What can plate tectonics teach us?

Can the often destructive forces of our planet shed light on its inner workings?
28 February 2023
Presented by Will Tingle
Production by Will Tingle.


Cracked Earth


The news is awash with the destructive side of these geological processes, but are there aspects of these events that can help us learn more about our planet’s inner workings?

In this episode

a crack in the earth

01:05 - How do plate tectonics work?

How does the ground beneath our feet move around and interact with each other?

How do plate tectonics work?
James Jackson, University of Cambridge

Perhaps the first place to start would be some surface level knowledge. More specifically, how does the ground beneath our feet operate? And if this is surface level knowledge, surely an expert isn’t needed to explain it. I mean, how hard could it be? Well, armed with too much self confidence and a strange array of demonstrations, I took it upon myself to become the teacher of tectonics. And they do say every day is a school day.

Amelia - Hello, my name's Amelia. I'm 16.

Will - Well, what do the words plate tectonics mean to you?

Amelia - I know that there's the ocean ones and there's the continental ones, but that's about it. And I know they're under the earth and they move around a bit.

Will - <laugh> You're already doing better than us. Way better than James, James had no idea what an oceanic crust was. But to demonstrate that, because these are on a scale unimaginable, massive earth plates and oceanic plates. What better way to demonstrate that, James, than with this...

James - An orange?

Will - I have an orange. Would you mind peeling this orange? Now listeners at home. I'm sure you think we're off our rockers.

James -
Have we got any tissues? <Laugh>. It's a bit disgusting actually.

Will - Well whilst he's doing that. I'm going to reveal the secret ingredient to this wonderful demonstration, which is a pot of jam, James, that looks lovely. And I'm about to ruin your perfectly peeled orange by smearing it in jam. But this shall all make sense in a second.

James - I didn't realize we were going to cause such a mess in this school.

Will - <laugh> Amelia, I have now smeared the orange in jam. Is any of this screaming plate tectonics to you currently?

Amelia - Um, no. It just looks a bit unappetizing right now. <laugh>.

Will - James, would you like to peel off your most Africa shaped bit of orange peel? Put that up back on the orange on top of the jam. And then we've got, let's call this Europe. So if you see here now I'm gonna push these two plates together, if you will. And what can we see is happening to the jam in between the two slices of orange?

Amelia - The jam is kind of coming up and making a bit of a mess really. But it's coming up in between the separate bits of the orange peel.

Will - And does that perhaps give you an idea as to what happens when two areas of land might collide with each other on top of the earth?

Amelia - I'm thinking maybe the jam is the magma under the Earth's crust. So maybe volcanic eruptions. I'm not entirely sure though.

Will - I would say this is already a smashing success. Volcanic eruptions or mountain ranges on other types of plate tectonics moving together. If we did this going the other way though, if we had two pieces of orange peel, which we consider to be one land mass, pulling them apart like this so that the jams spread back over, what would you imagine is happening there?

Amelia - Maybe an ocean, but a hole in the ground?

Will - Interesting. Interesting. So it's perhaps not as well represented by the orange. I was hoping to get there. The idea of the land shifting so that magma comes up through and creates new land. Theoretically that's what's happening there. However, it doesn't seem to have gone very well <laugh>.

James - Nice.

Ok, fair’s fair. It is a lot harder than it looks. Perhaps it would be better to hand it over to an expert. And who better to take up the mantle than the University of Cambridge’s Professor James Jackson.

James - A cross section of the earth looks a bit like an onion. So if you take an onion apart, it has layers of different material and the earth is like that. So if you were to look at the earth now, and you are familiar with the sort of things you see at the surface, all sorts of rocks which you would recognize at the surface, that goes down only about 30 kilometers under England. And that's not very much because the distance to the center of the earth is 6,300 kilometers. It's a long way down to the middle. So just the top 30 kilometers has all the rocks which you are used to seeing and we call that the crust. And then when you go down underneath that, you get different stuff. And that different stuff is pretty much the same all the way for the next 3000 kilometers down. And it looks green and it's always colored green in books because it is green. And it's green because it's made of a particular green mineral, which is called olivine, which is very common. And then there's a really big surprise because then in the center of the earth there's a ball. And that ball is about 3000 kilometers in radius. It's about the size of the planet Mars. And there it is, it sits inside the earth. And the big surprise is that the ball is liquid. It's made of molten iron and there's even another surprise inside that ball is another ball. And the ball inside that ball, which we call the core, is about the size of the moon. And that is where it goes solid again. So it's made of the same stuff, it's made of iron, but in the middle is a ball of solid iron. And outside it then is this ball of liquid iron, which is called the core, which is about the size of Mars. Then we have all this green stuff, which is a green and it's called the mantle. And right at the top is what we walk around on, which is called the crust, which is only about 30 kilometers thick.

