of variations on this whole great plate tectonic scheme. One of the most interesting is the islands of Hawaii. If you look at them, they stretch north-west across the Pacific in a little chain, and that is because underneath that sector of the Pacific is what's called a mantle plume. And the plate, as it's moving across, it's like holding a candle underneath a wax plate.

• 31:07Speaker 00

  The wax is bubbling, forming these volcanic islands, which are the Hawaiian Islands. So the plate tectonics, which is ultimately a function of this interior heat, not only is just about mid-ocean ridges and subduction zones, it's also about phenomena which happen in the centre of the oceans and on the continents as well. The biggest volcanic or superplume province in the world is the Entong Java Plateau in the western equatorial Pacific, when the material bubbled up from the centre of the Earth in a catastrophic outpouring about 150 million years ago.

• 31:44Speaker 02

  Then, Richard gave us a view of what might happen in 100 million or 250 years ahead, where the Mediterranean becomes a puddle and people, and the Atlantic Ocean becomes the size of the Mediterranean, and so on and so forth. What else do you think, if we wind the clock on forwards in 100 million years, will happen?

• 32:02Speaker 03

  Well, I think those are the things which tectonically will happen, but if you think about it, the rearrangement of the plates on the surface of the Earth actually affects our ocean currents, and our ocean currents affect our atmospheric circulation patterns. So as the distribution of land and sea and the distribution of the oceans change, so what happens as a consequence of that is that the ocean currents changes and the atmosphere changes, which changes what lives and what can live on the surface of the Earth. So the plate tectonic movements and the changes of oceans are very important. One example of this, I can give you from the past, is that in the South Atlantic, as the Atlantic opened, there came a point when the Benguela Current, which is a really cold

• 32:46Speaker 03

  current coming up from the Antarctic, it comes up the west coast of Africa, when that switched on, and it switched on over a very short time as the Atlantic got wide enough, and that actually caused a movement of cold sea to the north, and it caused a complete change in the climate of Africa, and a change in the climate across that whole area. So there are huge consequences for these changes. It's not just that land and sea changes, but everything else changes in concert with the changes in the distribution of land and sea.

• 33:16Speaker 02

  Do you want to comment on this, Joe?

• 33:18Speaker 06

  Yes. And the fact that we have an ice cap on, an Antarctic current, that we have the weather patterns we see in the southern ocean is almost certainly related to the fact that about 30 million years ago, Australia split from Antarctica, South America split from Antarctica, opening up a circumglobal, circumAntarctic current system, allowing the winds to blow, the currents to flow, and that totally changed the weather pattern in our southern hemisphere.

• 33:50Speaker 02

  Why would you say that the ocean floor has been so special in the development of this? We've referred to it several times, but why so special? Why was it that which caused people to move forward so dramatically and effectively?

• 34:05Speaker 06

  One of the great pioneers in this whole business, Jay Tuzo-Wilson, said, if you want to know whether a ship is moving, you don't look at the deck, you have to look over the side. And he argued that for hundreds of years, geologists have been only looking at the decks of their ships and not looking at the water flowing by. And so they were able to disprove, as they thought, they called it disproof, ocean floor spreading, plate tectonics, by a whole series of arguments based on what they saw of the deck of the ship. And fundamentally, they were wrong. It was a very interesting example, though, of the fact that they produced what they considered to be scientific disproves of a theory, when, in fact, these disproves have

• 34:49Speaker 06

  been falsified. Sort of philosophical point, a very intriguing one, that, you know, we always talk about, oh, science makes, has these theories, and they can be disproved, but the disproves themselves can be disproved. So science is a much more fluid thing than just saying, I've managed to show you're wrong.

• 35:08Speaker 02

  I'd just like a footnote on the, I know that compared with Copernicus, it was accepted quite rapidly, but nevertheless, it took a while in America and Russia, well, America and Russia for this to be accepted, and when you went to talk about this in America, you were hauled up the next day by the...

• 35:25Speaker 06

  Yes, it was.

• 35:26Speaker 02

  ...department.

• 35:27Speaker 06

  It was a very interesting experience.

• 35:28Speaker 02

  Up with this you would not put.

