A team of scientists has identified an old piece of the Pacific Ocean's seabed extending hundreds of miles under China. Apparently, it's getting pulled down into the Earth's mantle, into the transition zone.
The rocky place that used to reside in the bottom of the Pacific is a relic of the ocean's lithosphere (the outermost layer of the Earth's surface). The lithosphere consists of the crust and the solid upper parts of the mantle. We're saying that it's the outermost layer, but that doesn't mean it gets to enjoy the scenery up above. The upper layer includes in it a set of fragmented tectonic plates that slowly move around the surface, at times bumping and pressing into each other.
Some of the earthquakes originate from these plates interacting too. And these collisions sometimes lead to what geologists call a subduction. When two plates are forced towards each other, one of them sometimes slips down under, driving towards the nether layers.
In a recently published study, researchers from the U.S. and China unveiled a monumental phenomenon taking place at depths never before explored. Previously, the subduction phenomenon was recorded at depths of about 125 miles (200 km) below the surface.
How did they find it?
The researchers were able to witness the event at a much deeper point thanks to a huge network of over 300 seismic stations spread throughout northeastern China. They have imaged parts of a tectonic plate that used to reside beneath the Pacific Ocean being pushed towards the middle of the mantle's transition zone at about 254-410 miles (410-660 km) below the surface.
The team identified two seismic velocity discontinuities. The team says that one of these relates to the top and the other to the bottom end of the subducted plate.
Chinese Academy of Sciences Geophysicist Qi-Fu Chen explains: "Based on detail seismological analyses, the upper discontinuity was interpreted to be the Moho discontinuity of the subducted slab," and adds "The lower discontinuity is likely caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab."