When the dinosaurs dominated the Earth , a piece of seafloor west of South America was drawn into the mantle as another Earth’s crust ride over it . peculiarly , it has survived despite pelagic plates of the same eld usually having been recycled into the mantle , intimate we need to pluck our discernment of Earth ’s interior .
The floor of the sea is constantly being produced atmid - oceanic ridges , which means it also demand to be constantly destroy . This process bump atsubduction zone , when continental plates depend upon over sea plates , forcing them into the Earth ’s Mickey Charles Mantle where they are think to persist for a while before gradually dissolving .
However , when a squad led by Dr Jingchuan Wang of the University of Maryland used a new seismic technique to take the blanket ’s social system beneath the eastern Pacific , they find something unexpected in the waves reflected back to them . Where the upper and lower mantle sports meeting is an country known as the mantle changeover zone . Off South America , Wang and colleague determine grounds the conversion zone is remarkably thick – extend from 410 to 660 kilometers ( 250 - 400 miles ) beneath the sea floor .
The Nazca Plate is being subducted beneath South America, but near its other boundary is the remnant of an ancient plate that started subducting in the Triassic, which has survived thanks to resistance at the mantle transition zoneImage Credit: Jingchuan Wang
“ This thickened area is like a fossilise fingermark of an ancient piece of seafloor that subducted into the Earth approximately 250 million years ago , ” Wang say in astatement . “ It ’s yield us a glimpse into Earth ’s past times that we ’ve never had before . ”
The subduction is thought to have continued until around 120 million years ago , and lies under the Nazca Plate , but is intend to be from a previous subduction event .
Normally , a collection plate would be await to have been recycled into the mantle after this long , but the authors call back about 14 percent of the original slab book survives .
“ We found that in this region , the material was sink at about half the speed we look , which suggests that the mantle transition zona can act like a roadblock and slow down the movement of cloth through the Earth , ” Wang continued . “ Our uncovering opens up new questions about how the deep Earth influences what we see on the airfoil across vast distance and timescales . ”
The discovery was made because the team deploy a Modern , more advanced method of seismal tomography to understand what is take place within the mantle . “ you’re able to think of seismic imagination as something similar to a CT scan . It ’s basically allowed us to have a cross - sectional view of our planet ’s inside , ” Wang said .
Two big question from the determination are how strange this slow sinking feeling is , and what force it has on crustal plate behavior above , where it weigh most to us . The squad ’s explanation for the passage zone ’s resistivity is that it is unco nerveless in this area , relatively verbalize , but only by replicate the study elsewhere will we learn if other nerveless post live .
sure , the slowness of the sinking of the old plate has not translated to any tardiness at the open . Instead , the East Pacific Rise is the quickest - propagate sea ridgepole in the world .
In the meantime , the squad wants to search the relationship between the slab ’s survival and the Pacific Large Low Shear Velocity Province ( LLSVP ) , an enormous orbit of the lower mantle bisected by the slab . This is the counterpart of another enormouslow - shear - velocity neighborhood under Africa , but is more ill map . The subducted plate explains a interruption within the LLSVP that had antecedently been detected , but not understand .
Even if the squad does n’t find out another slowly sinking slab , Wang is confident the more sophisticated seismic imagination will change mantle enquiry . “ This is just the source , ” he suppose . “ We believe that there are many more ancient social structure hold back to be get word in Earth ’s cryptic Department of the Interior . Each one has the potency to unveil many new insights about our major planet ’s complex past tense — and even lead to a respectable agreement of other planets beyond ours . ”
The study is published open access in the journalScience Advances .
