0:03 without plate tectonics the Earth would
0:06 look very different than it does today
0:09 our mountain ranges oceans volcanoes and
0:11 even entire continents all get their
0:14 recognizable features from the shifting
0:16 Rock plates on the Earth's mantle and
0:18 those features are pretty good evidence
0:21 that plate tectonics works but knowing
0:24 if something works doesn't always mean
0:27 we know why or how it works and when it
0:30 comes to plate tectonics continents move
0:32 in complicated ways scientists have been
0:35 trying to solve the mystery of why plate
0:37 tectonics works the way it does for over
0:40 a 100 years and they might have just
0:42 uncovered a key to cracking it the
0:45 answer may be buried in a bizarre mirror
0:47 World deep inside the Earth where
0:50 movement beneath the surface looks a lot
0:52 like it involves the remains of former
0:55 crustal rocks pieces of crusts we
0:57 thought disappeared hundreds of millions
0:59 of years ago but now live inside the planet
1:00 planet
1:08 plates when scientists first embraced
1:10 plate tectonics around the 1960s the
1:12 theory explained many observations about
1:15 the planet the idea that earth's surface
1:17 is made of Rocky plates moving over a
1:20 molten mantle explains why large
1:22 mountain ranges form in the middle of
1:24 oceans for example because Earth's
1:27 mantle is creating new plates and why
1:30 deep trenches form at the edges of the
1:32 oceans where one plate is diving beneath
1:34 another tying together these
1:36 observations geologists realized that
1:39 tectonics works as a system plates are
1:41 created at mid ocean ridges then move
1:43 away from them and finally disappear
1:46 back into the Earth at subduction zones
1:48 creating most volcanoes and many large
1:51 earthquakes in the process but even
1:53 though the theory that recognized the
1:55 constant cycle by which land forms are
1:57 created was a breakthrough it didn't
2:00 exactly explain how thisen happens what
2:03 force could move massive plates which
2:06 can weigh a madeup sounding five
2:09 quillion tons around like Driftwood
2:11 early in the days of plate tectonic
2:14 theory geologists and geophysicists
2:16 assumed plates moveed because of mantle
2:19 convection currents of hot molten
2:22 material Rising inside the earth pushed
2:24 the plates dragging them along the
2:27 surface the way Rising bubbles move foam
2:30 in a pot of boiling water logically that
2:33 idea makes sense but scientists quickly
2:34 realize that the physics of mantle
2:37 convection don't totally add up now the
2:40 mantle definitely does have currents
2:43 since 1971 scientists have known that
2:45 Rising plumes of magma travel from deep
2:48 inside the Earth to the surface creating
2:50 islands like Hawaii and Iceland but
2:52 around the same time researchers also
2:55 realized something about the Mantle's
2:57 makeup that complicates the effect those
2:59 currents could possibly have see they
3:01 they were looking at how continents
3:04 moved during ice ages when the weight of
3:06 massive ice sheets pushed them down into
3:09 the mantle and how when the ice leader
3:12 melted the same continents began popping
3:14 back up so they used data from a
3:17 satellite to measure the gradual upward
3:19 movement of the continent relative to
3:22 the ocean's surface to see just how fast
3:24 the now de glaciated continents were
3:26 popping back up and even though they
3:29 couldn't see the mantle below the
3:31 continents knowing the rate at which the
3:33 mantle pushed continents up let
3:35 scientists calculate its viscosity that
3:38 is how runny it is they found that the
3:40 Earth's mantle is much runnier than
3:43 expected later researchers estimated
3:45 that the mantle is about a billion times
3:48 runnier than the lithosphere or crust
3:51 that sits on top of it so those mantle
3:53 currents simply couldn't be enough to
3:55 move the plates the way we know they
3:58 move there must have been another Force
4:00 helping Propel this movement even more
4:02 mysteriously geophysicists modeling
4:05 Earth's mantle and continents found the
4:07 Earth should have what's called a
4:10 stagnant lid where the lithosphere sits
4:12 in place on the surface as mantle
4:15 currents move underneath in fact Earth
4:18 is the only place we know for sure where
