Led by experts from the University of Münster, Germany, an international team of geologists has discovered that the solid core of the Earth is not, as previously believed, a simple solid ball of iron and nickel, but is made up of different … layers. Researchers have indeed found signs of a multi-layered structure that could explain why seismic waves behave so erratically as they pass through the depths of the planet.
The discovery, recently published in ‘Nature Communications’, joins a series of discoveries which, in recent years, have made it abundantly clear that the heart of our world is, in reality, a complex and still deeply mysterious place. Let us forever forget the classic inert metal sphere. The core of the Earth is alive, it changes shape, reverses its rotation and, far from being uniform, it rolls up in layers as if it were a gigantic metallic onion.
The mystery of the metallic onion
If we throw a tennis ball through a forest in which the trees are planted in orderly rows, it will move faster in one direction than the other. The same thing happens with seismic waves caused by earthquakes. Seismologists have long known that waves travel 3 to 4 percent faster when they travel parallel to the Earth’s axis of rotation (from pole to pole) than when they travel along the equatorial plane. This is called “seismic anisotropy” and so far it has proven very difficult to explain.
“There are several hypotheses on the origin of these anisotropies,” explains Carmen Sanchez-Valle, from the Institute of Mineralogy at the University of Münster and co-author of the study. In other words, the pieces of the puzzle don’t really fit together. To solve it, the researcher and her team decided to recreate the center of the Earth in their laboratory. They used a “diamond anvil cell,” a device capable of crushing tiny samples with a pressure a million times that of the atmosphere, while heating them to more than 800 degrees Celsius.
This discovery joins a series of discoveries which, in recent years, have shown very clearly that the heart of our world is a complex and still deeply unknown place.
The team tested an alloy of iron, silicon and carbon, the ingredients believed to make up the core’s “chemical cocktail.” And what they discovered was fascinating: By compressing the mixture, the crystals aligned in a specific way, which changed the speed of sound.
“The diffraction patterns,” explains Efim Kolesnikov, lead author of the study, “were then analyzed to deduce the plastic properties, in particular the yield strength and viscosity.” According to the researchers, the data suggests that the core is chemically stratified. That is, the center of the inner core would consist of almost pure iron (where the anisotropy is strong), while the outer layers would contain more silicon and carbon, thus mitigating this effect. “This depth-dependent anisotropy pattern – indicates the study – could be the result of chemical stratification during core crystallization.”
A changing core
That of a stratified core is only the latest chapter in a saga of astonishing discoveries. And in recent years, the center of the Earth has given us one humbling lesson after another.
For example, we learned that it is not a perfect sphere, but that it continually changes shape. This was revealed in 2021 by a study which found that the Earth’s inner core is growing asymmetrically, more quickly under Indonesia than under Brazil. It’s as if the planet “grew” on one side more than the other. And why don’t we live on a distorted planet? Because gravity is responsible for redistributing this excess iron, maintaining the sphere, but the process generates a flow of heat which feeds the magnetic field which protects us from solar radiation. If this lateral “engine” were to stop, our protection against space could be compromised.
If the shape of the nucleus proved strange, discovering its mode of rotation was even more so. In 2022, in fact, a team of researchers from the University of Southern California demonstrated that the Earth’s core oscillates, changing its direction of rotation from time to time, which goes against the most accepted models, according to which the core constantly rotates at a speed faster than the surface of the planet. The researchers demonstrated beyond doubt that the inner core changed direction over a six-year period, between 1969 and 1974, thereby affecting the length of days. More recently, as early as 2024, it was confirmed that the rotation of the inner core, for the first time in many decades, was slowing down.
To which are added the discoveries on its structure. Because it turns out that the core is not smooth, but has a “rough” texture like a dense forest full of iron crystals that grow like metallic stalactites.
Surface consequences
And why do we care about what happens there? The exploration of this abyssal world, located more than 5,000 kilometers beneath our feet, has shown us that everything that happens there affects us. The core, without going any further, is the “heart” that keeps the Earth’s magnetic field alive, an invisible shield without which the atmosphere would be swept away by the solar wind and life, as we know it, would be impossible.
That of a stratified core is only the latest chapter in a saga of astonishing discoveries. And in recent years, the center of the Earth has given us one humbling lesson after another.
Therefore, each of these discoveries, the last being the layered structure, are pieces of the same puzzle that attempt to explain how the “dynamo” of our world works. Today, by understanding the chemistry of silicon and carbon in the core, as the Münster researchers did, we not only learn geology, but we also discover how to operate our planetary “spaceship”.
On medieval and Renaissance maps, unexplored, dangerous, or unknown areas were marked with the phrase “Behold the dragons,” accompanied by figures of mythical creatures and sea monsters. Today, this frontier has changed and is no longer located at the limits of the oceans, but at the very center of our world, a place where the laws of physics are pushed to the extreme and whose secrets we are only beginning to know, despite all our technology.
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