Will - When we talk about oceanic plates and continental plates, what is a plate?

James - A plate is like a spherical cap. It's a curved cap which sits on top of the earth and it slides around and the whole earth is covered in a patchwork of these things. There are about 12 major ones and they all slide around, move past each other and bash into each other and separate from each other.

Will - And are there different kinds of plate?

James - The plates are all the same in that they are about a hundred kilometers thick, perhaps a bit more in some places and they move around. They are only different in that some of them have passengers on and some don't. So that the continents are not what really matters here. The edges of the continents are not the edges of the plates. The continents are sitting on top of the plates like a passenger or like a life raft if you like. Just moving with the plates sitting on top of them.

Will - And do these passengers affect the way that each plate interacts with one another?

James - Yes they do. Because when plates meet each other, generally what happens is one slides underneath the other. Imagine you are going up an escalator. When the escalator reaches the top, it just slides back into the underground and goes round again. And, and that's exactly what happens at the surface of the earth. Except you'll notice that when you reach the top of the escalator, you don't go back inside the building, you get scraped off on the top. And that is what happens to the continent. So the continents are sitting on the escalator and when they reach the edge, they don't go back inside the earth because they're too light. Imagine a cork floating on top of something which is heavier. So the continents get scraped off and they bash into each other.

Will - And what do the various interactions of these plates result in?

James - The plates, when they move past each other, have to slide past each other. And when rocks slide past each other, they break and they move on surfaces, which are called faults. So imagine a fault is like a knife cut in the earth, but a very big knife cut. I mean these ones, they go down maybe tens of kilometers, they may be hundreds of kilometers long, but as the rocks slide past each other, they vibrate and those vibrations are earthquakes. So one of the consequences of the edges of the plates is you get big earthquakes and you can also get volcanoes because as shared material moves back inside the earth or as material comes out of the earth, you get molten rock, which also moves around and comes to the surface. And those are volcanoes.

Will - What is it underneath the plates that's causing them to move?

James - Underneath the plates is not what you often see pictures of in books. It's not liquid. It's rock, but it's hot rock. And hot rock has no strength. Think of something like hot toffee or treacle or something of this sort which is moving because it's got no strength, right? And it's got no strength in this case because it's hot. And so that allows the cold rocks, which are at the surface of the earth, which are much stronger, can slide over the top of them even though they're not liquid, but they're just very soft solid. And what's making it all happen is actually the earth is losing heat like this. So if you have a cup of coffee or anything or a saucepan of liquid which you heat up. The rate, the way the heat gets from the bottom to the top is by the liquid moving. And that's what actually happens inside the earth is that hot material from the inside moves towards the surface. And as it moves, things have to get out of the way. And that's why these plates are sliding around, but it's not moving in the liquid state. It's moving as a very soft solid, but it works exactly the same way.

"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.

09:57 - How do we know what Earth's core looks like?

How can we be sure what the centre of the Earth looks like if we can't go down there?

How do we know what Earth's core looks like?
James Jackson, University of Cambridge

I remain undeterred with my demonstrations because it’s not just important to know about the surface of the Earth, but also what’s going on inside of it too. Starting with the obvious question, how can we find out what the centre of the Earth is made up of…

Will - I've got these two Easter eggs here in front of me, and I want James to be involved in this as well because I've surprised James with this demonstration. He doesn't know what's going on here. So between the two of you, does anything about these two scream the processes going on inside the earth to you?

James - I guess they're both sort of roughly spherical and, well, that one looks like it's got it's hollow inside, which I'm pretty sure the earth isn't hollow inside. I might be wrong.

Will - So we have two eggs in front of us and my question to the pair of you is what can you tell me about the inside of either or both of these eggs just by looking at them?

Amelia - You can't really tell anything.