4:20 continents move crash into each other
4:23 Rise and Fall so why does Earth have
4:26 such unique tectonics it all comes back
4:28 to the mantle which as scientists noted
4:31 back in the 1960s s shapes an important
4:34 feature of tectonic subduction that's
4:36 the process where plates slip underneath
4:39 another driving back towards the center
4:41 of the planet and while it's another
4:43 process that we can't directly see it's
4:46 one scientist inferred must be taking
4:49 place once they noted some odd features
4:51 of earthquakes with the installation of
4:54 big seismograph networks geophysicists
4:57 could suddenly tell exactly where and
4:59 how deep earthquakes happened and when
5:01 they looked at Big trimmers in places
5:03 like Alaska they saw earthquake
5:06 epicenters were far below the crust in
5:08 fact some earthquakes were happening
5:11 hundreds of kilometers down inside the
5:13 mantle that suggests that the stress of
5:15 the plate bending deep down is
5:18 triggering these earthquakes researchers
5:20 realized that these Quakes were like a
5:23 sonar scan revealing locations of where
5:26 plates were driving down into the mantle
5:28 they were seeing the edge of the
5:30 subducting plate that had disappeared
5:33 under another plate long ago Earth is
5:35 also the only planet we know where
5:37 subduction connects the surface to the
5:40 interior in a major way and scientists
5:42 soon figured out that subduction helps
5:45 explain why tectonics works the way it
5:48 does because as geologists reasoned if
5:51 plate edges sink into the mantle they'd
5:53 likely pull on the rest of the plate
5:55 dragging it along initially this was
5:58 purely logical speculation again we
6:00 can't see any of this but when
6:03 scientists plugged this new Force called
6:06 slab pull into models the calculations
6:08 worked better than mantle convection
6:11 alone but even with extra force from
6:13 slab pole added into the equation the
6:15 model still didn't quite fit Earth's
6:18 tonics plates still moved faster than
6:21 the models predicted there must be other
6:23 forces helping move the continents so
6:26 scientists kept digging in the mid 1970s
6:29 they noted that mid ocean ridges are
6:32 usually topographic highs so as new
6:34 plate material moves away from The
6:36 Ridges it travels downward as if
6:38 descending from a mountain this is due
6:41 to gravity pulling down adding a little
6:43 force in the process scientists call
6:46 Ridge push and scientists found this
6:48 does help move us closer than the two
6:51 forces of convection and slab pull alone
6:54 but according to the models the addition
6:56 of Ridge push still wasn't enough to
6:58 explain how fast our continents move
7:01 either there had to be something else at
7:03 least one more Factor scientists then
7:06 turned to another possible force acting
7:08 on continents in the 1980s something
7:11 they called slab suction slab suction
7:14 happens when the mantle flows around a
7:16 sinking plate Edge pulling it downward
7:19 more than just gravity alone like in
7:21 movies where people swimming away from a
7:24 sinking ship get sucked under water as
7:26 the boat disappears below the surface so
7:29 in the early 2000s scientists added slab
7:31 suction to their models and found this
7:33 matched observed plate movements a lot
7:36 more than before so now we have four
7:39 forces helping move continents mantle
7:42 convection Ridge push slab pull from the
7:44 weight of the plate sinking into the
7:47 Earth and slab suction pulling even
7:50 harder as those plates get sucked into
7:52 the lower mantle adding together all
7:55 these forces we get a better idea of why
7:57 Earth's tectonics Works plates are
7:59 created at mid ocean ridges and get
8:01 pushed away from those ridges by gravity
8:04 and mantle currents eventually plates
8:06 dive back into the mantle pulling the
8:09 plates along with suction then dragging
8:11 them down even harder but what makes
8:14 those pieces subduct and sink back into
8:16 the mantle this isn't something we see
8:19 happening on any other planets in our
8:22 solar system this is where the bizarre
8:24 mirror world comes in see seismic
8:27 studies done in the mid-20s show two
8:30 huge zones of something unusual at the
