The behavior of Earth’s mantle ultimately affects everything, from the evolution of our atmosphere to the distribution of volcanoes around the planet. A new study, published today in Science, reveals that there is an unexpected spike in viscosity at around 1,000 kilometers (621 miles) depth within the mantle. Although this may help to solve multiple longstanding geological mysteries, the ultimate origin of this zone remains unknown.

The viscosity (or gloopiness) of the mantle – a complex, semi-molten, mobile mush above the outer core and beneath the crust – controls how it moves around. Vast reservoirs of heat move up towards the crust; as they cool, they descend back towards the outer core. This cycle, known as mantle convection, controls the movement of the Earth’s tectonic plates, causing them to slowly drift across the surface.

It also controls volcanism: huge mantle upwellings are called plumes, and produce very effusive, long-lasting “hotspot” volcanism. Hawaii and Iceland both experience this type of volcanism. 

With all this in mind, the team of researchers wanted to investigate the possible changes in viscosity within the mantle. Direct measurements of the mantle are only possible through the highly altered lava that emerges from volcanoes, so it has to be imaged indirectly. For this study, the authors decided to reconstruct the geoid – the shape that the ocean surfaces would take under the influence of Earth’s gravity and rotation alone.

^{Image credit: This new layer may help to explain the distribution of hotspot volcanism across the world, including Iceland’s. Nathan Mortimer/Shutterstock}

Viscosity changes in the lower mantle have an effect on everything that occurs at the surface; the geoid is therefore a good way to detect any changes in the mantle’s viscosity. Combining ten years’ worth of satellite data, along with massive geophysical data sets that included long-term seismic imagining, the geoid was effectively simulated.

The authors noted a huge jump in viscosity at a depth of around 1,000 kilometers (621 miles) – far deeper than any previous estimates of any such major change. A separate study also published today in Science Advances, which modeled the subduction of slabs through the mantle, also noted a viscosity jump at the same depth.

“It’s not even necessarily a sudden increase,” Max Rudolph, an assistant professor at Pennsylvania State University and lead author of the paper, told IFLScience. “It could be gradual. Most of our calculations places it at around that depth, but there is actually quite a bit of variability.”

Either way, this viscous boundary solves many uncertainties that geoscientists have had for some time. It explains why subducting tectonic plates get “stuck” at this point, and stagnate. The subduction of plates controls the most explosive volcanoes on Earth, including Vesuvius and Mount Fuji, so understanding this underlying mechanism may help researchers explain the types of eruption styles seen at the surface.

In addition, mantle plumes may be thinned and even deflected by this apparent jump in viscosity. As they move upwards and encounter this layer, they will find it difficult to move smoothly past it, and may end up migrating elsewhere. This potentially means that the hotspot volcanism on the surface may appear more unpredictably than previous models suggest.

Currently, there are competing theories as to why this change in viscosity occurs, ranging from a change in oxidation state of the iron minerals to a deep, hot pool causing a viscosity imbalance – but the authors say that a conclusion is far from being made.