How do plate tectonics work?

How does the ground beneath our feet move around and interact with each other?
24 February 2023

Interview with 

James Jackson, University of Cambridge


a crack in the earth


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.


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