>> Earth, a 4.5-billion-year-
old planet, still evolving. As
continents shift and clash,
volcanoes erupt, and glaciers
grow and recede, the Earth's
crust is carved in numerous
and fascinating ways, leaving
a trail of geological
mysteries behind. In this
episode, the Marianas Trench,
the deepest point on Earth, is
explored. Its sheer walls cut
seven miles into the Pacific
Ocean. The mystery of what
created this deep, dark chasm
takes science detectives on
some of the most dangerous
dives ever attempted, deep
into the abyss. Scouring the
ocean floor, scientists
uncover a strange,
undersea world of fiery
mountains, bizarre mud
volcanoes and the largest
geological structure on Earth.
Discoveries from this unique
underwater world will
revolutionize our
understanding of the powerful
forces that shape not just the
trench, but the Earth itself.
S01x02 The Deepest Place on Earth Original Air Date on February 17, 2009
-- Sync, corrected by elderman -- -- for www.addic7ed.com --
Hidden deep beneath the waves
of the western Pacific lies
the Marianas Trench, the
deepest point of all the
oceans.
The first step on the journey
of what created this
mysterious scar in the Earth's
crust, and how it continues to
mold the planet, takes us
back to 1872, when a British
research vessel, HMS
Challenger, set out on the
first ever mission to map the
ocean floor.
>> Throughout most of recorded
history, men had just assumed
that, beyond a certain level,
the sea was pretty flat,
pretty dead, pretty lifeless.
They weren't expecting to find
anything very interesting.
>> For four years, the
Challenger crisscrossed the
oceans, covering 70,000 miles,
a third of the distance to the
moon.
The crew plumbed the depths
every 140 miles, using a total
of 249 miles of rope, and
hundreds of pounds of
lead weight. It was tedious,
backbreaking work, but at the
time, it was the only way to
measure the depth of the ocean
floor.
When they got to the western
Pacific, 200 miles off the
island of Guam, the crew
routinely lowered the rope for
a measurement.
But the weight kept on
dropping and dropping.
>> It's a big surprise! Nobody
thought the ocean was this
deep. So all of sudden we've
got scientists saying, "Why
is that?"
>> Eventually, the weight
struck the bottom at 4,475
fathoms, nearly five miles
beneath the ocean's surface.
>> The scientists would be
going, "Wow, we've found
something and what does it
mean? Is it a little hole? Is
it a big hole? What kind of
feature is it down there?"
There--there's a whole lot
of questions you get when you
find this one spectacular
reading.
>> The Challenger expedition
marked the birth of modern
oceanography, and provided the
first crude map of the ocean
floor.
It showed how the ocean floor
gently slopes away from the
land, and then plummets
thousands of feet into vast
flat plains. But the western
Pacific is different. It drops
off again, into the five mile
deep hole, a hole that blew
right out of t water the
long-held belief that the sea
floor was flat and
featureless.
And it spawned a mystery,
because nobody could
understand how this strange
underwater feature came about.
It would be 75 years before
any answers emerged. It took a
revolutionary new technology,
sonar, to push the
investigation forward to the
next crucial stage.
Sonar was first developed in
the early 1900s and then
perfected during the 1940s to
detect submarines lurking in
the deep.
The system works by pumping
sound waves through the water.
The waves bounce off solid
objects and are reflected back
to a detector. By measuring
the time it takes for the
sound waves to bounce
back, scientists realized they
could build a remarkably
accurate picture of the world
beneath the waves.
>> The world's major navies
spend a lot of time and effort
developing submarine hunting
technology, then the
hydrographers discover that
you can use this to chart the
bottom of the sea and it's an
awful lot cheaper and easier
than using large numbers of
sailors pulling on ropes.
>> In 1951, a British Navy
research ship returned to the
deep hole found by the
Challenger expedition.
But, this time, they were
armed with sophisticated new
sonar equipment.
And the results were amazing.
Detailed sonar maps revealed
that the deep hole in the
Pacific Ocean floor isn't a
hole at all, but part of a
massive trench, 30 times
deeper than the Empire State
Building is high.
It runs twice the length of
California, 1,500 miles from
the southeast of Guam to the
northwest of the Mariana
Islands.
