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{♫Intro♫}

When you learned about the Earth's interior

in elementary school, you were probably shown

a diagram that looked

like a perfect layer cake.

You had the thin crust, the thicker section of the mantle,

the outer core, and the inner core,

and everything was smooth and even:

perfect nested spheres.

I hate to burst your bubble, but…

we've known for a long time that that diagram

just isn't true.

At least, when it comes to the mantle.

In reality, the Earth's mantle

is far from a perfect, smooth layer.

Instead, it has some

gigantic blobs the size of continents in it.

And we're now learning that those irregularities

may actually be fundamentally important to

what's happening up here on the surface,

more than 2000 km away.

To do this kind of work, scientists use seismometers:

instruments about the size of a gallon of

paint that measure motion in the ground.

They're most famous for studying earthquakes,

but they can also be used to examine other

vibrations in the Earth.

And if you know what you're looking for,

seismometer data can also help you figure

out what the planet's interior is like.

For instance, when a big earthquake happens,

it releases energy that radiates out from

the epicenter in waves.

These waves travel through the Earth

in all directions, but they don't go in a

straight line,

and they don't all travel the same speed.

Instead, their speed depends on the temperature

and density of the rock they're moving through.

For example, hot, molten rock slows down the

seismic waves a lot, while cold, dense material

transmits them faster.

So by using seismometers to monitor the arrival

times of these waves at different locations,

scientists can figure out the density of rock

layers inside the Earth and what they're made of.

This kind of work is called seismic tomography,

and it's the main way we know what the inside

of our planet looks like.

It's also how we discovered that the mantle

isn't a perfect sphere.

The mantle is a thick layer of solid-ish rock,

and it's super weirdly shaped.

It's blobby and uneven,

and there are two areas in particular

that are very different

than the rest of it: one blob below

the Pacific Ocean, and one beneath Africa

and the Atlantic Ocean.

These two arm-like protrusions were discovered

in the late 1970s, and they're the size

of entire continents.

They sit right above

the core-mantle boundary and extend up toward

the surface for hundreds of kilometers.

Scientists can see them in tomographic images because they're

low-velocity zones, meaning

when seismic waves hit them,

the waves slow down a lot.

Then, when the waves exit the blobs, they change speed again.

This weird behavior likely has to do with what the blobs are made of.

Scientists haven't figured out their composition

for sure, but an experiment from 2017 suggested

that the blobs might be iron peroxide.

And they might have formed when iron-rich rock

from the mantle reacted with seawater under

enormous pressures and high temperatures.

Seawater sometimes gets into the mantle as

tectonic plates move underneath each other.

According to preliminary studies, a composition

like this would give us the kind of seismic

wave data we measure from earthquakes.

Of course, scientists want to know more than

just what these things are made of. They also

want to know how they fit into the larger scheme of things on Earth.

There are a bunch of questions to answer here,

but at least right now, these blobs seem to

be related to volcanic centers.

Like, nearly all the hotspots on Earth — that

is, all the volcanic centers not associated

with tectonic plate boundaries —

seem to be located above these blobs.

Hawai'i is pretty much centered over the one below the Pacific Ocean,

so it might be

the cause of the plume of molten rock

that feeds the Hawaiian volcanoes.

The other big blob may be related to older,

extinct volcanic fields, like some in Africa

that erupted huge volumes of lava called flood basalts.

And in addition to these two main blobs, there

are also some smaller ones, including one

that might be contributing to the volcanic system of Iceland.

Overall, though, there's still a lot to learn about how the blobs formed

in the first place.

Like, we're still not sure if they're leftover

from Earth's formation, or if they

started out at the surface and sank down

through the mantle at some point.

And if they did sink down there —

well, is there any possibility they might become

buoyant enough to rise,

like wax in a lava lamp?

The answer depends on their chemical compositions.

If that composition could cause them to

slowly rise and sink over millions of years,

then those movements would affect

how heat is circulated in the Earth.

And maybe — just maybe —

it could explain why the Earth's magnetic field

sometimes reverses.

But don't worry.

A magnetic reversal caused

by a rising mantle blob would take

millions of years to happen,

so don't go buy a new compass just yet.

As seismic tomography improves

and more scientists get in on the work,

our understanding of Earth's

interior will get better and better.

And with every new image,

we'll learn more about these mantle blobs,

how they came to be,

and how they affect the surface of Earth thousands of kilometers away.

If you want to learn more about seismology

and how scientists study the inside of our planet,

you can check out the Waves and Light course from Brilliant.

It teaches you about all kinds of waves,

including sound and light,

but it also talks a lot about seismic waves.

In one quiz, you even learn how

to figure out where an earthquake started

just by knowing what the vibrations

looked like at the surface.

Like all of Brilliant's other courses,

Waves and Light comes with a bunch of great diagrams

and explanations,

so even if you're not a geology expert,

you won't feel lost.

Besides this one,

Brilliant has other courses about science,

engineering, computer science, and math.

So no matter what you want to learn about,

you've got options.

You can learn more at Brilliant.org/SciShow.

And as a thank-you to our audience,

Brilliant is giving the first 200 people

to sign up at that link 20% off

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If you check them out, let us know what you think!

And as always, thanks for watching

this episode of SciShow.

{♫Outro♫}