High Pressure Experiments Duplicate Conditions 1,800 Miles Underground

Public domain image from Wikipedia.

As a dedicated Hollow Earth Nerd, I’m always fascinated by the idea of structures hundreds or thousands of miles beneath the surface of the Earth, especially if they involve bright lights and colors, not to mention bad acting. Unfortunately for dinosaurs and Victorian explorers alike, Earth ain’t hollow, but every time I hear about something a few thousand miles underground, my mind bends.

This latest gem (!) is that UC Berkeley and Yale Scientists are doing research into the unusual phase of magnesium silicate perovskite in the extreme reaches of the Earth’s mantle. The mantle is the layer between the crust (think “dirt”) and the outer core, made of nickel and iron. Perovskite, which forms most of the mantle, is compressed at the bottom of that layer into a 125-mile thick boundary, about 1,800 miles down, of “post-perovskite,” a phase of the substance that only exists at extreme pressure and depth.

As reported in this week’s issue of the journal Science (the abstract is public — full text requires a subscription) and summarized on Indian website domain-b.com, The Yale-UCB team compressed perovskite glass to a pressure of two million atmospheres (two million times the air pressure on the surface) and a temperature of 3,500 Kelvin or 6,000 Farenheit to create post-perovskite, then hit it with X-rays at Lawrence Berkeley National Laboratory.

What they’re trying to find out is why different kinds of seismic waves propagate differently between the mantle and the core. The answer is anisotropy — polarization of the waves parallel with or in opposition to the crystal structures of the ultra-compressed post-perovskite.

This is the seismic wave equivalent of light the old polarized-filter trick, where two polarized filters will form a transparent surface when the polarization is lined up. Turn them at a slight angle and the pair of filters becomes opaque. That’s because light travels in waves like seismic energy, and polarized filters (or post-perovskite) block waves along a single orientation. This affects the propagation of seismic waves through the mantle.

Plus, of course, the Lava Men are skeet-shooting our seismic waves with Negatroid Rays. But they’re really lousy aim.

If you’re a geology nerd, you may already know that while the core-mantle boundary is called the CMB, the boundary between the mantle and crust is known as the Moho, for Mohorovičić Discontinuity.

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