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An international research collaboration has harnessed supercomputing power to better understand how massive slabs of ancient ocean floors are shaped as they sink hundreds of kilometers below Earth's surface.

Sophisticated computer models developed by researchers in the UK, Switzerland and the U.S. have cast new light on the complex physical interactions which govern the sliding and sinking of the ancient ocean floor, also referred to as subducted slabs, through Earth's mantle, a process known as subduction.

Researchers from the University of Glasgow led the study. Their paper, "The Role of the Overriding Plate and Mantle Viscosity Structure on Deep Slab Morphology," is in Geochemistry, Geophysics, Geosystems.

The team's findings help explain how the conditions at a critical point in the mantle and the forces from the plate above the subduction zones determine how these ancient slabs are transported through Earth.

Previous research using seismic tomography to image deep inside the planet has shown that some subducted slabs flatten out at 660 km, while others continue to pierce through the lower mantle to depths beyond 1,000 km to eventually reach the core-mantle boundary.

At 660 km below Earth's surface, the mantle undergoes what scientists call the "endothermic phase transition," where untransformed material within the cold core of the subducted slab resists its further sinking.

This, together with a general increase of stiffness in the mantle at depths between 660–1,000 km and deeper, make it more difficult for the subducted slab to continue its onward journey.

The team's modeling demonstrates that whether the slab can continue its journey towards the core-mantle boundary is influenced both by whether at the surface the slab is sinking under another oceanic plate or a continent and the specific depth at which the increase in mantle stiffness occurs.

When a thick continental plate sits above the subduction zone, and the mantle becomes more resistant at a depth of around 1,000 km, the sinking slabs first deflect at 660 km but then continue deeper, forming a distinctive "stepped" shape similar to a flight of stairs.

In contrast, when a thinner oceanic plate sits atop the subduction zone, the sinking slabs tend to flatten at a depth of 660 km regardless of the depth at which the mantle stiffness increases.

This indicates that the presence of a continent at the subduction zone's surface provides enough extra forcing for the subducted slab to overcome the resistance to its sinking provided by the untransformed minerals at depth.

Dr. Antoniette Greta Grima of the University of Glasgow's School of Geographical & Earth Sciences led the research and is the paper's lead author. She said, "The continents that we live on don't just shape the landscapes that we see around us, but they also influence how the 'earth's engine' that runs deep below the surface works as ocean plates are pulled into Earth's interior over millions of years.

"Investigating how the relationship between the interior and the exterior of the planet works helps us better understand why certain regions on Earth are more prone to powerful earthquakes and volcanic eruptions than others, governed by this relationship between the surface and deep Earth properties."

The team used the UK's supercomputer ARCHER to run two-dimensional models showing how cold, old subducted oceanic plates are shaped both by Earth's mantle structure and by the surface properties at subduction zones.

A key metric which guides the model's prediction is the "slab bending ratio," a new concept developed by Dr. Grima, which helps to quantify the behavior of a subducting slab and helps determine whether this will flatten around 660 km or continue piercing further down.

The model's results aligned closely with what geologists already know about two slabs located below South America and Asia.

In South America, the Nazca slab beneath Peru plunges in a "stepped" fashion, a signature of continental influence. Meanwhile, the Izu-Bonin slab near Japan flattens at depth, just as expected for the subduction of an oceanic plate under another oceanic plate.

Dr. Grima added, "In medical settings, we use techniques like X-rays or CT scans to look inside bodies. In geology, we can use seismic tomography models to look deep inside Earth by creating images based on how the vibrations from earthquakes are absorbed and reflected by the differing densities of materials hundreds of kilometers below our feet.

"Our models do not just reproduce what we see in seismic tomography scans, they explain it. They show that the surface of Earth sitting above a subduction zone can strongly influence what happens thousands of kilometers below. Where continents overlie a slab, their thickness and strength help drive the slab deeper into the mantle, below 1,000 km depth.

"When only oceanic plates are involved, slabs flatten out instead. This reveals a direct link between the surface of Earth and the way its interior mantle resists flow, a connection that shapes how Earth works and one that scientists have long sought to understand."

Researchers from the University of California Los Angeles in the U.S., and Undertone Design and the International Space Science Institute in Switzerland, contributed to the research and co-authored the paper.