Earth's mantle (dark red) lies below the crust (brown layer near the surface) and above the outer core (bright red). (Credit: by Argonne National Laboratory )
Researchers at 抖阴旅行射 Boulder report that they may have solved a geophysical mystery, pinning down the likely cause of a phenomenon that resembles a wrench in the engine of the planet.听
In a , the team explored the physics of 鈥渟tagnant slabs.鈥� These geophysical oddities form when huge chunks of Earth鈥檚 oceanic plates are forced deep underground at the edges of certain continental plates. The chunks sink down into the planet鈥檚 interior for hundreds of miles until they suddenly鈥攁nd for reasons scientists can鈥檛 explain鈥攕top like a stalled car.
抖阴旅行射 Boulder鈥檚 Wei Mao and Shijie Zhong, however, may have found the reason for that halt. Using computer simulations, the researchers examined a series of stagnant slabs in the Pacific Ocean near Japan and the Philippines. They discovered that these cold rocks seem to be sliding on a thin layer of weak material lying at the boundary of the planet鈥檚 upper and lower mantle鈥攔oughly 660 kilometers, or 410 miles, below the surface.
And the stoppage is likely temporary: 鈥淎lthough we see these slabs stagnate, they are a fairly recent phenomena, probably happening in the last 20 million years,鈥� said Zhong, a co-author of the new study and a professor in 抖阴旅行射 Boulder鈥檚 Department of Physics.
Going stagnant
Graphic of stagnant slabs under the Japanese island of Honshu (top) and the Mariana Trench (bottom) using seismic data (left) and computer simulations (right). Stagnant slabs (blue) plunge down to the border between the upper and lower mantle then screech to a stop. (Credit: Mao & Zhong 2018,听Nature Geoscience)
The findings matter for tectonics and volcanism on the Earth鈥檚 surface. Zhong explained that the planet鈥檚 mantle, which lies above the core, generates vast amounts of heat. To cool the globe down, hotter rocks rise up through the mantle and colder rocks sink.听
鈥淵ou can think of this mantle convection as a big engine that drives all of what we see on Earth鈥檚 surface: earthquakes, mountain building, plate tectonics, volcanos and even Earth鈥檚 magnetic field,鈥� Zhong said.听
The existence of stagnant slabs, which geophysicists first located about a decade ago, however, complicates that metaphor, suggesting that Earth鈥檚 engine may grind to a halt in some areas. That, in turn, may change how scientists think diverse features, such as East Asia鈥檚 roiling volcanos, form over geologic time.听
Scientists have mostly located such slabs in the western Pacific Ocean, specifically off the east coast of Japan and deep below the Mariana Trench. They occur at the sites of subduction zones, or areas where oceanic plates at the surface of the planet plunge hundreds of miles below ground.
Slabs seen at similar sites near North and South America behave in ways that geophysicists might expect: They dive through Earth鈥檚 upper mantle and into the lower mantle where they heat up near the core.听
But around Asia, 鈥渢hey simply don鈥檛 go down,鈥� Zhong said. Instead, the slabs spread out horizontally near the boundary between the upper and lower mantle, a point at which heat and pressure inside Earth cause minerals to change from one phase to another.
Slab sliding
To find out why slabs go stagnant, Zhong and Mao, a graduate student in physics, developed realistic simulations of how energy and rock cycle around the entire planet.听
They found that the only way they could explain the behavior of the stagnant slabs was if a thin layer of less-viscous rock was wedged in between the two halves of the mantle. While no one has directly observed such a layer, researchers have predicted that it exists by studying the effects of heat and pressure on rock.听
If it does, such a layer would act like a greasy puddle in the middle of the planet. 鈥淚f you introduce a weak layer at that depth, somehow the reduced viscosity helps lubricate the region,鈥� Zhong said. 鈥淭he slabs get deflected and can keep going for a long distance horizontally.鈥�
Stagnant slabs seem to occur off the coast of Asia, but not the Americas, because the movement of the continents above gives those chunks of rock more room to slide. Zhong, however, said that he doesn鈥檛 think the slabs will stay stuck. With enough time, he suspects that they will break through the slick part of the mantle and continue their plunge toward the planet鈥檚 core.听
The planet, in other words, would still behave like an engine鈥攋ust with a few sticky spots. 鈥淣ew research suggests that the story may be more complicated than we previously thought,鈥� Zhong said.
