Is the Earth's core slowing down?

The Earth's core is appearing to slow down, or even reverse. Should we be concerned?...
24 February 2023

Interview with 

John Vidale, University of Southern California


A diagram of the core of a planet


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.


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