In a study that gives new meaning to the term “rock bottom,” seismic researchers have discovered the underside of a rocky plate from the earth’s surface layer or lithosphere that has been pulled more than 400 miles below northeast China by a tectonic subduction process.
The study, published by a team of Chinese and American researchers in Nature Geoscience, offers news evidence of what happens to water-rich oceanic tectonic plates as they pull through the Earth’s mantle beneath the continents.
Rice Fenglin Niu University seismologist, co-author, said the study provides the first high-resolution seismic images of the upper and lower boundaries of a rocky or lithosphere tectonic plate in a key region known as the mantle transition zone, which begins about 410 kilometers away. below the Earth’s surface and stretches for about 410 miles (660 kilometers).
“A lot of research shows that the plate actually deforms a lot in the transition zone of the mantle, that it becomes soft, so it deforms easily,” Niu said. The extent to which the plate deforms or retains its shape is important for explaining whether and how it mixes with the mantle and what cooling effect it has.
The ground mantle is convected as heat in an oven. The heat from the Earth’s core rises through the mantle in 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 returns back to the core to collect more heat, rise and complete the convective cycle.
Previous studies have examined the boundaries of mantle subduction plates, but few have looked deeper than 200 miles (200 kilometers) and none with the resolution of the current study, which uses more than 67,000 measurements collected from 313 regional seismic stations in the northeast. China. This work, carried out in collaboration with the China Earthquake Administration, was led by co-author Ki-Fu Chen of the Chinese Academy of Sciences.
The study explores fundamental questions about the processes that have shaped the Earth’s surface for billions of years. Mantle convection drives the movements of the Earth’s tectonic plates, hard blocked pieces of the earth’s surface that are in constant motion as they float over the asthenosphere, the top layer of the mantle, and the most fluid part of the inner planet.
Where tectonic plates meet, they collide and grind together, releasing seismic energy. In extreme cases, this can cause devastating earthquakes and tsunamis, but most seismic movements are too weak for people to feel without tools. Using seismometers, scientists can measure the magnitude and location of seismic disturbances. And because seismic waves accelerate in some types of rocks and slow down in others, scientists can use them to create images of the Earth’s interior in the same way that doctors can use ultrasound to image what’s inside. patient.
Niu, a professor of Earth, Environment and Planetary Science at Rice, has been at the forefront of seismic imaging for more than two decades. When he completed his doctorate while studying in Japan more than 20 years ago, researchers used dense networks of seismic stations to collect some of the first detailed images of submerged Pacific plate boundaries, the same plate that was depicted in a study published this week.
“Japan is located 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 leads to arc volcanoes that helped create Japan. But we are still debating whether this water is completely released at this depth. There is growing evidence that the water stays inside the plate to go much, much deeper. “
Northeast China offers one of the best perspectives to explore whether this is true. The area is about 1,000 kilometers from the Japanese trench, where the Pacific Plate begins its immersion in 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, the Institute for Earthquake Research at the University of Tokyo and the Earthquake and Volcanic Eruption Research Center at Tohoku University in Japan began installing broadband seismometers in the region.
“We put 140 stations there and of course, the more stations, the better the resolution,” Niu said. “The Chinese Academy of Sciences has set up additional stations so they can get a finer and more detailed image.”
In the new study, the data from the stations reveal both the upper and lower limits of the Pacific plate, sinking at an angle of 25 degrees in the transition zone of the mantle. The location in 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 phase changes. These phases of minerals behave very differently in seismic profiles, just as liquid water and solid ice behave very differently, even though they are made up of identical molecules. Because phase changes in the mantle transition zone occur at specific pressures and temperatures, geologists can use them as a thermometer to measure mantle temperature.
Niu said the fact that both the top and bottom of the slab were visible was evidence that the slab had not completely blended in with the surrounding mantle. He said thermal signatures on partially melted parts of the mantle under the slab also provide circumstantial evidence that the slab transported some of its water to the transition zone.
“The problem is to explain how these hot materials can be released into the deeper part of the mantle,” Niu said. “It’s still a question. Because they’re hot, they’re floating.”
This buoyancy should act as a savior, pushing upwards from the underside of the sinking slab. Niu said the answer to this question could be that holes have appeared in the deforming plate, allowing the heat to rise until the plate sinks.
“If you have a hole, the melt will come out,” he said. “That’s why we think the slab could deepen.”
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 slab’s boundary,” Niu said. “We really don’t understand the mechanism of this kind of volcano. But the melting rising from holes in the plate may be a possible explanation.”
Different interfaces of the plates found in the transition zone of the mantle
Xin Wang et al, Various interfaces of plates depicted in the transition zone of the mantle, Nature Geoscience (2020). DOI: 10.1038 / s41561-020-00653-5
Provided by Rice University
Quote: Former piece from the bottom of the Pacific Ocean, shot deep below China (2020, November 16), retrieved on November 16, 2020 from https://phys.org/news/2020-11-piece-pacific-ocean- floor-imaged.html
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