October 17, 2024
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Ancient ocean floor discovered slowly sinking into Earth’s mantle
A huge ancient slab of ocean floor sank beneath the Pacific Ocean and remained in the Earth’s mantle for more than 120 million years, a new study suggests.
The Nazca Plate is located west of the Pacific coast of South America. At the western edge of the Nazca Plate is the East Pacific Rise, and at its eastern edge runs the Nazca Subduction Zone.
Naeblys/Alamy Stock Photo
An ancient slab of ocean floor that existed when Earth’s earliest known dinosaurs emerged has been discovered beneath the Pacific Ocean, where it appears to have floated in a kind of submergence for more than 120 million years.
In addition to revealing geological processes deep within the Earth, the cold, descending slabs of dense rock located some 410 to 660 kilometers below the planet’s surface are two parts of a gigantic mass of the mantle layer. This may explain the mysterious gap between them.
“This study provides the first modern example of how cold downwells from above disrupt deep chunks of the mantle,” said the Cambridge University Press, which was not involved in the discovery. says Sanne Kottard, professor of geoseismology at the university. The paper was published online on September 27th. scientific progress.
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Deep beneath our planet’s surface, two giant continent-sized chunks of scorching material are rising from Earth’s hot, liquid outer core into the rock-filled mantle layer. Scientists cannot directly view these gigantic structures, which are hundreds of kilometers high and thousands of kilometers wide. Instead, researchers infer their presence from imaging techniques that rely on how seismic waves pass through them. Seismic waves slow down within the blob, leading to its more technical name: Large Low Shear Velocity Region (LLSVP). The larger, better-understood LLSVP, known as the African Blob, lies beneath the East African Rift Valley, which runs from the Red Sea to Mozambique. There, the two plates slowly move apart, potentially causing continents to eventually break apart.
“In the East African Rift Valley, there is a current example of how large hot upwelling mantle plumes originating from these deep mantle clumps (appropriately named LLSVPs) can begin to break up continents. Yes,” Cotard said.
Scientists are wondering how these LLSVPs formed (some studies suggest they are relics of the impact that formed the Moon), what they are made of, It is also unclear exactly how they contribute to surface phenomena such as volcanic activity. “The general idea is that mantle clumps are likely to be swept away by subducted slabs. The two main clumps are surrounded by ‘graveyards’ of subducted slabs,” Cotard said. said, referring to the edge of an oceanic plate that dips or subducts beneath another plate.
Jingchuan Wang, a geologist at the University of Maryland, College Park, and his colleagues were interested in studying the mantle mass beneath the Nazca Plate in the Pacific Ocean off the coast of South America. Previous research suggested there was a structural anomaly there that appeared to split the mass in half. In a new analysis that included measurements of the properties of seismic waves that travel deep underground, researchers found evidence that something cold and dense is stuck between the cracks of the mantle.
“The simplest explanation for the low temperatures and high seismic velocities is the presence of a subducted slab,” Wang said. “However, there is no active subduction in this area, and the imaged slabs have already peeled away from the surface. Therefore, we believe we are observing ancient stone slabs.”
This illustration shows what the research team envisioned as an ancient submerged “slab.” This ancient ocean floor slab directly influences large-scale structures called mantle masses.
The researchers describe two possible scenarios for how this ancient ocean floor became wedged in the middle of the Pacific mantle mass. One of them was a broken edge of an ancient ocean floor that fell between the predecessor of the Nazca Plate and part of the ancient supercontinent Gondwana, which became South America about 250 million years ago. . The broken plate section, which served as the ocean floor during the early Mesozoic Era, is thought to have subducted beneath these two plates, and their boundary now forms the world’s fastest-spreading ocean ridge, known as the East Pacific Rise. are.
Alternatively, a descending slab could have sunk beneath the precursor to the Nazca Plate during ancient tectonic movements, Wang said.
Regardless of how it got there, that part of the ocean floor is creeping downward very slowly, at a rate of about half a centimeter to one centimeter a year. This is approximately half the rate at which a similar object would sink if it remained just below this zone of the mantle. .
The thickness of the slab and the viscosity (or stickiness) of this region of the mantle could explain the slow rate of subsidence, Wang said.
“Our findings help connect the past 250 million years of plate tectonic history to the present-day mantle structure,” Wang said, adding, “Earth’s complex past, especially geologically identified “It provides clues about what was happening underground, where possible traces are often not left.” On the surface. ”