>> People were probably
astounded by what they were
seeing, because, clearly, the
ocean floor had enormous
changes in relief. It was very
mountainous in some places,
had great deeps in other
places. To a geologist, this
would be extremely exciting.
Even within the trench itself,
there are remarkable
variations. At its southern
end lies the greatest surprise
of all.
The sea floor drops down
another two miles to its
lowest point, a staggering
seven miles beneath the waves.
Scientists had discovered the
deepest part of the oceans.
Even today, it is the lowest
known point on the planet.
They named this part of the
trench the Challenger Deep, in
honor of the ship that
discovered it.
>> To get a sense of just how
deep trenches are, if we take
the heig
we would still have about a
mile of water above us before
we get to the ocean surface.
>> But how the Marianas Trench
was formed remained a mystery.
Investigators decided the best
way to find the answer was to
dive to the bottom of the
trench, to see for themselves
the lowest point on the
planet, the Challenger Deep.
But they faced a major
problem. At the bottom of the
trench, they would have to
contend with pressure a
thousand times stronger than
at the surface, that's the
equivalent of being squeezed
on all sides by the weight of
50 jumbo jets.
To demonstrate the effects of
such pressure, scientists
use a dummy head.
>> Today, what we are going
to do is actually put one of
these Styrofoam wig heads in
the, uh, pressure chamber and
expose it to the, uh, pressure
we would see in the Marianas
Trench. That's about 16,000
psi.
>> A human skull would be
crushed to a pulp, but the
rubbery head will only have
all the air squeezed out.
>> Wow, the head's smaller.
Here's what the original size
was, just for comparison.
[LAUGHS]
Quite dramatic! Pretty stark
difference between, uh,
something that hasn't been
seven miles deep in the ocean
and something that has.
Glad I'm not going there.
[BOTH LAUGH]
At the Mariana Trench, human
life is impossible, we're not
equipped to resist those kinds
of pressures, and so it's
necessary to protect humans
from that type of an
environment.
>> The challenge to engineers
was how to accomplish this.
In 1953, Swiss scientist
Auguste Piccard designed the
Trieste, a pioneering vehicle
that could withstand the
crushing pressures.
The submersible was dominated
by a 50 foot long hull, filled
with light aviation gasoline
and lead weights to control
buoyancy. Slung underneath it
was a tiny six foot spherical
cabin with five inch thick
steel walls.
Finally, after seven years of
modifications and manned test
dives no deeper than three and
a half miles, the Trieste was
ready to attempt the seven
miles to the bottom of the
trench. The commander of this
perilous undertaking was US
Navy Lieutenant and deep sea
explorer Don Walsh.
>> I know the astronauts that
go through this all the time.
"Why do you have to be there?
Why can't we just put up a
robot to do things?" You've
got to be there because that's
what we do.
>> Only a few officers and
scientists knew about the
risky mission, which was
launched in January 1960 from
the western Pacific island of
Guam.
>> Guam in those days was kind
of a backwater, it was just
right for us because we were
trying to do this project sort
of out of sight, because we
weren't too sure it was gonna
work. The navy just didn't
want to be embarrassed by a
failed science spectacular.
>> Accompanying Walsh was the
son of the Trieste's designer,
engineer and oceanographer
Jacques Piccard. The two men
would spend the next nine
hours squeezed inside the
cramped sphere.
>> And we had, erm, 20 cubic
feet of space inside, that's
about the same as a
household refrigerator, and
the temperature was almost
that cold inside. It was a
drama.
>> The story of how the
Marianas Trench came to be is
beginning to take shape.
In 1874, British surveyors
were the first to discover
a five mile deep hole in the
ocean. 75 years later, sonar
mapping revealed the hole to
be a vast, 1,500-mile long
trench, with the deepest part
seven miles beneath the
surface waves of the Pacific.
To gather further evidence,
two courageous men were about
to undertake the most
dangerous dive in history.
They would venture into the
abyss and go to the bottom of
the Marianas Trench.
The Marianas Trench is one of
the most remote, inhospitable
places on Earth.
In January 1960, two deep sea
explorers, Don Walsh and
Jacques Piccard, plunged into
its depths on board the
submersible, the Trieste.
