Something strange is happening 1,700 miles beneath the Earth's surface

Something strange is happening deep within the Earth’s mantle, around 1,700 miles below the surface, where seismic waves are accelerating in a way that has puzzled scientists for decades. This bizarre phenomenon occurs at a boundary known as the D'' layer, located between the hot magma of the lower mantle and the Earth’s liquid outer core.

While it was once believed that the mineral perovskite was responsible for this acceleration, new research has revealed an even more complex cause, linked to a unique type of crystal.

The D'' layer has long been a mystery to geoscientists. As seismic waves travel through the Earth's interior, they behave differently upon reaching this boundary, accelerating in a way that defies simple explanations. This acceleration was attributed to the transformation of the mineral perovskite into a form called post-perovskite, which occurs under extreme pressure and temperature deep within the Earth.

However, advanced research led by geoscientist Motohiko Murakami has shed new light on the situation. Murakami and his team from Switzerland and Japan conducted laboratory experiments and computer simulations to investigate the cause behind the acceleration of seismic waves. Their findings suggest that the true cause is not just the transformation of perovskite, but also the alignment of post-perovskite crystals at the core-mantle boundary.

Post-perovskite crystals are anisotropic, meaning their physical properties vary depending on the direction in which they are measured. These crystals can form in two distinct ways: one through a phase transformation (from perovskite to post-perovskite) and the other through deformation (when crystals align in specific directions due to pressure).

Murakami’s team discovered that it is this deformation, not just the transformation, that causes the acceleration of seismic waves. “When crystals undergo plastic deformation, their orientations align in specific directions, creating a structure that affects the speed at which seismic waves can travel through them,” explained Murakami.

What drives the deformation of post-perovskite crystals? According to Murakami, the answer lies in the convective movements that occur deep within the Earth's mantle. As hotter material rises and cooler material sinks—similar to the way storms form on the Earth’s surface—this causes post-perovskite crystals to align in specific directions. This convective process, which was previously only theoretical, is now confirmed through experimental evidence.

The research team used a material known as MgGeO3 to simulate the behavior of post-perovskite crystals in the lab. By compressing and heating the crystals and then exposing them to high-pressure sound waves, they were able to measure how seismic wave speeds changed. The results showed that seismic waves moved significantly faster when passing through aligned post-perovskite crystals, just as the theory predicted.

Murakami’s study marks the first experimental verification of the hypothesis that convection plays a critical role in the deformation of post-perovskite crystals. “Our results, obtained through in situ measurements of post-perovskite velocities under high pressure, represent the experimental confirmation of this hypothesis, bridging the gap between theory and observation,” said Murakami.

This major discovery not only explains the strange acceleration of seismic waves in the D'' layer but also offers valuable insight into the dynamic forces shaping the Earth’s deep interior.

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