December 6, 2023
Earth's core is changing: Little-known phenomenon is creating a mysterious new layer

Water from Earth’s surface can find its way deep into the planet, and new research shows how it alters the outermost region of the metallic liquid core.

The discovery could explain the presence of a thin layer of material inside the planet that has mystified geologists for decades.

The Earth’s crust is made up of tectonic plates that grind and slide beneath each other; Over billions of years, these subduction zones have transported water into the lower mantle.

When this water reaches the core-mantle boundary, about 2,900 kilometers (1,800 mi) below the surface, it begins a powerful chemical interaction. A team from South Korea, the US and Germany showed that it creates a top core layer rich in hydrogen, and sends silica into the lower mantle.

“For years it has been assumed that the exchange of material between the Earth’s core and mantle is small,” says Dan Shim, a materials scientist at Arizona State University.

“Yet, our recent high-pressure experiments tell a different story. We found that when water reaches the core-mantle boundary, it reacts with silicon in the core, forming silica.”

Illustration of the Earth’s interior, showing underlying water. (Yonsei University)

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

The Earth’s core-mantle boundary changes quite rapidly from silicate to metallic, and not much is known about chemical exchange.

Decades ago, researchers recording seismic waves through the Earth’s interior documented a thin layer more than a few hundred kilometers thick, but until now no one knew where this proposed ‘E prime’ layer came from. She has come.

“We suggest that such chemical exchange between the core and mantle during deep transport of water over gigayears likely contributed to the formation of the E prime layer,” the team writes.

Seismologists have mapped some unusual features that suggest this altered liquid metal layer will be less dense and have slower seismic speeds. These density differences are thought to involve different concentrations of light elements such as hydrogen or silicon.

But increasing the concentration of a single light element will increase the velocity while decreasing the density, making it difficult to reconcile seismic observations and the dynamic stability of the E prime layer.

Increasing the concentration of one light element while decreasing the concentration of another has been put forward as a possible explanation. However, scientists were not aware of such an exchange process.

The team used laser-heated diamond-anvil cells to mimic the pressure-temperature conditions at the core-mantle boundary.

They showed that water that had seeped into the Earth’s core could react chemically with the materials there, turning the outer core into a hydrogen-rich film and spreading silica crystals that rose up and were incorporated into the mantle. Become.

Illustration of silica crystals erupting from the liquid metal of Earth’s outer core as water triggers a chemical reaction. (Dan Shim/ASU)

The layer of hydrogen-rich, silicon-poor material forming on top of the core will have low density and low velocity, which matches seismic wave observations.

The altered core film could in turn have significant effects on the deep water cycle, and the team says their results suggest a more complex global water cycle than we thought.

Shim says, “This discovery, along with our previous observation of diamonds forming from water by reacting with carbon in iron fluids under extreme pressure, points to a more dynamic core-mantle interaction, which could allow substantial material exchange.” suggests.”

The study has been published in nature geology,


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