Former bottom piece of the Pacific Ocean taken deep beneath China

In a study that gives new meaning to the term “rock bottom,” seismic researchers discovered the underside of a rock slab of the earth’s surface layer, or lithosphere, that was dragged more than 400 miles below northeastern China. from the tectonic subduction process.

The study, published by a team of Chinese and US researchers in Nature Geoscience, offers news evidence of what happens to water-rich ocean tectonic plates as they are dragged through the Earth’s mantle beneath the continents.

Rice University seismologist Fenglin Niu, a corresponding co-author, said the study provides the first high-resolution seismic images of the upper and lower boundaries of a rocky or lithospheric tectonic plate within a key region known as the mantle transition, which begins approximately 254 miles (410 kilometers) below the Earth’s surface and extends to approximately 410 miles (660 kilometers).

“Many studies suggest that the plate actually deforms a lot in the transition zone of the mantle, which becomes soft, so it deforms easily,” Niu said. How much the sheet deforms or keeps its shape is important in explaining if and how it mixes with the mantle and what kind of cooling effect it has.

The earth’s mantle summons like heat in an oven. Heat from the Earth’s core rises through the mantle to the center of the oceans, where tectonic plates form. From there, the heat flows through the mantle, cooling as it moves to the continents, where it falls back to the core to collect more heat, rise and complete the convective circle.

Previous studies have probed the boundaries of subducting plates in the mantle, but few have looked deeper than 125 miles (200 kilometers) and none with the resolution of the current study, which used more than 67,000 measurements collected from 313 regional seismic stations. in northeastern China. That work, which was done in collaboration with the China Earthquake Administration, was led by co-correspondent author Qi-Fu Chen of the Chinese Academy of Sciences.

The research investigates fundamental questions about the processes that have shaped the earth’s surface over billions of years. Mantle convection drives the movements of Earth’s tectonic plates, rigid interlocking pieces of the Earth’s surface that are in constant motion as they float on the asthenosphere, the topmost layer of the mantle and the most fluid part of the inner planet.

Where tectonic plates meet, collide and rub together, releasing seismic energy. In extreme cases, this can cause destructive earthquakes and tsunamis, but most of the seismic motion is too weak for humans to feel without tools. Using seismometers, scientists can measure the amplitude and location of seismic disturbances. And because seismic waves accelerate in some types of rock and slow down in others, scientists can use them to create images of the Earth’s interior, in much the same way a doctor might use ultrasound to imagine what’s inside. a patient.

Niu, a professor of earth, environmental and planetary sciences at Rice, has been at the forefront of seismic imaging for more than two decades. When did he get his Ph.D. in Japan more than 20 years ago, researchers were using dense networks of seismic stations to collect some of the first detailed images of the boundaries of the submerged plates of the Pacific plate, the same plate that was captured in the study published this week.

“Japan is roughly where the Pacific plate reaches a depth of about 100 kilometers,” Niu said. “There is a lot of water in this plate and it produces a lot of partial melting. This produces arc volcanoes that helped create Japan. But we are still debating whether this water is fully released at that depth. There is growing evidence that some of the water remains inside the dish to go much, much deeper. “

Northeast China offers one of the best vantage points to investigate if this is true. The region is located about 1,000 kilometers from the Japan trench, where the Pacific plate begins its plunge into the interior of the planet. In 2009, with funding from the National Science Foundation and others, Niu and scientists from the University of Texas at Austin, the China Earthquake Administration, Tokyo University’s Earthquake Research Institute, and the Research Center for Prediction of Earthquakes and Volcanic Eruptions at Japan’s Tohoku The university has started installing broadband seismometers in the region.

“We put 140 stations there, and obviously the more stations the better the resolution,” Niu said. “The Chinese Academy of Sciences has put additional stations so they can get a finer and more detailed picture.”

In the new study, station data revealed both the upper and lower boundaries of the Pacific plate, dropping at a 25-degree angle within the mantle transition zone. Positioning within this zone is important for the study of mantle convection as the transition zone is located below the asthenosphere, at depths where increased pressure causes specific mantle minerals to undergo dramatic changes. phase. These mineral phases behave very differently in seismic profiles, just as liquid water and solid ice behave very differently even if they are composed of identical molecules. Since phase changes in the mantle transition zone occur at specific pressures and temperatures, geoscientists can use them as a thermometer to measure the temperature in the mantle.

Niu said the fact that both the top and bottom of the slab are visible is evidence that the slab has not completely mixed with the surrounding mantle. He said the heat traces of partially melted portions of the mantle under the slab also provide indirect evidence that the slab carried some of its water into the transition zone.

“The problem is to explain how these hot materials can be dropped into the deepest part of the mantle,” Niu said. “It’s still a question. Why are they hot, they are floating.”

That buoyancy should act as a lifesaver, pushing up on the underside of the sinking slab. Niu said the answer to this question could be that holes have appeared in the warping plate, allowing the hot melt to rise as the plate sinks.

“If you have a hole, the fusion will come out,” he said. “That’s why we think the slab can go deeper.”

The holes could also explain the appearance of volcanoes such as Changbaishan on the border between China and North Korea.

“It’s 1,000 kilometers from the plate border,” Niu said. “We don’t really understand the mechanism of this type of volcano. But the melt rising from the holes in the plate could be a possible explanation.”