Something Strange Is Happening 1,700 Miles Beneath Your Feet. Now We Know Why.
Here's what you'll learn when you read this story:
Over a thousand miles from the surface, in Earth's D' layer—right on the edge of the liquid metal outer core—there is a weird acceleration of seismic waves.
Experiments recreating the phenomenon in a lab found that this is the result of post-perovskite crystals, which form from perovskite.
The alignment of these crystals determines their hardness, which then determines how fast seismic waves can move through them.
Deep beneath Earth's surface are layers of soil, rock strata often embedded with fossils, gurgling magma, and—back up. Before your Journey to the Center of the Earth mission can get any further, you're going to have to get past flows of solid rock.
The D' layer—located between layers of magma above and the liquid rock of the outer core below—has been mystifying scientists for decades. This is in part because if you were to plunge down 2,700 kilometers (1,700 miles), you would be jump-scared by seismic waves that accelerate when they hit the threshold of the D' layer. It used to be thought the reason for this was the mineral perovskite, found in the lower mantle, morphing into a form known as post-perovskite close to the D' layer. But that still wasn't enough to explain the phenomenon.
Geoscientist Motohiko Murakami wanted to investigate what could possibly be going on to cause the strange seismic wave acceleration known as the D' discontinuity. Because trekking to the core-mantle boundary (CMB) where the D' layer lies is obviously not an option, he led a team of researchers from Switzerland and Japan in running lab tests and computer simulations to find out what post-perovskite had to do with he unusual increase in seismic waves.
Post-perovskite crystals are anisotropic, meaning their physical properties are different when measured in different directions. They have two different types of textures—one comes from transformation (the phase transition from the perovskite phase to post-perovskite), and the other is a result of deformation (when post-perovskite crystals turn to face in the same direction). Murakami and his team found out that it isn't just transformation that causes the rumbling. It actually happens with deformation.
'The deformation-induced texture forms when crystals undergo plastic deformation, causing their orientations to align in specific directions. It is mainly produced by dislocation slip or creep,' Murakami said in a study recently published in the journal Communications Earth & Environment.
How post-perovskite crystals are aligned determines their hardness, and the speed at which seismic waves move through them depends on how hard they are. Materials called perovskites can be created from any substances capable of being arranged into the same cubic crystal structure. Perovskite is a calcium titanium oxide mineral (CaTiO3), while post-perovskite is a form of magnesium silicate (MgSiO3) achieved at extremely high pressures. Its crystal structure is orthorhombic, meaning that the right angles of the cubes have unequal axes.
For post-perovskite crystals to align with each other, the axes all have to be in the same position. Murakami used MgGeO3 to create crystals analogous to post-perovskite. Like perovskite, MgGeO3 crystals deform easily when pressure is applied, so how they behaved would reflect was is going on over a thousand miles underground. The crystals were heated by a laser, compressed, and heated again to synthesize post-perovskite. They were then exposed to high-pressure sound waves, and the wave velocity was measured once those waves passed through the crystals.
It turned out that sound waves can experience a substantial increase in velocity when passing through aligned post-perovskite crystals. Researchers also discovered that the cause of this alignment—which determines the hardness of the material, and therefore the speed of sound waves in the lab and seismic waves deep in Earth—is convection. As hotter material rises, cooler material sinks, as it does in convective storms like hurricanes.
Murakami thinks that convection of materials in the mantle (such as plumes rising and slavs sinking) is behind the deformation in the D' layer. This is the first time any evidence—even lab-based evidence—has been found for Earth's innards moving.
'While previous theoretical work has suggested that anisotropy could explain the observed seismic discontinuities,' he said. 'Our results, obtained through in situ measurements of post-perovskite velocities under high pressure, represent the experimental verification of this hypothesis, bridging the gap between theory and observation.'
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