Most people tend to think of the Earth in terms of crust, mantle and core, and while those are indeed the largest “layers” (you can’t properly call the mantle a layer though), each one of them is made from other, thinner layers. Now, researchers from the University of Utah have identified another one of these thinner layers, 930 miles beneath our feet.

An illustration of a slab of rock sinking through the upper mantle above, through the boundary between the upper and lower mantle at 410 miles depth, then stalling and pooling at a depth of 930 miles. Image credits: University of Utah.

“The Earth has many layers, like an onion,” study co-author Dr. Lowell Miyagi, an assistant professor of geology and geophysics at the University of Utah, said in a written statement. “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.”

Their finding actually has some significant implications. Geophysicists had previously observed that many slabs that are otherwise active and “move around” inside the Earth get stuck at around 930 miles underground – a phenomenon thought to lead to seismic activity and increased volcanic activity. This layer seems to be the cause.

“The result was exciting,” Miyagi said in the statement. “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.”

Miyagi holding a press that houses the diamond anvil, in which minerals can be squeezed at pressures akin to those deep within the Earth.

Oceanic plates are denser and when they collide with continental plates, they typically subduct, triggering earthquakes and volcanism. The subduction process takes place at very long times even geologically, with some estimates putting a 300 million years time stamp for a full subduction. Miyagi and fellow mineral physicist Hauke Marquardt, of Germany’s University of Bayreuth wanted to see why some slabs dive all the way through the mantle, and some stop. This is important because when they stop, they “bump”, potentially creating a deep earthquake.

“Anything that would cause resistance to a slab could potentially cause it to buckle or break higher in the slab, causing a deep earthquake,” Miyagi said.

They used a device a diamond anvil to simulate how the mineral ferropericlase reacts to high pressure – they squeezed thousands of crystals of ferropericlase at pressures up to 960,000 atmospheres. They focused specifically on ferropericlase, a magnesium/iron oxide virtually inexistent on the surface, but one of the main constituents of the lower mantle. They found that the stiffness (or viscosity) of the mineral increased threefold by the time it was subjected to pressure equal to what’s found in the lower mantle (930 miles below Earth’s surface) compared to the pressure at the boundary of the upper and lower mantle (410 miles beneath the surface). The results were quite surprising because it was thought that viscosity varies only slightly in the depths of the mantle.

Miyagi says the stiff upper part of the lower mantle also may explain different magmas seen at two different kinds of seafloor volcanoes and may mean that the lower mantle is hotter than previously believed.