At a speed of just three miles
per hour, they began their
slow descent into the twilight
zone.
By 3,000 feet, the darkness
was total. The only
illumination was from the
Trieste's powerful lights.
>> At the depths we were
operating at, it was always
black. The only thing that lit
up the abyss was the
bioluminescence from animals
and plankton. Like fireflies,
they carry their own light
sources with them.
>> Encased in their five-inch
thick steel sphere, Walsh and
Piccard quickly passed their
test dive record of 18,000
feet. Everything appeared to
be going to plan. At the rear
of the cabin, the crew were
protected by a double layer of
glass. But, two hours into the
dive, the outer pane cracked.
>> We, um, had a great big
bang. We didn't know what it
was. We were at about 20,000
feet, and we looked around and
checked everything,
>> Every square inch of their
tiny life-supporting capsule
was fighting back eight tons
of pressure. With the outer
pane broken, the only thing
between the men and instant
death was a single pane of
glass.
>> If the inner window had
cracked, erm, we would have
been instantly dead, maybe
even before we knew it.
>> But, incredibly, the inner
pane remained watertight.
Walsh and Piccard decided to
continue the descent.
After a tense, claustrophobic
four hours and 48 minutes,
they approached the bottom of
the trench, only to be
startled by movement on the
sea floor.
>> Just before we landed, we
saw a flatfish about a foot
long, and that's a
bottom-dwelling fish, so if
you see one there are others,
>> Nobody expected to see life
at these crushing depths, but
it meant the explorers had
reached their goal, the very
bottom of the Marianas Trench.
The depth gauge, with a
reading of 35,800 feet, nearly
seven miles below the surface,
confirmed the sonar findings.
Squeezed inside their bubble
of breathable air, the two
explorers were closer to the
Earth's centre than man had
ever been.
We took a self-portrait,
that's the picture that you
see. We said we were going
to do it, and we did it.
>> But there was work to be
done. Walsh and Piccard wanted
to make detailed observations
of the enormous trench.
Unfortunately, the Trieste
stirred up a cloud of fine,
powdery sediment from the sea
floor that obscured their
view.
>> WALSH: It was like being in
a bowl of milk at that point.
So, realizing that we weren't
gonna see anything, we decided
to go on back up to the
surface.
>> ANNOUNCER: Off the island
of Guam, the Trieste surfaces
after a descent into the
Marianas Trench.
>> After nine grueling hours
underwater, Walsh and Piccard
returned to the surface on
January 23rd 1960 and
officially entered the record
books for the deepest dive of
all time. To this day, their
extraordinary feat has never
been repeated.
The mission was a success, but
the mystery remained.
Geologists still didn't
understand what could have
formed the immense trench.
And if they couldn't find the
answer inside the trench,
they would have to look
elsewhere.
Perhaps there was something,
somewhere, on the ocean floor
that might explain the
trench's origins.
Throughout the '50s and '60s,
a team of geologists led by
Princeton's Harry Hess
compiled sonar data from all
of the world's oceans.
It was as though they had
pulled out a giant plug, to
drain away all the water, and
expose the ocean floor.
Their maps revealed that the
Marianas Trench is just a tiny
fraction of a network of
enormous underwater canyons
stretching right around the
planet. But that wasn't all.
Running parallel to the
trench, on the other side of
the Pacific, the maps showed a
giant underwater mountain
range, the East Pacific Ridge.
And this too is part of a
global network, a 40,000 mile
long chain of mountain ranges
that ring the globe like the
seams of a baseball, to make
the largest geological feature
on Earth.
It was a major development in
the investigation, one that
scientists hoped might explain
the trench's formation.
The next step was clear.
Investigators needed to
understand whether there was a
connection between the trench
and the East Pacific Ridge.
The breakthrough came from the
unlikeliest of sources. During
the Cold War, the US built a
vast network of underground
seismometers to pick up atomic
bomb testing around the world.
Inadvertently, the
seismometers also detected
naturally occurring
earthquakes. When geologists
plotted these on a map, a
pattern emerged.
The earthquakes were clustered
along the ocean's ridges and
trenches. It was a discovery
that transformed our
understanding of the Earth.
Geologists realized the
friction that cause
earthquakes comes from
movements that must be
occurring deep beneath the
ridges and trenches.
