ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: The Graveyard at the Center of the Earth

Without plate tectonics, the Earth would look very different than it does today.

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Without plate tectonics, the Earth would look very different than it does today.

Our mountain ranges, oceans, volcanoes,

and even entire continents all get their recognizable features from the shifting rock plates on the Earth's mantle.

And those features are pretty good evidence that plate tectonics works.

But knowing if something works doesn't always mean we know why or how it works.

And when it comes to plate tectonics,

continents move in complicated ways.

Scientists have been trying to solve the mystery of why plate tectonics works the way it does for over a hundred years,

and they might have just uncovered a key to cracking it.

The answer may be buried in a bizarre mirror world deep inside the Earth,

where movement beneath the surface looks a lot like it involves the remains of former crustal rocks.

Places of crusts we thought disappeared hundreds of millions of years ago,

but now live inside the planet,

like a graveyard of ancient plates.

When scientists first embraced plate tectonics around the 1960s,

the theory explained many observations about the planet.

The idea

that Earth's surface is made of rocky plates moving over a

molten mantle explains why large mountain ranges form in the middle of oceans, for example.

as Earth's mantle is creating new plates.

And why deep trenches form at the edges of the oceans,

where one plate is diving beneath another.

Tying together these observations, geologists realize that tectonics works as a system.

Plates are created at mid-ocean ridges,

then move away from them and finally disappear back into the Earth at subduction zones,

creating most volcanoes and many large earthquakes in the process.

But even though the theory that recognized the constant cycle by which landforms are created was a breakthrough,

it didn't exactly explain how this happens.

What force could move massive plates,

which can weigh a made-up sounding 5 quintillion tons around like driftwood?

Early in the days of plate tectonic theory,

geologists and geophysicists assumed plates move because of mantle convection.

Currents of hot, molten material rising inside the Earth push the plates,

dragging them along the surface the way rising bubbles move foam in a pot of boiling water.

Logically, that idea makes sense.

But scientists quickly realized that the physics of mantle convection don't totally add up.

Now, the mantle definitely does have currents.

Since 1971, scientists have known that rising plumes of magma travel from deep inside the Earth to the surface,

creating islands like Hawaii and Iceland.

But around the same time,

researchers also realized something about the mantle's makeup that complicates the effect those currents could possibly have.

See, they were looking at how continents move during ice ages,

when the weight of massive ice sheets pushed them down into the mantle.

And how, when the ice later melted,

the same continents began popping back up.

So they used data from a satellite to measure the gradual upward movement of the continent relative to the ocean's surface,

to see just how fast the now deglaciated continents were popping back up.

And even though they couldn't see the mantle below the continents,

knowing the rate at which the mantle pushed continents up let scientists calculate its viscosity,

that is, how runny it is.

They found that the Earth's mantle is much runnier than expected.

Later researchers estimated that the mantle is about a billion times runnier than the lithosphere,

or crust, that sits on top of it.

So those mantle currents simply couldn't be enough to move the plates the way we know they move.

There must have been another force helping propel this movement.

Even more mysteriously, geophysicists modeling Earth's mantle and continents found the Earth should have what's called a stagnant lid,

where the lithosphere sits in place on the surface as mantle currents move underneath.

In fact, Earth is the only place we know,

for sure, where continents move,

crash into each other, rise, and fall.

So why does Earth have such unique tectonics?

It all comes back to the mantle,

which, as scientists noted back in the 1960s,

shapes an important feature of tectonic subduction.

That's the process where plates slip underneath another,

driving back towards the center of the planet.

And while it's another process that we can't directly see,

it's one scientists inferred must be taking place once they noted some odd features of earthquakes.

With the installation of big seismograph networks,

geophysicists could suddenly tell exactly where and how deep earthquakes happened.

And when they looked at big tremors in places like Alaska,

they saw earthquake epicenters were far below the crust.

In fact, some earthquakes were happening hundreds of kilometers down inside the mantle.

That suggests that the stress of the plate bending deep down is triggering these earthquakes.

Researchers realized that these quakes were like a sonar scan,

revealing locations of where plates were driving down into the mantle.

They were seeing the edge of the subducting plate that had disappeared under another plate long ago.

Earth is also the only planet we know where subduction connects the surface to the interior in a major way.

And scientists soon figured out that subduction helps explain why tectonics works the way it does.

Because as geologists reasoned, if plate edges sank into the mantle,

they'd likely pull on the rest of the plate, dragging it along.

Initially, this was purely logical speculation.

Again, we can't see any of this.

But when scientists plugged this new force,

called slab-pull, into models, the calculations worked better than mantle convection alone.

But even with extra force from slab-pull added into the equation,

the models still didn't quite fit Earth's tectonics.

Plates still moved faster than the models predicted.

There must be other forces helping move the continents.

So scientists kept digging.

In the mid-1970s, they noted that mid-ocean ridges are usually topographic highs.

