A new 'superviscous' layer in Earth’s lower mantle has been discovered. The breakthrough could explain why parts of Earth’s sinking tectonic plates sometimes stall and thicken 930 miles (1,500km) underground. It could also explain why earthquakes occur deep inside the Earth’s interior, suggesting it is hotter than previously thought. Shown here is a simplified image of a slab from one of Earth's tectonic plates sinking through the upper mantle above,  then stalling and pooling at a depth of 930 miles, where University of Utah experiments suggest the existence of an extremely stiff or viscous layer in Earth . The research was carried out by scientists at the University of Utah by crushing minerals between diamonds to replicate conditions in Earth's interior. Earth's main layers are the thin crust four to 50 miles (six to 80km) deep, a mantle extending 1,800 miles (2,900km) deep and the iron core. But there are subdivisions. The crust and some of the upper mantle form tectonic or lithospheric plates 60 to 90 miles (97 to 145km) thick that are like the top side of conveyor belts carrying continents and seafloors. They are responsible for the formation of continents, and are still active - as earthquakes and volcanic eruptions show. However, researchers have never been able to work out why the Earth developed tectonic plates, but other planets did not. Last year, Yale said it had solved the mystery - and said the key is tiny minerals within rocks. The research suggests how and when Earth came to develop one of its most distinct features - rigid tectonic plates - and why Venus, Earth’s twin-like neighbor, never has. 'We think it all comes down to the behavior of tiny grains of minerals within rocks,' said Yale geophysicist Dr David Bercovici, lead author of research published  in the journal Nature. The researchers argue that in Earth’s early days, mantle convection caused weak zones in the lithosphere - the outermost shell of the planet. These zones persisted, developed into plate boundaries, and ultimately connected to form a global network of tectonic plates. The weak zones, they argue, resulted from the tendency of mineral grains in rocks to shrink as the rock deforms. This new layer is a ‘superviscous’ region, where liquids are under such intense pressure that they are extremely stiff. It extends from 410 miles (660km) underground to 930 miles (1,500km). In laboratory experiments, the researchers found that an abundant mineral in the mantle called ferropericlase began to increase in strength at pressures equivalent to those 410 miles (660km) underground - the upper-lower mantle boundary. And this strength increased threefold at depths of 930 miles (1,500km) deep. By simulating how it behaves with another dominant mineral at the same level - bridgmanite - the researchers calculated that the viscosity or stiffness of the mantle rock at 930 miles deep is 300 times great than at 410 miles deep. To put that in perspective, on the pascal-second scale the viscosity of water is 0.001, peanut butter is 200 and this stiff mantle layer is 1,000 billion billion. ‘The result was exciting,' said Dr Lowell Miyagi, an assistant professor of geology and geophysics at the University of Utah. ‘This viscosity increase is likely to cause subducting slabs [ones that move under each other] to get stuck - at least temporarily - at about 930 miles underground. ‘In fact, previous seismic images show that many slabs appear to “pool” around 930 miles, including under Indonesia and South America's Pacific coast. ‘This observation has puzzled seismologists for quite some time, but in the last year, there is new consensus from seismologists that most slabs pool.’ Earth's main layers (illustrated) are the thin crust four to 50 miles (six to 80km) deep, a mantle extending 1,800 miles (2,900km) deep and the iron core. But there are subdivisions. The crust and some of the upper mantle form 60- to 90-mile-thick tectonic or lithospheric plates that are like the top side of conveyor belts . University of Utah geophysicist Dr Lowell Miyagi holds a press that houses a diamond anvil, in which minerals can be squeezed a pressures akin to those deep within Earth. Miyagi and a colleague in Germany squeezed mineral crystals in a diamond anvil under pressures like those deep underground . Dr Miyagi added that the stalling and buckling of sinking slabs due to this stiff layer in the mantle may explain some deep earthquakes higher up in the mantle; most quakes are much shallower and in the crust. ‘Anything that would cause resistance to a slab could potentially cause it to buckle or break higher in the slab, causing a deep earthquake,’ he said. Dr Miyagi also said the stiff upper part of the lower mantle may explain different magmas seen at two different kinds of seafloor volcanoes. Recycled crust and mantle from old slabs eventually emerges as new seafloor during eruptions of volcanic vents along mid-ocean ridges - the rising end of the conveyor belt. The magma in this new plate material has the chemical signature of more recent, shallower, well-mixed magma that had been subducted and erupted through the conveyor belt several times. But in island volcanoes like Hawaii, created by a deep hotspot of partly molten rock, the magma is older, from deeper sources and less well-mixed. Dr Miyagi said the viscous layer in the lower mantle may be what separates the sources of the two different magmas that supply the two different kinds of volcanoes. Another implication of the stiff layer is that 'if you decrease the ability of the rock in the mantle to mix, it's also harder for heat to get out of Earth, which could mean Earth's interior is hotter than we think,' he said. The findings could also indicate that Earth's interior is hotter than thought, averaging 2,150°C (3,900°F) at the lower mantle boundary rather than 1,540°C (2,800°F) as previously predicted . Scientists believe the average temperature and pressure 410 miles (660km) deep at the upper-lower mantle boundary is 1,540°C (2,800°F) and 235,000 times the atmospheric pressure on Earth's surface. Dr Miyagi said that at the newly discovered viscous layer's stiffest area, 930 miles (1,500km) deep, the temperature averages 2,150°C (3,900°F) and the pressure is 640,000 times the air pressure at Earth's surface. Such conditions prevent geophysicists from visiting Earth's mantle, so 'we know a lot more about the surface of Mars than we do Earth's interior,' according to Dr Miyagi. 'We can't get down there, so we have to do experiments to see how these minerals behave under a wide range of conditions, and use that to simulate the behavior of the Earth.' The finding was a surprise because researchers previously believed that viscosity varied only a little bit at temperatures and pressures in the planet's interior. The study's simulations also determined that just below the 930-mile-deep zone of highest viscosity, slabs sink more easily again as the lower mantle becomes less stiff. ‘The Earth has many layers, like an onion,’ said Dr Miyagi. ‘Most layers are defined by the minerals that are present. ‘Essentially, we have discovered a new layer in the Earth. This layer isn't defined by the minerals present, but by the strength of these minerals.’
Research was carried out by scientists at the University of Utah . They crushed minerals between diamonds to mimic Earth's core . And they found there must be a new region 930 miles underground . The superviscous region could explain why some plates don't sink . And the findings also indicate Earth's interior is hotter than thought .