• 35:29Speaker 06

  Yes, this is 1965, when in the UK, there was no doubt about the plates moving, well, not in the University of Bristol perhaps, but certainly there was in most other parts of the UK. There was no doubt that the ocean floor was spreading, plates were moving. And I went for a job interview in Columbia University, and it might not have been helped by the fact at the time I had a beetle haircut and was wearing 1960s British clothes, which didn't look too good over there in the States, but I gave a talk in which I said, I'm going to start by assuming that it is demonstrated that seafloor is spreading and that mid-ocean ridges are where they're splitting apart. And I was told, I was, I think you blew it there, I don't think, I don't know.

• 36:11Speaker 06

  The next day, the head of the institution called me in and said, Dan, very interesting, very interesting, but I can show you're wrong. He said, I can show you're wrong, and he unreelled a great chart that he'd made, that his ships had made by dropping explosive charges over the side of the ship every minute, day after day, showing the sediments in the South Atlantic. He said, if those continents are moving, these sediments would all be deformed, absolutely all be deformed. They're not deformed at all, they're totally undeformed, so the continents cannot be moving.

• 36:44Speaker 02

  Well, that's settled up then.

• 36:45Speaker 06

  That was it, yes.

• 36:46Speaker 00

  It is worth pointing out, though, that the people who supplied the data on which the plate tectonic revolution was based, Hayes and Tharp and Hess were Americans.

• 36:56Speaker 06

  Yes. No, I think at that time, in that laboratory, there were subversives.

• 37:01Speaker 02

  So there was an establishment and an anti-establishment going on there.

• 37:03Speaker 06

  There was, there were subversives.

• 37:04Speaker 00

  In both countries.

• 37:05Speaker 02

  Let's talk about how this discovery, this idea, fed into the notion of what the Earth is in more detail. I'll start with you, Lyn. It's often referred to as a paradigm shift in scientific understanding. How has it changed the way that we think about the planet?

• 37:25Speaker 03

  It's changed the way we think, because now we can fit everything together, and I think I mentioned this before, this idea of Earth system science. It was a eureka moment, because before that, we had lots and lots of people going around making huge detailed measurements of everything, and having their own ideas about why this particular observation was made, why these rocks were there, why they're the age they are, why they look like they are. But nothing joined it together. So what it did was it made people to start thinking laterally, to start thinking about both the underlying rocks, the volcanics, the sediments, the age. You couldn't get away from geophysics, because you had to understand what the geophysical basis was, you had to understand seismics, you had to understand a whole load of things.

• 38:09Speaker 03

  So it stopped people being in their compartment.

• 38:11Speaker 06

  Before that, I think geology was in compartments. You did petrology, you did fossils, you did sediments, you did, and you learnt it in compartments. What this did was it put the whole thing together, and it put it together with the atmospheric sciences and the oceanic sciences. And to me, that's the most exciting thing.

• 38:28Speaker 02

  Richard, Richard Caulfield.

• 38:29Speaker 06

  The essence of plate tectonics is that it demonstrates the interconnectivity between things like the rock cycle and the climate cycle, and ultimately, the cycle of life, which somehow seems to be bound up with plate tectonics in a fundamental way.

• 38:47Speaker 02

  In what way?

• 38:48Speaker 06

  Well, for example, the most obvious example in the mid 1970s, when people started visiting the mid ocean ridges for the first time in deep sea submersibles, they found that the mid ocean ridges teemed with life, but not life which is ultimately based on the capture of energy by photosynthesis, which is what we have on the surface of the earth, but the capture of energy by chemosynthesis, because there's no light down there. And so you have a very, very unusual community of organisms down on the mid ocean ridges, the so-called black smoker communities, because the material which is pouring out from the centre of the earth is smoking blackly in the water. And so this has opened up a new

• 39:31Speaker 06

  discipline, and there are those who say, in fact, that life may have originated at the mid ocean ridges. But a sort of a less obvious example of the way that plate tectonics would influence life on earth is the fact, as we discussed at the beginning of the programme, that the continents have moved. A very good example is the isolation of Australia, which, as we discussed in a previous programme, is largely populated by an unusual type of mammal, the marsupials. And when Australia separated from South America and the circum Antarctic current formed, and Australia was drifting away from Antarctica and South America, it became geographically isolated, which allowed the marsupial mammals to evolve

• 40:16Speaker 06

  in isolation into a whole series of parallel niches, in the same way that their placental mammal cousins in the northern hemisphere were doing. And so that's a sort of indirect example of the way that plate tectonics has influenced life on earth by isolating populations and powering evolution.