>> With this great investment
in seismology, it became
possible to locate very
precisely where earthquakes
had occurred. And it was these
things, the precise location,
the depth and the motion that
really gave the outlines of
plate tectonics.
>> It was the birth of an
extraordinary new theory. The
solid layer of rock, the
crust, on which the land and
ocean sits, is broken up into
a series of vast slabs, that
geologists call tectonic
plates. It's these plates that
are moving, grinding past each
other, and triggering
earthquakes.
The underwater ridges and
trenches sit on the boundaries
between tectonic plates.
The East Pacific Ridge and the
Marianas Trench lie on
opposite edges of the Pacific
Plate.
The journey to discover what
formed the Marianas Trench is
accumulating additional
evidence. The Trieste dived to
the bottom of the trench,
and confirmed that it is the
deepest point on the planet.
Sonar maps then revealed the
East Pacific Ocean Ridge,
running parallel to the
trench. To solve the mystery
of the Marianas Trench,
investigators needed to find
out exactly what was happening
at the East Pacific Ridge, and
that meant exploring these
vast mountains, 8,000 feet
underwater.
The pieces of the Marianas
Trench puzzle are falling into
place with the knowledge that
it lies on the western edge of
the Pacific tectonic plate.
On the opposite side of the
plate lies the East Pacific
Ocean Ridge, part of an!
enormous chain of underwater
mountain ranges that ring the
globe to create the largest
geological feature on Earth.
Scientists had a hunch that
this colossal ridge might help
explain how the trench was
formed. And they found a major
clue halfway round the globe,
where the ridge passed beneath
the middle of the Atlantic
Ocean.
During the Cold War, the US
Navy developed a new technique
to spot Soviet submarines.
They scanned the seas with a
tool called MAD, a magnetic
anomaly detector, which could
pinpoint steel hulls lurking
in the deep. But they stumbled
across something else. Running
parallel on either side of the
ridge, they found strange
stripes of magnetic rocks,
alternating positive and
negative away from the ridge's
peak.
Here's the Mid-Atlantic Ridge
coming down through here.
Almost perfectly symmetric on
either side of that are these
white and black stripes, these
have often been called zebra
stripes.
>> Geologists know that the
Earth is like a giant magnet
with a north and a south pole.
But the magnetic poles aren't
fixed. Every 300,000 years or
so, the magnetic field
suddenly flips 180 degrees.
When the field flips, a
compass that was previously
pointing north will swing to
the south.
>> This reversing of the
Earth's magnetic field is a
very interesting and exciting
but very puzzling phenomenon
for a geophysicist to explain.
>> Scientists think this
reversal explains the stripes
either side of the ocean
ridge. In the 1960s,
geologists discovered that
molten volcanic rock, known as
magma, swelled up from deep
underground to create the
ridges in the Atlantic and
Pacific.
As magma wells up between the
tectonic plates, it pushes the
sea floor up, and forms the
colossal mid-ocean ridge,
thousands of feet high.
When the rock is hot and
molten, its magnetic minerals
line up with the north-south
direction of the Earth's
magnetic field. As the magma
cools, the minerals are locked
in position.
These rocks act as a permanent
record of the magnetic poles'
location when the rocks were
formed. As more and more magma
is forced up, the old crust is
pushed away from the ridge and
records the reversals in the
Earth's magnetic polarity.
>> If you have reversals of
magnetic polarity, then the
sea floor acts sort of like a
tape recorder and records
these changes in
magnetization, then the
pattern of magnetic stripes
allows people to calculate the
speed at which the plates are
moving apart.
The zebra stripes are proof
that, over time, the sea floor
in both the Atlantic and the
Pacific, is spreading away
from the ridges at a rate of
more than two inches a year.
But geologists needed proof
that magma created the ridge.
If red-hot molten rock is
forming the enormous mountain
range in the Pacific, the
surrounding water should be
warm.
In 1977, a team of scientists
set out to discover whether
this warm water really
existed.
Dudley Foster was the pilot
for these historic dives.
>> It's been an exciting
occupation because you're on
the cutting edge of science,
uh, new discoveries all the
time. Every cruise, there's a
new group of scientists with
new scientific objectives and
there's the exploration and
the discovery and that's
really what puts the thrill in
the job.
