What's going on in Earth's core?

16 June 2020

Interview with 

Claire Nichols, University of Cambridge

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We’ve cracked open the crust and mused over the mantle, but now let’s get to the core of the matter. What’s actually at the centre of the earth? Claire Nichols is a paleomagnetist formerly of Cambridge University and now working at MIT, and she spoke to Chris Smith...

Claire - We only indirectly know what's in the core, but there are lots of lines of evidence that tell us that it's predominantly made of the element iron.

Chris - How do you know that?

Claire - Two main ways. One, we know from the mass of rocks that we see at the surface of the Earth, that what's in the interior must be a lot denser. And the other reason we know is because we know it must be conductive because it's generating Earth's magnetic field.

Chris - When you say the density gives this away, is that because we have some inkling as to how much the Earth "weighs", in inverted commas. And so given that if you know what proportion of it is the lighter rocks on the outside, the heavier stuff sinks, we're inferring, there must be really heavy stuff in the middle?

Claire - Right. Exactly.

Chris - Now, in terms of the heat that's down there, I mentioned at the top of the program that it's about 6,000 odd degrees, isn't it, but where's all that heat coming from. Because the Earth's four and a half thousand million years old, it's been around a long time. You'd have thought it would have cooled down by now. So why is the Earth still so hot? Why is the core still so hot?

Claire - Yeah. So all of that heat, as you say, it does come actually from the very, very beginning of the planet's formation, billions of years ago. And the reason it's still so hot in the middle is because there's just so much crust and mantle that all that heat has to be extracted through. So it's like the core is wearing the thickest winter coat of all time. So it is cooling down, but slowly.

Chris - And as it cools down, does it harden because right at the very centre, it is solid, isn't it? And it's the outer part of the core that's the liquid bit.

Claire - Yeah. Yeah. So we think that it started to solidify right in the middle and the inner solid core is now growing through time.

Chris - And do we have any insights into how that is spawning the magnetic field that we have?

Claire - Yeah. So what we think is happening today, is that the solid inner core is ejecting other elements. So light elements, like things like oxygen and sulfur into that liquid part of the core, and that's driving really, really vigorous convection.

Chris - I'm just picturing this. Therefore, I've got right in the centre of the Earth, I've got the inner core. And that's the bit you're saying has gone hard, that solid. There's stuff coming out of that into the liquid bit that surrounds it, and that's spinning, and that spinning is in some way creating this magnetic effect.

Claire - Yeah, exactly. So you can kind of think of this liquid part as like a lava lamp. So things are mixing around and because it's made of a conductive material, so something that electricity can travel through, by moving that charge around, that's actually generating the magnetic field.

Chris - Obviously you're trying to work out how something that is 6,000 kilometres below our feet is working. How do you do that?

Claire - So what we look at is rocks that form on the surface of Earth, very conveniently when they cool. So like lava flows coming out of volcanoes. They trap a record of the magnetic field at the surface. So we can look at what the magnetic field is doing today, and what it's doing back in time. And that tells us about what the core is doing.

Chris - Oh, right. So when the stuff spawns as liquid magma from within the Earth, before it goes hard, it can move in any direction. But because it's got stuff in it that is susceptible to the Earth's magnetic field, it will sort of line up with whatever the field is doing at that time.

Claire - Yeah. So there's just those little magnetic blobs in those lava flows. And as those little blobs cool down, they will align with the direction of the magnetic field.

Chris - How do you know what way they're pointing? So if I hand you a pebble, how do you know that the pebble was orientated in a certain direction relative to the Earth's magnetic field then?

Claire - So well, a pebble is tricky because we don't know how it was oriented on the surface, but if we go to, let's say a cliff face or something that we know it's original orientation, then we can take oriented samples of that to a laboratory. And then we can measure the direction of the magnetic field in that sample very, very accurately. And then we can use that to tell us about the ancient magnetic field.

Chris - And because you can date the rocks, you know how old it was, and therefore what the magnetism was doing in rocks that age. So you can wind the clock back.

Claire - Yeah exactly.

Chris - And if you do that then, what do we learn about the Earth's magnetic field through time?

Claire - So we have learned that we've had a magnetic field for billions of years, and also that the magnetic field wobbles around. So it's not perfectly North and South all of the time. And sometimes it even flips. So it's actually very dynamic.

Chris - When did it last flip round?

Claire - So it last flipped hundreds of thousands of years ago. So it doesn't happen very often.

Chris - Do we have any insights into the consequences of that? Because obviously hundreds of thousands of years, there were our human ancestors walking around on the Earth at that time. So presumably when this happens, it's not terribly catastrophic for life as we know it.

Claire - No. So we don't think it is. So actually there's no evidence going back in time of humans or fossils being made extinct by a reversal, but one thing it will affect is technology. So things like your mobile phone will not work very well during a reversal.

Chris - Why?

Claire - It's because our magnetic field is shielding our planet from cosmic radiation. So basically radiation coming in from the Sun. And if our magnetic field is flipping, it's much weaker and that means we get a lot more radiation and that will interfere with satellites and all sorts of things for technology.

Chris - And do you know how quickly these flips happen? Can we see evidence of the field collapsing and then reestablishing in the new direction? And does that happen really quickly, or does it happen geologically really quickly? Meaning over thousands of years.

Claire - Yeah, exactly. So it happens geologically quickly. So a reversal would take well beyond our lifetimes. But in the rocks it looks instantaneous.

Chris - And in terms of actually what's causing this flip, can we work this one out or do we just have to say, well, it's something to do with some convulsion in the core at some point, with the movement of things, and it causes this to happen. Do we have an idea as to why this does what it does?

Claire - So it's still a bit of a mystery. We know it happens on a fairly regular timescale, but we're not entirely sure what's driving it. But it's basically, it's indicating to us that the flows within the core are quite complicated and something must trigger a change in their behavior.

Chris - And as more of the core hardens, which is happening with time, does that mean that the frequency of this happening may change too?

Claire - It might do. Yeah. That's something that we'll have to look for evidence of, as and when it happens.

Chris - Let's hope it's not too soon. I like my mobile. It's bad enough, the signal where I live already. And one last question, Claire, because obviously Mars is quite a similar size to the Earth, but Mars doesn't seem to have a magnetic field anymore. Many people blame the absence of a magnetic field for the fact that it is now a prune of a planet with almost no water left, where previously it was a Waterworld. So why has Mars lost its magnetic field, and we haven't?

Claire - That's a good question. Partly it's probably because Mars is much smaller than the Earth. So it cools down much, much quicker. But we also think that something a bit weird happened that made the Martian magnetic field switch off so early. So that's something else that scientists are actively looking at today.

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