8:32 base of the mantle one below the Pacific
8:35 Ocean and another underneath Africa when
8:37 seismic waves hit these zones they slow
8:39 down unlike how they move through the
8:42 rest of the mantle geophysicists call
8:46 these massive 15,000 km wide features
8:49 large low velocity provinces or
8:53 lvps they can tell from the way lvps
8:55 react seismically that they're probably
8:58 denser than the surrounding mantle and
8:59 possibly made of different different
9:02 material and the strangest thing is
9:05 these lvps seem to move around much the
9:07 same way continents do on the Earth's
9:10 surface they Collide assemble and
9:12 possibly even break apart just like
9:15 continents in fact scientists recently
9:17 uncovered evidence that some of them
9:20 might actually contain the remains of
9:21 continents that used to be on the
9:24 Earth's surface conents we thought
9:26 disappeared hundreds of millions of
9:28 years ago the geophysicists running the
9:30 models to figure out what lvps might be
9:33 made of found that they could be the
9:35 remains of old surface plates that sunk
9:37 towards the core of the Earth in a giant
9:40 pile like a graveyard the idea of this
9:43 slab graveyard could explain a few
9:45 things we see at the surface like how
9:48 erupting lavas in places like Hawaii are
9:51 oddly enriched in the isotope helium 3
9:53 or how volcanoes in both the South
9:56 Pacific and off the West African Coast
9:59 erupt unusually high amounts of uranium
10:01 those are both elements we'd expect to
10:04 see on Earth's surface rather than in
10:07 plumes from the mantle but helium and
10:09 uranium makes more sense if remnants of
10:11 Earth's surface sunk deep into the
10:14 mantle are feeding plume eruptions so
10:16 those mirror slabs inside the lower
10:19 mantle are probably at least partly the
10:21 remains of old plates that got subducted
10:24 and then sank and they're invisible to
10:26 us at the surface it seems they continue
10:29 to play a role in plate tectonics like
10:32 they probably determine mantle currents
10:34 when plotting major mantle plumes that
10:36 have erupted onto Earth's surface
10:38 geologists noticed that they all seem to
10:41 burst forth from lvp zones and these
10:43 could also be the reason we have
10:45 subduction although most subduction
10:48 zones lie well away from these deep
10:50 Earth structures it might be the case
10:53 that mantle currents rise above llv PS
10:56 pushing plates off to the sides then
10:58 once the plates are pushed away from the
11:01 upwelling plume above lvps they start to
11:04 sink subducting when researchers plug in
11:06 all the data we have on the mantle and
11:09 the movements of continents that
11:12 scenario is exactly what they see and it
11:14 all comes full circle when the sinking
11:17 plates absorb back into the lvp
11:19 graveyard deep in the mantle and the
11:22 cycle continues now not all scientists
11:25 agree with this theory for example
11:26 there's debate about whether mantle
11:30 plumes rise from the center of llv PS or
11:33 from their edges scientists also argue
11:36 about whether lvps stay in one place or
11:38 migrate around the mantle another
11:41 hypothesis argues that lvps are actually
11:43 the remnants of an ancient planet that
11:45 crashed into early Earth which we talk
11:48 about in our episode when Earth ate a
11:51 planet overall lvps are pretty
11:53 mysterious even after Decades of
11:55 research into plate tectonics and a
11:57 solid understanding of how it works the
12:00 question of why it works has been harder
12:03 to pin down but it appears that from
12:05 deep in the Earth this ancient mirror
12:08 world has something to do with driving
12:10 tectonics the thing that makes our
12:13 planet unique as far as we know and
12:15 given that the mirror plates seem as
12:17 active as the surface continents the
12:20 future of tectonics could be as
12:23 surprising as this bizarre Discovery [Music]
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14:52 time continental drift came out in 1912
14:54 and everybody was like y'all crazy y'all
14:56 crazy and then around the 1960s they're
14:58 like H maybe you're not as crazy as we
15:00 thought you were so it took like 50
15:03 years for scientists to be like no this
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