Will - James?

James - Well, yeah. Ignoring the picture of the egg on one of the boxes. <laugh>, nothing.

New Speaker - So if I were to put a challenge to you two as to, how would you be able to find out what might be inside without breaking the egg open? What would you do?

Amelia - Maybe give it a shake and have a look at what's inside?

Will - Maybe give it a shake?

James - She's good.

Will - She's good. So if you were to shake this egg here... <Silence> Nothing. Nothing. However, were you to shake this egg here <rattling>

Amelia -
It's not hollow.

Will - It's not hollow. So therefore, could we assume that by vibrating this 'egg earth' here, you could find out where the different densities inside the middle might be? You could find out what constitutes the center of this 'egg earth'?

Amelia - Potentially. Yeah.

James - Yeah. Seems plausible.

Will - Seems plausible. Would you say I'm onto something here?

James - Yeah. Who can tell us more?

Will - So glad you asked <laugh>.

We’ll call that a partial success, but obviously there’s only so much you can explain with a chocolate egg. And so I put the question again to James Jackson, just how could we know so much about the centre of our Earth?

James - How can we possibly know? Because you can't go down there. How we know is by looking at sound, how sound travels through the earth. So we can't go to the center of the earth, but sound can, and this is actually something you are familiar with. You see pictures all the time of mothers who go to hospital and they get ultrasound scans of the baby inside them. And this is just by sending sound through the body and the sound bounces off things. It goes around things and you look at how it goes through the inside of the body and that allows you to construct exactly what is inside there. And that is exactly the same technology that we use to look inside the earth. In fact, the geologist got there a long time before because the geologist worked all this out about 1920, which is long before anyone thought of using this to look inside the human body. But the science is exactly the same.

Will - There is a slight discrepancy in size between the entire earth and a human baby. How big do the vibrations involved have to be?

James - So if you are looking inside a human body, you are in a good position because you can completely cover the body with microphones. You can have little sound sources going, ping, ping, ping all the way around. You're in complete control of where the sound sources are and where the sound receivers are. The microphones on the earth we're not in such control of. The sound sources we use are earthquakes, natural earthquakes. So a decent sized earthquake will sens sound all the way through the earth. You can record it on the other side easily. But of course your sensors, your receivers are instruments called seismometers, which is a smart way of saying it's a microphone which picks up vibrations in the earth. But of course they're all on land cuz that's where we live. They're very few of them under the sea. And furthermore, the earthquakes are only in special places. The earthquakes are not everywhere, they're on the edges of these plates. And so we are much less in control of what's going on. We don't have earthquakes everywhere we would like them, and we don't have receivers everywhere we would like them. But we do have a lot of them and we do the best we can. But it means that we know some places in better detail than other places. That's all.

Will - And how does this work then? The vibrations pass through the earth? Do they get delayed in areas that are of different density?

James - Yes. The speed with which they travel depends on the density of the material and so that gives you some idea of what's going on. But the density of the material also depends on its temperature. So where it's hotter, they go slower, and where it's cooler, they go faster. So there are other tricks we can use to see exactly what the variations are inside the earth.

Will - When the vibration passes through areas of different density, how big is the delay between these different materials for you to be able to go 'ah, this is polymer A or substance B'?

James - It takes, for example, about 18 minutes for the sound to travel from here to the other side of the world. So if there's an earthquake in Britain it would take about 18 minutes for it to go to New Zealand. And the sort of delays you see on average are plus or minus, say 10 seconds. That sort of delay. It's quite easy to spot, 'Ooh, it's coming a bit earlier than it should, or it's a bit later than it should. That's because it's gone through something on the way, which is different'. And that may not seem much, but actually it's quite a lot. If you have thousands and thousands of different paths through the earth going between different earthquakes and different receivers, you can really put this together in three dimensions to see where that thing is, which is causing the delay or causing something to speed up.

A diagram of the core of a planet

16:10 - Is the Earth's core slowing down?

The Earth's core is appearing to slow down, or even reverse. Should we be concerned?

Is the Earth's core slowing down?
John Vidale, University of Southern California

The idea of using vibrations to map out the inside of the Earth isn’t just useful to find out the Earth’s structure, but also to see when that structure is changing. A recent story in the news came out with the alarming sounding claim that the Earth’s core, that’s the bit made of iron who’s magnetic field helps protect us from solar flares, might be slowing down or even reversing. So are we in any danger? I spoke to the University of Southern California’s John Vidale.

