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Why is it known that the cores of the earth rotate?

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Depiction of the core, mantle and crust of the earth.

The earth is made up of more or less concentric layers, broadly consisting of the crust, mantle and core. Each of them, in turn, is divided into others. As you state in your question, the earth has a core made up of two main layers: what we call the outer core, which is liquid, and what we call the inner core, which is solid. The core rotating with the rest of the earth would not be a problem if the entire earth were solid, since everything would rotate synchronously and coupled. The problem arises because the outer core is liquid. This means there is a separation between what is happening above and below this level. The earth could spin and not transmit its rotation to the inner core.

We know the outer core is fluid because it is not traversed by seismic S-waves, which create shear motion that cannot propagate in fluids. We also know that it is composed of iron, nickel and some lighter elements (oxygen, silicon, sulphur). It has a very high temperature, around 3000 °C. All this makes this liquid of low viscosity and therefore very mobile. And that provokes a series of movements, sometimes very organized and sometimes less so. Dominating among them is the formation of a type of vortex called a “tangential cylinder,” which is essentially coaxial with the Earth’s axis of rotation. This creates most of the Earth’s magnetic field because, as we’ve seen, the outer core is made of metals, which are possibly the most magnetic elements. In addition, the outer core has other, more chaotic motions, controlled by temperature changes, Coriolis acceleration, and the law of conservation of angular momentum of a rotating body. They all create the rest of the earth’s magnetic field. All of the magnetic field generated in the outer core is coupled to the inner core, which is also mostly composed of iron and nickel. The process is similar to that of a magnet attracting a piece of iron. As the Earth rotates and the outer core rotates with it, the magnetic field generated by the latter interacts with the inner core, causing it to rotate.

And here we come to the most interesting. The rotation of the earth slows down mainly due to the action of the tides, which are large masses of water that move differently according to latitude and longitude and cause a lot of friction as they move. This has various consequences. For one thing, the moon is getting farther and farther away. Another reason is that the slowing of our planet’s rotation is not immediately transmitted to the inner core, as it is connected to the rest of the Earth by a layer of fluid more than 1000 km in radius that does not transmit the efforts as a solid. Therefore, the inner core continues to rotate slightly faster than the rest of the earth. In fact, it is this motion that creates the “tangential cylinder” we described above, in a sort of feedback process.

And how do we know all this? The origin of the magnetic field is known, since magnetohydrodynamic models are built with great precision, which allow us to fully understand the consequences of the dynamics of the outer core, that is, the formation of the Earth’s magnetic field. These models were released in 1995, so they are relatively new. On the other hand, seismologists have proven that the inner core is indeed spinning faster than the rest of the Earth by comparing the arrival times of seismic waves generated at high latitudes (earthquakes in polar regions) and recorded in their antipodes. These variations in transit times were explained by changes in the relative position between the inner and outer core precisely because of the faster rotation of the former. However, there is no consensus as to how much the inner core over-revs the outer core.

Although all of this data is relatively recent, there are some earlier models from the 1970s that already suggested that due to the law of conservation of angular momentum, the inner core must be rotating faster than the outer core and the rest is distant from Earth.

Puy Ayarza Arribas She holds a PhD in Geological Sciences and is Director of the Department of Geology at the University of Salamanca.

Question emailed by Viladim Pujols

Coordination and editing:victory bull

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