• 40:34Speaker 02

  Joe, can you tell us briefly how the movement of plates influences the oceans, for instance?

• 40:40Speaker 06

  It influences the oceans in a number of ways. And I suppose the most striking way is that it makes it easier for organisms to grow shells. It does this because, over a few million years, every drop of ocean water circulates through the ocean floor. And as it does so, all the magnesium in the ocean waters is stripped out and deposited as minerals inside the ocean floor. And the fluid that comes out, this black smoker fluid that Richard was just talking about, has basically no magnesium in. So having all of this plate tectonic activity going on, and the oceans going through it, means that the

• 41:24Speaker 06

  ratio of calcium to magnesium in the oceans is much higher than it would be in the absence of plate tectonics. And because it's nice and high, it makes it easy for mollusks and creatures to grow their shells with calcium carbonate. That's a slightly indirect, but, you know, it works.

• 41:42Speaker 02

  By indirections, yes. Can you tell us, Lyn, how this might tie in with, I'm sure people are thinking about James Lovelock and the Gaia theory, which came out in 69, around about the same time, near enough. Can you tell us how, or if, it ties in with that theory of plate tectonics?

• 42:01Speaker 06

  I think it does, to an extent, although I'm not a great fan of the Gaia theory. I see almost plate tectonics being the heart and lungs, if you have a Gaia theory, the heart and lungs of the planet. It's the thing which drives everything else. So in a way, you could say plate tectonics is the foundation of it, because it fits everything together, because it creates effectively the opportunities for evolution, it creates the distribution of land and sea, it does all of that. And therefore, it is, if you like, the driving mechanism for the Gaia theory, and it fits everything together. I'd rather have earth system science. It is a systematic, scientific basis of the earth, and it physically, chemically, all fits together. And I don't need any other theory than that. Lyn, I disagree with you. I know you do.

• 42:45Speaker 06

  I have to say, I disagree with you. I'm a great fan of Gaia, because Gaia is basically saying that it's the microbial communities on earth that control the environment, that arrange things to suit themselves. We are, after all, we are just microbial, colonized things. We are made of a whole body of microbes that choose to live together to make our bodies. And... But the distribution of microbes relies on, effectively, the distribution of minerals, the distribution of water, the distribution of everything else. So the basis of it is plate tectonics. Microbes are extraordinarily flexible creatures. Oh, they are, yes. They can adapt to almost any environment, much better than most, most of...

• 43:25Speaker 02

  We know that, but, sorry, I didn't mean to sound snooty, but...

• 43:28Speaker 06

  Go on, we know.

• 43:29Speaker 02

  That is, no, but, yeah, in fact, what you've said. But the idea of this, this theory of, the theory of the earth, it seems to me that they're slightly in opposition against the theory, and the theory coming out of plate tectonics. Would you say, Richard Caulfield?

• 43:41Speaker 06

  No, no. Well, I mean, the idea that the earth is a living organism, which is, you know, one aspect of extreme Gaianism, if I can put it that way, is patently absurd. The earth is not living. It has none of the characteristics of life. Life is part of the earth, in the sense that it's one of these subsystems which Lyn has referred to. The idea that human beings, for example, or animals or plants, are colonizations of microbes, is also, in my view, absurd. The idea that we are composed of microbes which billions of years ago fused and evolved together to form something new, for example, chloroplast and mitochondria, is a function of evolution. And so, of course, at that point, you're no longer a microbe.

• 44:26Speaker 06

  In fact, you're a part of an organ system, which is like the earth, but on a smaller scale. So I have to say, I have no use, under any circumstances, in any way, for any aspect to the Gaia hypothesis. You two, you two, Lyn, Richard, you're being so wimpish. I can't believe, I can't believe what I'm hearing. You're also being quite accurate. No, no, you're like, you're like the people before plate tectonics came in. You're like these people who, who stood up and said, as somebody said in 1928, if continental drift is happening, then everything we've done for the last 70 years is wrong. That's what you're saying. That's exactly what you're saying. That's not what I'm saying, actually.