>> For weeks, the crew scanned
the undersea mountains without
success. And then, they hit
the jackpot, a bizarre pillar
of rock, spewing hot toxic
gas.
>> And we saw the water was
sort of shimmering, sort of
like, uh, bubbling in a glass
teapot or something.
We stuck the temperature probe
in there and it measured 38,
39 degrees Fahrenheit, which
was really amazing, 'cause
the--the ocean's a huge heat
sink, and so to see something
warm like that was kind of
startling.
>> In these pillars of rock,
the expedition had found the
heat from the magma surging up
from deep inside the Earth.
It wasn't warming the water
evenly along the ridge, it was
channeled up through strange
hydrothermal vents.
>> FOSTER: When you make these
discoveries, you don't know
how significant they are. The
true significance of 'em maybe
takes several years to
appreciate, and this was one
of those times.
>> For the investigation into
the Marianas Trench, these
vents are a decisive piece of
evidence.
They confirm that magma is
continually creating new crust
at the Pacific Ocean Ridge.
And magnetic zebra stripes
prove that old crust is pushed
away from the ridge towards
the other side of the Pacific
Plate, towards the Marianas
Trench. But this presents
scientists with a puzzle.
If new crust is being created
at the ocean ridge, and the
Earth isn't expanding, then
the old crust must be
disappearing somewhere else.
>> The reason that the Earth's
not getting bigger with sea
floor spreading is because the
same amount of sea floor is
being destroyed in the
Pacific.
>> Something in the Pacific
Ocean is devouring the sea
floor. And all the evidence
points to the Marianas Trench.
In the hunt to discover what
formed the Marianas Trench,
scientists now know crust
created at the ocean ridge is
being devoured somewhere and
by something in the Pacific
Ocean.
They suspect the Marianas
Trench is involved. But the
proof would come, not from the
trench, but from these, the
Mariana Islands, a chain of
volcanoes that break through
the ocean's surface 200 miles
west of the trench. Scientists
noticed the island chain
mirrors the trench's exact
shape. This led them to think
the trench was responsible for
the islands' creation.
>> If, uh, you see pictures of
the Marianas Trench, it's
curved, and the line of
volcanoes that it generates is
curved exactly parallel to it,
>> Geologists believe that the
trench formed the volcanoes
via a process called
subduction. Subduction occurs
where two tectonic plates
collide. As they grind past
each other, the heavier plate
is pushed beneath the lighter
plate. The descending plate is
forced down into the Earth's
intensely hot interior, called
the mantle. It takes with it
water and sediment built up
over millions of years.
>> Volcanoes form above
subduction zones not because
the Earth is hotter there but
because this is where we're
taking the water that once was
in the ocean. It gets taken
into the mantle and gets
sweated out, causes the mantle
to melt and this magma is what
then rises and erupts
explosively out these
volcanoes.
>> The water in the sediment
forces magma to swirl up and
push through the plate above.
And when it breaks the
surface, it creates volcanoes,
like the volcanoes that form
the Mariana Islands.
It was subduction that formed
the islands west of the trench
and gave investigators the
breakthrough they'd been
looking for. Because here, at
last, was a process powerful
enough to create the Marianas
Trench. As the descending
plate dives down, it digs into
the mantle. Here, the
colliding plates form a
trench, a giant crease in
the ocean floor.
It seemed that scientists had
finally explained how the
trench was formed.
There was just one problem. A
very large problem. Around the
world, subduction zones cause
violent earthquakes and
catastrophic tsunamis.
>> We know subduction is
happening because of the
active earthquakes and these
are the most devastating
earthquakes. This is the
earthquake that generated e
tsunami in Sumatra, also the
other very large earthquakes
in Alaska and Chile.
>> But the Marianas Trench,
the deepest subduction zone in
the world, hasn't caused a
devastating earthquake since
records began in the 17th
century. Investigators needed
to know why.
>> Ah, that's--that's, uh, the
$60,000 question.
>> They hoped the trench's
shallower western edge might
provide the answer.
Here, they found an intriguing
chain of underwater hills two
miles below the surface of the
sea.