So as new plate material moves away from the ridges,

it travels downward, as if descending from a mountain.

This is due to gravity pulling down,

adding a little force in the process scientists call ridge push.

And scientists found this does help move us closer than the two forces of convection and slab pull alone.

But according to the models,

the addition of ridge push still wasn't enough to explain how fast our continents move either.

There had to be something else,

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at least one more factor.

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Scientists then turned to another possible force acting on continents in the 1980s,

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something they called slab suction.

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Slab suction happens when the mantle flows around a sinking plate edge,

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pulling it downward more than just gravity alone.

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Like in movies, where people swimming away from a sinking ship get sucked underwater as the boat disappears below the surface.

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So in the early 2000s,

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scientists added slab suction to their models,

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and found this matched observed plate movements a lot more than before.

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So now we have four forces helping move continents.

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Mantle convection, ridge push, slab pull from the weight of the plate sinking into the Earth,

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and slab suction pulling even harder as those plates get sucked into the lower mantle.

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Adding together all these forces,

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we get a better idea of why Earth's tectonics works.

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Plates are created at mid-ocean ridges,

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and get pushed away from those ridges by gravity and mantle currents.

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Eventually, plates dive back into the mantle,

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pulling the plates along, with suction then dragging them down even harder.

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But what makes those pieces subduct and sink back into the mantle?

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This isn't something we see happening on any other planets in our solar system.

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This is where the bizarre mirror world comes in.

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See, seismic studies done in the mid-2010s show two huge zones of something unusual at the base of the mantle,

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one below the Pacific Ocean and another underneath Africa.

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When seismic waves hit these zones,

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they slow down, unlike how they move through the rest of the mantle.

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Geophysicists call these massive 15,000-kilometer-wide features large, low-velocity provinces, or LLVPs.

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They can tell from the way LLVPs react seismically that they're probably denser than the surrounding mantle,

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and possibly made of different material.

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And the strangest thing is,

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these LLVPs seem to move around much the same way continents do on the Earth's surface.

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They collide, assemble, and possibly even break apart, just like continents.

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In fact, scientists recently uncovered evidence

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that some of them might actually contain the remains of continents that used to be on the Earth's surface.

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Continents we thought disappeared hundreds of millions of years ago.

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The geophysicists running the models to figure out what LLVPs might be made of found

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that they could be the remains of old surface plates that sunk towards the core of the Earth in a giant pile,

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like a graveyard.

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The idea of this slab graveyard could explain a few things we see at the surface.

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Like how erupting lavas in places like Hawaii are oddly enriched in the isotope helium-3.

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Or how volcanoes in both the South Pacific and off the West African coast erupt unusually high amounts of uranium.

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Those are both elements we'd expect to see on Earth's surface,

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rather than in plumes from the mantle.

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But helium and uranium makes more sense if remnants of Earth's surface,

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sunk deep into the mantle, are feeding plume eruptions.

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So those mirror slabs inside the lower mantle are probably,

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at least partly, the remains of old plates that got subducted and then sank.

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And there, invisible to us at the surface,

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it seems they continue to play a role in plate tectonics.

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Like, they probably determine mantle currents.

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When plotting major mantle plumes that have erupted onto Earth's surface,

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geologists notice that they all seem to burst forth from LLVP zones.

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And these could also be the reason we have subduction.

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Although most subduction zones lie well away from these deep earth structures,

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it might be the case that mantle currents rise above LLVPs,

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pushing plates off to the sides.

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Then once the plates are pushed away from the upwelling plumes above LLVPs,

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they start to sink, subducting.

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When researchers plug in all the data we have on the mantle and the movements of continents,

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that scenario is exactly what they see.

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And it all comes full circle when the sinking plates absorb back into the LLVP graveyard,

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deep in the mantle, and the cycle continues.

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Now, not all scientists agree with this theory.

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For example, there's debate about whether mantle plumes rise from the center of LLVPs or from their edges.

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Scientists also argue about whether LLVPs stay in one place or migrate around the mantle.

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Another hypothesis argues that LLVPs are actually the remnants of an ancient planet that crashed into early Earth,

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which we talk about in our episode When Earth Ate a Planet.

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Overall, LLVPs are pretty mysterious.

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Even after decades of research into plate tectonics,

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and a solid understanding of how it works,

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the question of why it works has been harder to pin down.

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But it appears that from deep in the Earth,

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this ancient mirror world has something to do with driving tectonics,

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the thing that makes our planet unique,

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as far as we know.

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And given that the mirror plates seem as active as the surface continents,

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the future of tectonics could be as surprising as this bizarre discovery itself.

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So we're still figuring out the details of plate tectonics,

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Join Michelle Barbosa-Ramirez and Blake DePastino on a field trip to explore the epic natural history saga of LA in our episode,

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How Plate Tectonics Transformed Los Angeles.

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So it took like 50 years for scientists to be like,

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nah, this is real.

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This is, this is it.

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