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  • Thanks to Brilliant for supporting

  • this episode of SciShow.

  • Go to brilliant.org/SciShow to grab a gift subscription

  • to help your loved ones spark a lifelong love of learning.

  • {♫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 Earththat

  • 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 maybejust 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

  • 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♫}

Thanks to Brilliant for supporting

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B2 中高級

這張地球圖是個謊言 (This Diagram of Earth Is a Lie)

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    林宜悉 發佈於 2021 年 01 月 14 日