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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.
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their annual Premium subscription.
If you check them out, let us know what you think!
And as always, thanks for watching
this episode of SciShow.
{♫Outro♫}