Engineers drilled down into
the hills and collected core
samples.
An$ when the scientists
analyzed the samples, they
discovered the hills were
actually volcanoes, and they
spewed out not lava, but mud.
The fine, powdery mud is made
up of a soft type of rock that
has been ground up in the
subduction zone. It seemed
this soft rock might explain
why there have been no
major earthquakes at the
Marianas Trench.
>> Everybody has a sense of
what a volcano is but not all
volcanoes erupt igneous rocks,
there's some volcanoes that
erupt mud. And a certain kind
of unusual kind of mud in the
Marianas is made out of
serpentine, and serpentine is
a very weak rock and it can be
scratched with a knife or
something like that.
>> Investigators realized the
grinding plates crush the soft
rock to form a lubricating mud
that prevents large
earthquakes. Then the mud
bubbles up to the ocean floor,
where it forms the strange mud
volcanoes found along the
trench's western edge.
>> Other parts of the world,
like the Andes or maybe
Indonesia, you've got two
plates that are grinding
together and the--one
of the plates is quite strong,
and it takes a big earthquake
to rupture that plate
interface. But if these rocks
are weak like they are in the
Marianas, where you've got
these serpentinites, those
are very weak and it doesn't
take much energy at all to get
the two plates to glide one
past the other.
>> At last, geologists had
discovered what created the
Marianas Trench.
50 million years ago, the
Pacific Plate slipped under
the edge of the Philippine
Plate. As it bent and dived
into the Earth's mantle, it
formed the colossal Marianas
Trench. And the plate is still
moving. Like a giant conveyor
belt, the Earth's crust
travels slowly across the
Pacific Plate, from its
birthplace in the East
Pacific Ridge to its
graveyard, 10,000 miles away
in the Marianas Trench.
Today, the Pacific Plate's
movement can be tracked in
real time.
>> Confirmation has come from
GPS technology, where we can
actually put a transmitter on
an island and come back year
after year and actually follow
it moving a few centimeters a
year towards the trench.
>> It's devouring the crust at
a rate of three inches a year,
about as fast as a human
fingernail grows.
Every four million years, it
swallows an area the size of
the United States. By
consuming the crust created at
the Pacific Ocean Ridge,
the ravenous Marianas Trench
is the world's largest
recycling plant.
But there was one remaining
and major piece of the puzzle
to find. Scientists still
didn't know why it is the
deepest trench on Earth.
They suspected the age of the
sea floor at the bottom of the
trench may provide the answer.
>> It turns out there's a
really strong relationship
between the age of the sea
floor and its depth in the
water.
>> In 1999, a team of deep sea
drillers returned to the
trench to collect core
samples.
>> PLANK: One great thing
about drilling this ocean
crust is we actually got
pieces of it. So, we're
holding in our hands here the
material that's actually getting
subducted at the Marianas
Trench, and it turned out to
be 170 million years old. So
we can say with confidence
that's the oldest ocean floor
before it's getting swallowed
up in the mantle at the
trench.
>> But why is this piece of
rock the oldest on the ocean
floor?
>> PLANK: The sea floor at the
Marianas Trench is so old
because it's been so long
since it was born, so it was
born in the equivalent of the
eastern Pacific today and it's
just been going on longer
than--than any other place in
the oceans before it's been
subducted.
>> The Pacific Plate is the
planet's largest tectonic
plate, covering an area 11
times the size of the United
States.
When crust bubbled up at the
ridge 170 million years ago,
it was light and buoyant.
But as it traveled 10,000
miles across the plate, it
cooled and became compact and
dense. Over millions of years,
the dense crust got heavier
and began to sink into the
mantle below.
Scientists realized that,
because the crust at the
Marianas Trench is the oldest
ocean crust, it's also the
heaviest and so has sunk
deeper into the mantle than
any other area of ocean crust.
Here, at last, was the
explanation for the trench's
extraordinary depth. The
picture of the Marianas Trench
is almost complete.
Volcanic islands mirroring the
trench's exact shape lead
scientists to believe it runs
along a subduction zone.
And slippery mud volcanoes
explain why it doesn't create
large earthquakes. But one
question remains unanswered.
