water seeping from outside

2023-11-21 03:38:32

It was an enigma that had haunted geologists for decades. A thin layer, just a few hundred km thick, right around the molten metal that makes up the Earth’s outer core. Known as ‘primary layer E’, its origin has always been a mystery… until now.

In an article recently published in ‘Nature Geoscience’, In fact, an international team of researchers has just revealed that the ‘blame’ of this thin layer lies with water, which is capable of penetrating to the deepest depths of the planet and altering, 3,000 km beneath our feet, the composition of the outermost region of the core, creating a layer of different material. Researchers refer to this layer hundreds of km thick as a simple ‘film’ around the Earth’s core, which in total is 6,970 km in diameter.

In their study, the researchers explain that the filtration of water to these depths is a slow but continuous process, like a drop of water that, given enough time, ends up piercing a rock. Only in this case it is not an endless drip, but a different process. For billions of years, in fact, water from the Earth’s surface has been transported to the depths of the Earth on descending, or subducting, tectonic plates. That is, those that go under other plates and return again to the depths from which they once emerged.

Unexpected chemical reactions

Upon reaching the boundary between the core and the mantle, about 2,900 kilometers below the surface, all that water triggers a deep chemical interaction that alters the very structure of the core.

Under the direction of Yong Jae Lee of Yonsei University in South Korea, researchers have shown, through high-pressure experiments, that subducted water reacts chemically with core materials. This reaction forms a layer rich in hydrogen and depleted in silicon, and has the ability to alter the upper region of the outer core, forming a film-like structure around it. In addition, the reaction generates silica crystals that rise and integrate into the mantle. The study predicts that this modified liquid metallic layer will be less dense and have reduced seismic velocities, something that coincides with anomalous characteristics mapped for many years by seismologists.

As is known, the mixture of iron and nickel in the outer core plays an important role in generating the Earth’s magnetic field, which protects life on the planet from winds and solar radiation. That is why it is important to understand how the interior of the planet works and how it has evolved over time.

At 3,000 km depth, at the boundary between the mantle and the outer core, the composition changes abruptly from silicates to metals, and until now not much was known about the chemical exchanges between them.

Silica crystals emerge from liquid metal in Earth’s outer core due to water-induced chemical reaction Dan Shim/ASU

The mystery, solved

But the issue finally seems to have been clarified. “We suggest – says the article – that this chemical exchange between the core and the mantle over ‘giga years’ of deep water transport may have contributed to the formation of the primary layer E.”

«For years – says Dan Shim, from Arizona State University and co-author of the article – it has been believed that the exchange of material between the core and the mantle of the Earth is small. But our recent high-pressure experiments reveal a different story. We discovered that when water reaches the boundary between the core and the mantle, it reacts with the silicon in the core and forms silica. “This discovery, together with our previous observation of diamonds forming from water reacting with carbon in liquid iron under extreme pressure, points to a much more dynamic core-mantle interaction, suggesting substantial exchange of materials.”

In addition to improving our understanding of the Earth’s internal processes, the study points to a much more extensive global water cycle than previously assumed. The altered ‘film’ around the core, in fact, has profound implications for the geochemical cycles that connect the surface water cycle to the deep metallic core.

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