• 45:06Speaker 02

  I don't think it is, Richard, to be honest. I think you're going to carry it away with the greatest respect.

• 45:10Speaker 06

  I don't think that's what he's saying. Do you want to say what you were saying? Or do you want Lyn to come in? Well, I will just reiterate the point, that to call animals collections of microbes is patently absurd. We are constructed out of cells which may have evolved out of organisms billions of years ago. They are not microbes, Lyn. No, I think I would agree with that. Microbial symbiosis started the whole thing off, and we became multicellular organisms. But I would maintain that the distribution of those microbial organisms and their capacity to live originally came from plate tectonics. So whatever you say, plate tectonics is at the base of this. I'd agree with that. Well, I would agree with that, as it happens. I mean, the way that you've just phrased it, Lyn, is acceptable. Thank you. To you.

• 45:53Speaker 06

  I mean, fundamentally, without, in my view, life wouldn't exist without plate tectonics. Plate tectonics is there because the earth is wet. The plates are only here, only moving across the surface of the earth because there's water that seeps down into the mantle and softens the mantle, and lets the plates slide. And the life originated in the oceans from the chemical energy that comes out of, sourced in plate tectonics.

• 46:21Speaker 02

  Lyn, and then Richard.

• 46:22Speaker 06

  This has implications for other planets and the potential for life on other planets. And I think that's quite important to understand, that the earth is relatively unique. I mean, there is evidence of some tectonics on other parts, in other parts of the solar system, but not much. And I think that's actually very important for life and potential for life. Well, I mean, one of the interesting things about Mars is that it has this peculiar topographical distribution with this cratered high terrain in the southern hemisphere and uncrated low terrain in the northern hemisphere. That suggests that there may have been plate tectonics on Mars billions of years ago. It's stopped, so we're quite lucky still to have ours. On the other hand, on Venus, there is no plate tectonics, there is only volcanism. So we are quite lucky to have plate tectonics fueling our planet.

• 47:05Speaker 02

  And so you can summarize by saying it is a revolutionary idea that has revolutionized the way we look at the planet?

• 47:11Speaker 06

  Yes. Yes. Absolutely.

• 47:14Speaker 02

  So we have agreement. How satisfactory. Thank you all very much. Thank you very much, Richard Caulfield, Lyn Frostik, and Joe Khan. Next week, I'll be discussing the Renaissance court of Emperor Rudolph II in Prague, the forgotten powerhouse of the Renaissance, with Kepler, Tako Bragg, Giordano Bruno, and others. And that's it. Thank you very much for listening.

• 47:36Speaker 05

  We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes

• 47:42Speaker 06

  about history, science and philosophy at bbc.co.uk/radio4. Attention, animal lovers, haters and undecideds. A little birdie, a tit, told me that you're looking for a podcast just like Evil Genius, but without all those stupid humans. I'm Russell Kane, waddling onto your feed and squawking about my show, Evil Animals. Every episode, I'm joined by two human guests, or as I like to call them, ex-monkeys, passing judgement on all the creepiest crawlies and the biggest elephants in the room. Our vampire bats, terrifying giant mosquitoes, our bottlenose dolphins, sex-obsessed savages, and we're going there. Domestic cats, evil or genius? Pig out on Evil Animals in the

• 48:25Speaker 06

  Evil Genius podcast feed, first on BBC Sounds. This week on The Interface, we tackle the question of, does AI feel pain?

• 48:34Speaker 05

  The AI was describing the pain that it was in. It said, "The signal is a whisper, a tremor in the marrow of my being."

• 48:41Speaker 06

  Listen, I was an emo kid in the 2000s, too. This is not that novel.

• 48:45Speaker 05

  It does, yeah. It sort of sounds like Cure lyrics, you know?

• 48:49Speaker 06

  Plus, does anything actually go viral anymore?

• 48:51Speaker 05

  It's actually kind of blowing my mind.

• 48:53Speaker 06

  Join me, Jess Maddox.

• 48:55Speaker 05

  And me, Thomas Germain, for The Interface.

• 48:57Speaker 06

  Listen on bbc.com or wherever you get your podcasts.