John - Well, we know something's changing down with the inner core. The ideas are generally that it's moving differently than the mantle above it, just slightly. It's moving so that it might move 10 kilometers faster or slower than the mantle above it. It can move because the outer core around it is a liquid and doesn't hold it in place. But they're different ideas about the pattern with which it's moving. This latest paper argues that it's oscillating with a 70 year period that it had been going a little faster than the rest of the Earth, and now it's changed speed, and it's going, started to go a little bit slower and just oscillates every 70 years.

Will - How can a big ball of molten iron slow down in relation to the rest of the Earth?

John - Earth? Well, again, there's several ideas for this. One is that the outer core drags on it. It's highly magnetic down there. It's liquid iron in the outer core and solid iron for the most part in the inner core. And so the magnetic field lines connect the inner and outer core.

Will - And the big question is, therefore, should we be concerned?

John - Yeah, there's no reason to be concerned except that if there's processes down there, we don't understand, we'd like to know what they are before we completely relax. But I have heard no interpretations that would make me lose any sleep.

Will - Does this have any effect on what we notice day to day?

John - I'd say the overwhelming odds are no. I mean, conceivably something down there could be a sign that there's some sort of instability, but that's very unlikely. And if the inner core speeds up a fraction of a degree per year or slows down it'd be impossible to notice up here. The one thing we can measure up here is it affects the length of day. People argue that one sign of the inner core moving is that the length of the day gets a 10th of a millisecond shorter and longer over the years. But that's way too small to notice, except with the very best clock.

Will - Yeah. So a problem for atomic clocks, but perhaps not for the rest of us, then.

John - That's right.

Will - I'm sure as a seismologist you are sick to death of all the misrepresentation of earthquakes in movies, like when a main character is chased down the street by an earthquake. But there's also films where the center of the earth's core stops, and we have to dig down into it and restart it With nukes. You're saying that not only would this never be a problem, but also it's basically impossible for us to get down there anyway?

John - Yeah, that's pretty hard to get more than a few miles down into the earth for one thing. For another thing, if the course stopped, something would have to stop it. And that's just, uh, beyond any forces we have. And who knows what would happen to the outer core in a magnetic field if the intercourse stopped. So, yeah, it's completely implausible, but it's fun. I guess lots of movies we watch have things that would never happen in real life, so all we can do is laugh.

Will - So how often does this take place, this slowing on this reversing?

John - Well, it would probably be happening all the time, it's just a gradual practice. One of the ideas that I like is that it oscillates every six years, not every 70 years.And there's several kinds of evidence we look at, and some of it points in one direction, some points in another. There's another theory that it moved around the year 2001 by about half a degree and otherwise has been kind of fixed in place. The idea of something dramatic happened at the base of the outer core that year but the rest of the time it's calm. So we don't know the pattern very well. That's what we're currently discussing, but this latest paper is very good. The argument for the 70 year oscillation does have a lot of data behind it.

Will - How did we know that this core reversal of slowing down was taking place?

John - Actually, the authors of the paper this month wrote a paper back in 1996 that they could see signals changing. They had earthquakes that repeated years apart, and the signals from the pairs of earthquakes were different. And ever since we've been arguing just what pattern of change explains it. Seismic waves have come in at a 10th or two tenths of a second differently than they did years prior.

Will - So we're using the tremors of earthquakes to measure what the center of the earth is consisting of.

John - That's right. Earthquakes, and I've used pairs of nuclear tests as well. Nuclear tests are very powerful signals that can light up things that are otherwise hard to see.

Will - Is there any experiment that the general public could do to try and understand this process that doesn't involve being near an earthquake or being near a nuclear destination?

John - No, this is something where it takes very, very precise timing and rare occurrences. We're looking at magnitude five earthquakes for a lot of these studies, and they only repeat in a few places on the earth. And the best instruments are arrays of instruments. So if the public were doing this, it would take decades long experiments with 20 or 50 instruments. It wouldn't be a fun hobby.

