New research led by Dr Hauke Marquardt of the University of Bayreuth, Germany, suggests the existence of a previously unknown superviscous layer inside our planet: part of the lower mantle where the rock gets 3 times stiffer. Such a layer may explain why tectonic plate slabs seem to pool at 930 miles (1,500 km) under Indonesia and South America’s Pacific coast.
A simplified image of a slab from one of Earth’s tectonic plates sinking through the upper mantle above, through the boundary between the upper and lower mantle 410 miles deep, then stalling and pooling at a depth of 930 miles, where the new study suggests the existence of an extremely viscous layer in Earth. Image credit: Lowell Miyagi / University of Utah.
“The Earth has many layers, like an onion. 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,” said Dr Lowell Miyagi of the University of Utah, the second author on the
paper published in the journal Nature Geoscience.
The discovery also may explain some deep earthquakes, hint that Earth’s interior is hotter than believed, and suggest why partly molten rock or magmas feeding midocean-ridge volcanoes such as Iceland’s differ chemically from magmas supplying island volcanoes like Hawaii’s.
Earth’s main layers are the thin crust 4 – 50 miles (6.4 – 80.5 km) deep, a mantle extending 1,800 miles (2,900 km) deep and the iron core.
But there are subdivisions. The crust and some of the upper mantle form 60- to 90-mile-thick (100 – 145 km) tectonic or lithospheric plates that are like the top side of conveyor belts carrying continents and seafloors.
Oceanic plates collide head-on with continental plates offshore from Chile, Peru, Mexico, the Pacific Northwest, Alaska, Kamchatka, Japan and Indonesia. In those places, the leading edge of the oceanic plate bends into a slab that dives or subducts under the continent, triggering earthquakes and volcanism as the slabs descend into the mantle, which is like the bottom part of the conveyor belt. The subduction process is slow, with a slab averaging roughly 300 million years to descend.
Dr Marquard and Dr Miyagi identified the likely presence of a superviscous layer in the lower mantle by squeezing the mineral
ferropericlase between gem-quality diamond anvils in presses. They squeezed it to pressures like those in Earth’s lower mantle.
Bridgmanite and ferropericlase are the dominant minerals in the lower mantle.
The team found that ferropericlase’s strength starts to increase at pressures equivalent to those 410 miles (660 km) deep and the strength increases threefold by the time it peaks at pressure equal to a 930-mile (1,500 km) depth.
And when the scientists simulated how ferropericlase behaves mixed with
bridgmanite deep underground in the upper part of the lower mantle, they calculated that the viscosity or stiffness of the mantle rock at a depth of 930 miles is some 300 times greater than at the 410-mile-deep upper-lower mantle boundary.
“This viscosity increase is likely to cause subducting slabs to get stuck at about 930 miles underground,” Dr Miyagi said.
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.
“How viscous is the new layer of the lower
mantle? On the pascal-second scale, the viscosity of water is 0.001, peanut butter is 200 and the stiff mantle layer is 1,000 billion billion (or 10 to the 21st power),” Dr Miyagi said.
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Hauke Marquardt & Lowell Miyagi. Slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity. Nature Geoscience, published online March 23, 2015; doi: 10.1038/ngeo2393