Towards the trench's southern
end, the vast chasm drops a
further two miles to its
lowest point, the Challenger
Deep, seven miles beneath the
waves. The question is, what
makes it plunge so deep?
The investigation into the
Marianas Trench has one final
puzzle to solve.
At the trench's southern end,
the sea floor plummets a
further 10,000 feet into a
seven mile deep chasm called
the Challenger Deep. It's the
lowest point on the planet,
but so far, scientists have
been unable to explain why
this one section of the trench
is so deep. Now, they believe
the shape of the descending
tectonic plate may hold the
answer.
>> The Challenger Deep, in
addition, is a little bit
deeper, because of some
peculiarities relating to how
the slab that's going down is
behaving.
>> A narrow slab of crust has
torn away from the Pacific
Plate's descending edge.
>> STERN: Well, it's basically
got to do with how the slab
pushes the mantle out of the
way. Where you have a narrow
slab, like you have at the
Challenger Deep, it can sink
almost vertically, because the
mantle that it's trying to
displace can move around out
of the way.
Investigators have finally
unraveled the mysteries of the
Marianas Trench.
And in the process, they've
made a discovery with
implications that stretch far
beyond the trench itself.
Studying the ocean ridges led
geologists to believe that
magma, welling up at the
ridges, was pushing the plates
apart.
>> How much weight is that--
>> But the exploration of the
Marianas Trench has changed
this idea forever.
>> People used to think that
maybe the magma would kind of
push the plates apart, and
that idea is largely
discounted now.
>> As the ocean crust travels
from the Pacific Ocean Ridge
to the trench, it changes from
a buoyant, red-hot magma into
a colder, denser and heavier
crust. The plate's leading
edge becomes so heavy that it
drags the rest of the plate
along behind it.
>> The heavy cold plates at
the trenches are sinking down
into the mantle and pulling
the plates apart, uh, at the
ridges, and the magma just
passively, uh, fills in the
gaps.
>> The investigation into the
Marianas Trench has
revolutionized our
understanding of how the
Earth's plates move.
We now know a worldwide
network of subduction zones
drag tectonic plates around
the globe, powering the
movement of continents over
millions of years and moving
the very Earth we stand on.
>> The plates that are moving
fastest on the Earth are
the ones that have all the
trenches.
>> The Pacific Plate is the
fastest moving of the nine
major plates on the planet,
because it is surrounded by
dozens of destructive trenches
like the Marianas. They are
consuming the ocean crust
faster than the Ocean Ridge
can produce it. Over millions
of years, the Pacific Plate
will shrink until, some time
in the distant future, the
largest ocean on Earth will
disappear. Australia will
crash into the United States,
reshaping our planet.
Perhaps one day, downtown
Seattle will compete for real
estate with a suburb of
Sydney, Australia. And
all because of subduction
zones like the Marianas
Trench.
But for all its significance,
man has only ever dived to the
bottom of the trench once, and
there are no immediate plans
to return.
>> Imagine asking someone,
"What is the flora and fauna
of California?" and saying
that someone's spent ten
minutes there, picked up two
ants, come back and said
they've sampled California.
That's probably how well we
know the Marianas Trench.
>> To date, less than five per
cent of the world's oceans
have been explored. But only
by returning to the oceans'
very deepest reaches will we
fully comprehend the
incredible forces that recycle
and rebuild our world.
>> The way I like to think of
it is that ocean exploration
leads to new research
questions. And if we don't
have exploration, we don't
even know the right questions
to ask.
It is now known what a
geological wonder the Marianas
Trench is. Since this deep
chasm in the Earth's crust was
first discovered with a length
of rope and a lump of lead
more than a century ago,
evidence has piled up. A
record-breaking dive to the
lowest point on Earth. Giant
undersea mountain ranges with
bizarre magnetic zebra
stripes, proof that the ocean
crust is spreading towards
the hungry Marianas Trench,
lined with slippery mud
volcanoes which prevent
devastating earthquakes.
And the planet's oldest ocean
crust, the reason that the
Marianas Trench is the deepest
point in the oceans.
In the darkest and most remote
place in the world, scientists
have added to their knowledge
about the powerful forces that
contribute to the dynamic
story of our planet.
-- Sync, corrected by elderman -- -- for www.addic7ed.com --