Researchers were able to image the top and bottom of an oceanic slab, a former piece of the Pacific seafloor, sliding deep into the mantle, 400 miles beneath the surface of northeastern China. Photo by F. Niu/Rice University
Nov. 16 (UPI) -- Using high-resolution seismic imaging, geologists have successfully surveyed the top and bottom of a water-rich oceanic tectonic plate, a former piece of the Pacific Ocean floor, sliding some 400 miles beneath northeastern China.
The feat, described this month in the journal Nature Geoscience, marks the first time scientists have characterized the top and bottom of a rocky tectonic plate inside the mantle transition zone, located between the depths of 254 and 410 miles beneath Earth's surface.
"A lot of studies suggest that the slab actually deforms a lot in the mantle transition zone, that it becomes soft, so it's easily deformed," study author Fenglin Niu, seismologist at Rice University, said in a news release.
Understanding how tectonic plates deform as they are subducted into the mantle is key to properly modeling the cooling and mixing effects of newly subsumed tectonic plates.
Heat originates within Earth's core, rising through the mantle toward the ocean floor. As the heat moves across the mantle, from the middle of the ocean tower the continents, the thermal energy dissipates. Cooling mantle material sinks as it nears the continents, completing the convection circle -- the pattern that drives the planet's plate tectonics.
While the jostling of adjacent tectonic plates can release deadly amounts of energy in the form of earthquakes and tsunamis, most tectonic and seismic activity goes unnoticed. Because seismic waves travel at different speeds through different types of rock, slight seismic disturbances can be used to study Earth's interior.
Researchers in China and the United States were able to find and create an image of the oceanic rock slab by targeting a portion of the mantle transition zone in East Asia using a dense network of seismic stations.
"We put 140 stations there [in Northeastern China], and of course the more stations the better for resolution," Niu said. "The Chinese Academy of Sciences put additional stations so they can get a finer, more detailed image."
Images from the network revealed the contours of a piece of the Pacific plate plunging into the mantle transition zone at a 25-degree angle.
Because the top and bottom of the slab are still visible at such depths, researchers are confident the plate hasn't become totally mixed with the surrounding mantle. Heat signatures also suggest the oceanic plate has still not relinquished all of its water.
"The problem is explaining how these hot materials can be dropped into the deeper part of the mantle," Niu said. "It's still a question. Because they are hot, they are buoyant."
Scientists suspect holes in the slab allowed the interior melt to escape and rise even as the plate itself sinks deeper into the mantle. The theory could explain why a series of volcanoes line the border between China and North Korea.
"It's 1,000 kilometers away from the plate boundary," Niu said. "We don't really understand the mechanism of this kind of volcano. But melt rising from holes in the slab could be a possible explanation."