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… …pieces of crust 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—because 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  realized 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 five  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 moved 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 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 that 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, diving  back toward 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 work 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, at least one more factor.

Scientists then turned to another possible  force acting on continents in the 1980s,   something they called “slab suction.” Slab suction happens when the mantle  flows around a sinking plate edge,   pulling it downward more than just gravity alone.

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

So, in the early 2000s, scientists  added slab suction to their models   and found this matched observed plate  movements a lot more than before.

So now we have four forces helping  move continents: mantle convection,   ridge push, slab pull from the weight  of plate sinking into the Earth,   and slab suction pulling even harder as those  plates get sucked into the lower mantle.

Adding together all these forces, we get a  better idea of why Earth’s tectonics works.

Plates are created at mid-ocean  ridges and get pushed away from   those ridges by gravity and mantle currents.

Eventually plates dive back into the mantle,   pulling the plates along, with suction  then dragging them down even harder.

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

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

This is where the bizarre mirror world comes in.

See, seismic studies done in the mid-2010s  show two huge zones of something unusual   at the base of the mantle—one below the  Pacific Ocean and another underneath Africa.

When seismic waves hit these zones,   they slow down – unlike how they  move through the rest of the mantle.

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

They can tell from the way LLVPs  react seismically that they’re   probably denser than the surrounding mantle  and possibly made of different material.

And the strangest thing is, these LLVPs seem to  move around much the same way continents do on   the Earth’s surface.

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

In fact, scientists recently uncovered evidence   that some of them might actually contain  the remains of continents that used to be   on the Earth’s surface – continents we thought  disappeared hundreds of millions of years ago.

The geophysicists running the models  to figure out what LLVPs might be   made of found that they could be the  remains of old surface plates that   sunk towards the core of the Earth  in a giant pile.

Like… a graveyard.

The idea of this slab graveyard could  explain a few things we see at the surface… Like how erupting lavas in places like Hawaii are   oddly enriched in the isotope helium  3.

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

Those are both elements we’d expect to see  on Earth’s surface rather than in plumes   from the mantle.

But helium and uranium makes  more sense if remnants of Earth’s surface,   sunk deep into the mantle,  are feeding plume eruptions.

So those mirror slabs inside the lower  mantle are probably, at least partly,   the remains of old plates that  got subducted, and then sank.

And there, invisible to us at the surface,   it seems they continue to play  a role in plate tectonics.

Like, they probably determine mantle currents.

When plotting major mantle plumes that have   erupted onto Earth’s surface, geologists noticed  that they all seem to burst forth from LLVP zones.

And these could also be the  reason we have subduction.

Although most subduction zones lie well  away from these deep-Earth structures,   it might be the case that mantle currents rise  above LLVPs, pushing plates off to the sides.

Then, once the plates are pushed  away from the upwelling plumes   above LLVPs, they start to sink, subducting.

When researchers plug in all the data we have  on the mantle and the movements of continents,   that scenario is exactly what they see.

And it all comes full circle  when the sinking plates absorb   back into the LLVP graveyard deep in  the mantle, and the cycle continues.

Now, not all scientists agree with this theory.

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

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

Another hypothesis argues that LLVPs are  actually the remnants of an ancient planet   that crashed into early Earth, which we talk  about in our episode “When Earth Ate A Planet.” Overall, LLVPs are pretty mysterious.

Even after decades of research into plate  tectonics, and a solid understanding of   how it works, the question of why it  works has been harder to pin down.

But it appears that, from deep in the Earth,  this ancient mirror world has something to   do with driving tectonics– the thing that  makes our planet unique –as far as we know.

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