A Zircon

21:39 - Mineral reveals chemistry of young Earth

A mineral known as a 'Zircon' could shed light on the conditions that first formed life on Earth

Mineral reveals chemistry of young Earth
Dustin Trail, University of Rochester

The forces involved in moving the ground beneath our feet are massive and often very destructive. And the destruction of earth through tectonic forces can present unusual problems. And what are you supposed to do if you want to look back billions of years into the past to discover what Earth’s chemical conditions might have been at the very start of life, because there’s a good chance that the land you want to study has already been destroyed. Well that was the challenge faced by Dustin Trail and his team at the University of Rochester, New York, until they came across a very special and very hardy mineral called a ‘zircon’.

Dustin - We don't know how life started on Earth, despite our planet being essentially right beneath our feet. And part of what makes this problem so hard is that we live on this remarkably dynamic planet. So rocks are being created and destroyed at plate tectonic boundaries. And one can imagine that if you are thinking about a planet that's been active for 4.5 billion years, that the available resources, both in terms of rocks and minerals, are going to become more and more limited the further we travel back in time. So we have to find a way to transport ourselves back billions of years. And that's one of the major goals of this work. Charles Darwin put it in a letter to his contemporary Joseph Hooker back in the early 1870s, something along the lines of life could have emerged in a warm little pond. And so in some respects, this is what we are studying as part of this work. What was that warm little pond like?

Will - It sounds unfortunate like tectonics were getting in the way of this study.

Dustin - That is correct. Tectonics both creates and destroys rocks. So what we need first is a remarkably durable mineral. And it turns out that we have such a mineral. Zircon, which is a zirconium silicate. It's a physically and both chemically durable mineral. So it is able to withstand the tests of time. It also has a couple of other key properties. It incorporates radioactive uranium into its structure. So when zircon is crystallizing from a high temperature fluid or from a magma, it incorporates uranium which decays to lead. And so by measuring uranium and lead within the crystal, we can obtain an absolute age, and that's an important component if we want to connect the age with chemistry. And so the zircons that we were interested in studying as part of this study approach 4 billion years old.

Will - What is it about zircons that make them so durable, that allow them to survive this long?

Dustin - There are two ways to think about this. So zircon is a remarkably hard mineral, which makes it physically durable. So it is capable of being transported by wind and water without physically breaking down. The second part is that it is chemically durable. Once a zircon forms and locks in its chemistry from the time of formation, any later metamorphic event or in some cases a magma that interacts with that zircon in most cases does not modify its original chemistry.

Will - These zircons then when they form, do they create some kind of snapshot of earth's chemistry all that time ago?

Dustin - That is exactly right. The important thing to keep in mind is that the mere presence of the mineral itself is not diagnostic of a particular rock type or a tectonic environment. It is the chemistry, the trace constituents within the zircon structure that provide us with clues as to what its formation environment may have been like. So it's sort of like decoding that chemistry, getting that chemistry to tell a story about the physical and chemical conditions of our planet 4 billion years ago.

Will - So it would be to chemists what amber would be to biologists?

Dustin - Exactly right. It's a remarkable mineral that preserves chemistry from the time of its formation.

Will - And what did the ones that dated back all this time tell you about the chemistry of earth?

Dustin - They've told us a remarkable amount about our planet during its first 500 million years. For example, they have told us that there was likely water rock interaction on our planet as early as 4.3 billion years ago, including the alteration of preexisting rock to form sediments. And they've also told us that the volcanic emanations that were coming out of our planet at that time were dominated by CO2 and water and nitrogen, actually very similar to today, rather than methane or ammonia. So they have already provided us with clues about what the surface of our planet was like at that time.

Will - And so how do you fancy the odds then that that could be where life started forming?

Dustin - Well, I am certainly an optimist. I <laugh> we know for instance what the output was. We know that life started on our planet right now. We are struggling with what the input was. We don't know what the planetary conditions of our planet were like a billion years ago. And so that is really what we are after as part of this study is can we better constrain the inputs that resulted in this amazing output, life on our planet?

Will - If these zircons then can help us understand the chemical pathways that led to life starting on our own planet, is there perhaps scope for it to help us find life on others?

Dustin - I think we're at this amazing time in which humankind is searching for life on other planets and we still don't have an idea of how or when life started on our own planet. And I think this is work that will factor up prominently in the search for life outside of our planet.

Will - A remarkable time capsule then.

Dustin - Exactly.


Add a comment