{♫Intro♫}.

When you learned about the inside of the Earth,  you might have learned that it’s made of nice,   smooth layers: the crust, the upper  mantle, the lower mantle, and so on. But the more we discover, the more we  find out that that’s anything but true.

Like, scientists have found a whole mountain  range between the upper and lower mantle. And these mountains might be the key to  one of the biggest mysteries in geology:   the case of Earth’s missing elements. Compared to meteorites and other  nearby objects in our solar system,  .

Earth seems to have lower amounts  of certain elements, like silicon. Which is weird, because based on what we  know about how the solar system formed,   these objects should have come  from the same disk of material. So, geologists have been trying to figure out   literally where on Earth  those elements are hiding.

The lower mantle was a prime suspect,   because it and the core beneath it are the  only big places we haven’t been able to look. The problem is, the lower mantle begins   660 kilometers underground — way  too deep to just drill down to it. But it is possible to explore  this part of the

Earth:  . You just have to wait for a strong earthquake. Earthquakes send seismic waves through the planet,   and how they travel can be affected by things  like what kind of rock they’re passing through. So, measuring those waves can  tell us something about Earth’s   insides — including what the lower mantle is like.

Or more specifically, the boundary between  the lower mantle and the upper mantle. At this boundary, an increase  in pressure and temperature   causes the minerals there to change phase:. Their atoms become more densely-packed, and  30 times more viscous than those above them.

And because of this change,   some seismic waves can bounce off this  boundary and be detected by seismometers. Now, interpreting these reflections is complex.  But basically, if the boundary in the mantle is   smooth, the waves will bounce off it and  strike the seismometer at the same time. But if the boundary is rough, the  reflections will be scattered,   and they’ll strike the seismometer  at slightly different times.

So in a 2019 study, researchers  used reflected seismic waves to   piece together what that lower  mantle boundary looked like. Using data from 13 earthquakes, they mapped  the top of the lower mantle — and realized   it isn’t smooth at all. It’s full of huge  mountains.

Like, 30 to 40 kilometers high! Mount Everest is only nine kilometers tall! Now, big features like this could signal that   the mantle is hotter in one  place or cooler in another.

This could slightly shift the point  where the minerals change phase,   and make the mantle boundary all jagged. But the researchers were pretty sure that  wasn’t the case, because these mountains   weren’t just tall. They were also rugged, full  of bumps between one and three kilometers high.

And this can’t be explained  by temperature differences,   because fluctuations that small  should even out over time. Instead, scientists believe that these bumps  likely mark a chemical change — and that the   mountains of the lower mantle are made of  something different than the upper mantle. They hypothesized that these  mountains could be ancient slabs   of oceanic crust from the surface  that have subducted and sunk down.

And instead of fully melting and mixing back  into the mantle, they’re just sitting down there. If this is true, these slabs would  block the circulation between the   parts of the mantle and possibly  form a treasure trove of Earth’s   hidden elements — locked away  since our planet’s early days. And even if the missing elements  turn out to be somewhere else,   this is a reminder that Earth is  never as neat and tidy as it seems.

And that’s really a theme with everything  out there. Like, the mantle also has some   gigantic blobs in it, further disproving the idea  that Earth is made of neat, concentric circles. If you want to learn more about what  those blobs are and what they’re doing,   you can watch our episode about it after this. {♫Outro♫}.