article_snippet: The breakthrough could explain why parts of Earth’s sinking tectonic plates sometimes stall and thicken 930 miles (1,500km) underground.
article_snippet: 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 .
article_snippet: 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.
article_snippet: 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.
article_snippet: In fact, previous seismic images show that many slabs appear to “pool” around 930 miles, including under Indonesia and South America's Pacific coast.
article_snippet: This observation has puzzled seismologists for quite some time, but in the last year, there is new consensus from seismologists that most slabs pool.
article_snippet: Dr Lowell 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.
article_snippet: Anything that would cause resistance to a slab could potentially cause resistance to buckle or break higher in a slab, causing a deep earthquake,’ he said.
article_snippet: 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.
highlights_snippet: a new region 930 miles underground could explain why some plates don't sink .
  