![]() ![]() When volcanic rocks cool down, small grains of iron-oxide in them get magnetised and therefore save the direction and strength of the Earth’s magnetic field at that time and place. Lead author Yael Engbers is drilling a core on Saint Helena. These originate from two separate volcanoes and were erupted from between eight million and 11.5 million years ago. This island, where Napoleon was exiled to and eventually died in 1821, is made of volcanic rocks. To find out, we travelled to Saint Helena – an island in the middle of the South Atlantic Ocean. But it would require a specific explanation for what was causing the magnetic field to act strangely in this particular place. 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 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 a non-recurring feature, then its current location is not significant – it could happen anywhere, perhaps randomly. 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. This “ reverse flux patch” itself has grown over the last 250 years. ![]() Colours range from weak fields (blue) to strong fields (yellow). The black outline shows the limits of a large region of anomalously slow seismic velocity (implying hot mantle) sitting just on top of Earth’s core. The geomagnetic field at Earth’s surface with the South Atlantic Anomaly shaded in darkest blue and St Helena marked with a star. ![]() The strange region is thought to be related to a patch of 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 2,889 kilometres within the Earth. 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. Weak magnetic fields make us more prone to magnetic storms that have the potential to knock out electronic infrastructure, including power grids. ![]() Now our new study, published in the Proceedings of the National Academy of Sciences, has uncovered how long the field in the South Atlantic has been acting up – and sheds light on whether it is something to worry about. Weak and unstable fields are thought to precede magnetic reversals, so some have argued this feature may be evidence that we are facing one. This is called a reversal and last happened 780,000 years ago.īetween 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. The field can even change polarity completely, with the magnetic north and south poles switching places. This field changes over time, and also behaves differently in different parts of the world. Deep inside the Earth, liquid iron is flowing and generating the Earth’s magnetic field, which protects our atmosphere and satellites against harmful radiation from the Sun. ![]()
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