Why seismic waves spontaneously race inside the earth

Earthquakes, volcanic eruptions, shifting tectonic plates - these are all signs that our planet is alive. But what is revealed deep inside the earth surprises laymen and scientists alike: Almost 3000 kilometres below the Earth's surface, solid rock is flowing that is neither liquid, like lava, nor brittle like solid rock. This is shown by a new study by geoscientists led by Motohiko Murakami, Professor of Experimental Mineral Physics at ETH Zurich. The study has just been published in the journal Communications Earth & Environment.

Half a century of guesswork

For over 50 years, researchers have been puzzling over a strange zone deep inside the earth - the so-called D” layer, around 2700 kilometres beneath our feet. Earthquake waves suddenly behave differently there: their speed jumps as if they were travelling through a different material. What exactly happens at that layer of the mantle has been unclear for a long time, until now.

In 2004, Murakami, who has been a professor at ETH Zurich since 2017, discovered that perovskite, the main mineral of the Earth's lower mantle, transforms into a new mineral near the D” layer under extreme pressure and very high temperatures - so-called “post-perovskite”.

The researchers assumed that this change explained the strange acceleration of the seismic waves. But that was not the full story. In 2007, Murakami and colleagues found new evidence that the phase change of perovskite alone is not enough to accelerate earthquake waves.

Using a sophisticated computer model, they finally discovered something important: depending on the direction in which the post-perovskite crystals point, the hardness of the mineral changes. Only when all the crystals of the mineral point in the same direction in the model are the seismic waves accelerated - as can be observed in the D” layer at a depth of 2700 kilometres.

In an unusual laboratory experiment at ETH Zurich, Murakami has now proven that post-perovskite crystals align themselves in the identical direction under enormous pressure and extreme temperatures. To do this, the researchers measured the speed of seismic waves in their experiment and were also able to reproduce the jump that occurs at the D” layer in the laboratory. “We have finally found the last piece of the puzzle,” says Murakami.

Mantle flow aligns crystals

The big question is: what makes these crystals line up? The answer is that solid mantle rock that flows horizontally along the lower edge of the Earth's mantle. Researchers have long suspected that this movement - a kind of convection like boiling water - must exist but have never been able to prove it directly.

A new chapter in Earth research begins

Murakami and his colleagues have now also demonstrated experimentally that mantle convection of solid rock is present at the boundary between the core and the Earth's mantle, i.e. that solid - not liquid - rock flows slowly but steadily at this depth. “This discovery not only solves the mystery of the D” layer but also opens a window into the dynamics in the depths of the Earth,” Murakami explains.

It is not only a milestone, but also a turning point. The assumption that solid rock flows has been transformed from a theory into a certainty. “Our discovery shows that the Earth is not only active on the surface, but is also in motion deep inside,” says the ETH professor.

With this knowledge, researchers can now begin to map the currents in the Earth's deepest interior and thus visualise the invisible motor that drives volcanoes, tectonic plates, and perhaps even the Earth's magnetic field.