Between South America and southern Africa, there is an enigmatic magnetic region called the South Atlantic Anomaly, where the field is a lot weaker than we would expect. Weak and unstable fields are thought to precede magnetic reversals, so some have argued this feature may be evidence that we are facing one.
Weak magnetic fields make us more prone to magnetic storms that have the potential to knock out electronic infrastructure, including power grids. The magnetic field of the South Atlantic Anomaly is already so weak that it can adversely affect satellites and their technology when they fly past it. The strange region is thought to be related to a patch of the magnetic field that is pointing a different direction to the rest at the top of the planet’s liquid outer core at a depth of 1,795 miles (2,889 km) within the Earth.
Underneath our feet, deep down in the Earth, liquid iron is producing the magnetic field that we all take for granted. But every now and then that magnetic field reverses or flips its polarity. What was once the magnetic north becomes south – and vice versa. When these reversals take place – and why they do so – has been an enduring mystery.
This “reverse flux patch” itself has grown over the last 250 years. But we don’t know whether it is simply a one-off product of the chaotic motions of the outer core fluid or rather the latest in a series of anomalies within this particular region over long time frames.
If it is a non-recurring feature, then its current location is not significant – it could happen anywhere, perhaps randomly. But if this is the case, the question of whether its increasing size and depth could mark the start of a new reversal remains.
If it is the latest in a string of features reoccurring over millions of years, however, then this would make a reversal less likely. But it would require a specific explanation for what was causing the magnetic field to act strangely in this particular place.
When volcanic rocks cool down, small grains of iron-oxide in them get magnetized and therefore save the direction and strength of the Earth’s magnetic field at that time and place.
Not only might this relationship give us some idea of how many magnetic fields reverses occur over any time period, but it also enables us to understand how quickly the mantle (the layer of earth between the crust and the core) moves. This is important because mantle motion is ultimately responsible for producing nearly all earthquakes, volcanoes, and mountain chains. Hot plumes from the mantle may also be responsible for piles of earth’s major extinctions. If we can understand the workings of the mantle we can have better insight into long timescale geological phenomena that affect our species.
A window into Earth’s interior
So what could explain the odd magnetic region? The liquid outer core that is generating it moves (by convection) at such high speeds that changes can occur on very short, human timescales. The outer core interacts with a layer called the mantle on top of it, which moves far slower. That means the mantle is unlikely to have changed very much in the last ten million years.
Graveyard for surface plates
Plate tectonics is the scientific theory that the Earth’s “lithosphere” (the cold, uppermost mantle and the crust, which are welded together) is split into seven large plates and many smaller ones. Plates are formed by volcanism at mid-ocean spreading centers such as the Mid-Atlantic Rift. Once at the surface, the new lithosphere moves away from spreading centers and cools over millions of years. Over time it becomes denser and eventually, the lithosphere sinks back into the hot mantle at subduction zones, such as that found just west of the Andes.
At this point, the plates disappear from the earth’s surface. But seismologists claim that colder “slabs” of lithosphere can be identified deep down in the mantle up to 300 million years after they have disappeared from the surface. The slabs of the lithosphere have descended thousands of kilometers downwards, displacing vast volumes of the solid mantle in the process and forming a “slab graveyard” just above the much denser liquid iron outer core. This means that the slabs of lithosphere descend as much as 2,890 km and it is there that they might influence the motion of iron liquid in the underlying core.
But there is strong disagreement regarding the amount of time it takes for the slabs to sink down far enough to affect the core. Estimates have ranged from around 50m to 250m years.
Fossilised magnetic activity of the core
The Earth’s magnetic field has persisted for billions of years, though its polarity has flipped many times. Because the magnetic field leaves a fossilized magnetization in many rocks that form at Earth’s surface, we have a “palaeomagnetic record” of how Earth’s magnetic field has changed over time.
Earth’s magnetic field is generated in the liquid outer core by a dynamo process that converts the motion of electrically conductive fluid into electromagnetic energy. This process is similar in principle to the dynamo used in a wind-up torch. So our Earth’s core is sensitive to the rate at which it loses heat to the overlying cooler mantle.
When cold subducted slabs arrive in the lower mantle, they increase the rate of core cooling and so speed up the motion of the liquid iron within it. According to numerical models, this extra motion should cause the rate of reversals to also increase. So perhaps the magnetic field reversal rate increases when more subduction happens at the surface? If so, then we would expect to measure a time delay between the subduction and changes in reversal rate because the slabs have to sink a long way before they affect the core. Our study aimed to measure just how quickly slabs sink through the viscous solid mantle.
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