Researchers at CU 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.Ěý
In a , the team explored the physics of âstagnant slabs.â These geophysical oddities form when huge chunks of Earthâs oceanic plates are forced deep underground at the edges of certain continental plates. The chunks sink down into the planetâs interior for hundreds of miles until they suddenlyâand for reasons scientists canât explainâstop like a stalled car.
CU Boulderâs 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 planetâs upper and lower mantleâroughly 660 kilometers, or 410 miles, below the surface.
And the stoppage is likely temporary: âAlthough we see these slabs stagnate, they are a fairly recent phenomena, probably happening in the last 20 million years,â said Zhong, a co-author of the new study and a professor in CU Boulderâs Department of Physics.
Going stagnant
The findings matter for tectonics and volcanism on the Earthâs surface. Zhong explained that the planetâs 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.Ěý
âYou can think of this mantle convection as a big engine that drives all of what we see on Earthâs surface: earthquakes, mountain building, plate tectonics, volcanos and even Earthâs magnetic field,â Zhong said.Ěý
The existence of stagnant slabs, which geophysicists first located about a decade ago, however, complicates that metaphor, suggesting that Earthâs engine may grind to a halt in some areas. That, in turn, may change how scientists think diverse features, such as East Asiaâs roiling volcanos, form over geologic time.Ěý
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 Earthâs upper mantle and into the lower mantle where they heat up near the core.Ěý
But around Asia, âthey simply donât go down,â 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.Ěý
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.Ěý
If it does, such a layer would act like a greasy puddle in the middle of the planet. âIf you introduce a weak layer at that depth, somehow the reduced viscosity helps lubricate the region,â Zhong said. âThe slabs get deflected and can keep going for a long distance horizontally.â
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 doesnât 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 planetâs core.Ěý
The planet, in other words, would still behave like an engineâjust with a few sticky spots. âNew research suggests that the story may be more complicated than we previously thought,â Zhong said.