>> Earth--a unique planet--
restless and dynamic
Continents shift and clash.
Volcanoes erupt, glaciers grow
and recede.
Titanic forces that are
constantly at work, leaving
a trail of geological mysteries
behind.
This episode explores Yosemite
Valley, a magnificent
3,000-foot-deep canyon.
Traveling through time,
scientists are unlocking deep
Secrets trapped inside these
granite walls.
These rocks have literally been
to hell and back, frozen,
drenched, and battered by
earth-shattering forces.
One more chapter in the
incredible story of...
Yosemite Original Air Date on December 22, 2009
-- Sync, corrected by elderman -- -- for addic7ed.com ---
In the foothills of the mighty
Sierra Nevada, California, lies
a valley like no other on
earth--Yosemite, a 7-mile-long,
one-mile-wide granite canyon.
Here lie some of the most
awe-inspiring geological
features on the planet:
Half Dome--America's most
iconic peak; Yosemite Falls--
the highest unbroken waterfall
on the continent;
And El Capitan--one of the
biggest sheer cliffs in the
world.
The europeans discovered this
astonishing valley only 150
years ago.
>> These sheer walls and granite
cliffs and high waterfalls were
a marvel to them as it is to us.
>> Ever since it shot to fame,
Yosemite has been shrouded in
mystery.
>> It's almost as if there was
a higher power at work that
basically said, "that looks
really good right about there,
and let's just put a little bit
of grass right there, and some
oak, like, an oak woodland right
through there, and that's it!
That's perfect!" because it
doesn't seem like anything was
haphazard. It seems like this
was designed to overwhelm and
then leave people awestruck.
>> Yosemite's unique design has
intrigued scientists for
centuries.
>> I think when you see
something that is this
dramatic, you just have
questions, and the question is,
how did this happen? How do you
get monoliths, these huge
stone structures that are just
rising thousands of feet off the
valley floor? How does that come
to pass? And for over a hundred
years, people have been trying
to answer that question.
>> The story begins in 1870,
when amateur geologist John Muir
went hunting for the answer.
Founder of the Sierra Club, he
funded his obsession with the
valley by herding sheep and
writing articles about Yosemite.
He wrote that "no temple made
with hands can compare with
Yosemite."
Muir scoured the landscape for
clues as to how this unusual,
box-shaped canyon formed
and came up with a radical,
seemingly far-fetched theory.
He believed that the world's
climate was once extremely
different, and the lush canyon
where the Merced River flows was
filled by a gigantic glacier
thousands of feet thick.
Muir proposed that as this river
of ice slowly flowed downhill,
over thousands of years, it
gouged a deep canyon and sliced
vertical cliffs into the granite
walls. But that controversial
idea brought him into conflict
with a fearsome opponent--
the admired California state
geologist, professor Josiah
Whitney.
>> Josiah Whitney did not have
much respect for John Muir
because Josiah Whitney was the
State geologist, and at that
time, John Muir was herding
sheep. Certainly he's thinking,
what does this man know about
this range? How dare he?
>> Whitney believed that this
unusually square, steep-sided
valley could only have been
formed by a sudden, cataclysmic
event. He was adamant that giant
cracks in the earth caused the
valley to pull apart and
dranatically sink...
cleaving steep cliffs as it
fell.
This bitter Whitney-Muir
conflict raged on for decades,
but now scientists are hoping to
figure out this complex puzzle
once and for all. The first
stage of the investigation is to
understand how the rocks
themselves were created. The
canyon is made almost entirely
of Yosemite granite, one of the
hardest rocks in the world.
To understand how this formed,
scientists must travel back in
time to the beginning.
It's 250 million years ago, and
the landscape is very different.
Dinosaurs roam the land and
dominate the skies.
[dinosaurs screeching]
An incredibly rare remnant of
that time has been discovered
Northwest of the valley at
Mount Hoffman.
This patch of softer, reddish
rock is the oldest in the park.
It's sandstone, and it once
covered the entire region.
>> This rock is very different
than most of the rock in
Yosemite. It's much darker in
color, got a different texture.
This was originally a
sedimentary rock that was
deposited in layers.
>> On an ancient shore, sand and
sludge compacted together and,
over millions of years, formed
this sandstone. This tranquil
landscape was transformed by an
earth-shattering event deep
below the surface, and the
evidence lies here in the rock.
>> Here we have this older,
layered sedimentary rock, and
you see that many layers and
banding, which continues across
over in here. But, strangely,
yhis section of granite has cut
tight through the sedimentary
rock and split this section
in two.
>> How this unusual rock
formation formed can be
explained by a simple experiment
using sand and wax.
>> This red wax here represents
the granite, the sand represents
sedimentary rock, and right now
I'm heating it to see what's
going to happen when the wax
melts. Actually I start to hear
some--oh, here it goes. We're
actually getting a little bit of
the wax pushing up through the
sand. And the molten wax is
lighter, and so it's pushing its
way up through the sand, which
represents the sedimentary rock.
Yep, here's some more.
>> It proves the only way
granite could have cut through
this sandstone is if it was
originally molten and able to
flow like a liquid.
>> molten rock forms deep within
the earth, rises up through the
crust, in this case, forced its
way through a crack, expanding
it open, and then cooling and
solidifying to form this band
of granite.
>> This band of granite tells
scientists that Yosemite's
cliffs were once molten and
heated to 1,800 degrees
fahrenheit.
>> 95% percent of the rock in
Yosemite is granite, and I can
see with the naked eye all the
crystals in here. They're fairly
large, and that tells me that
this rock cooled very slowly.
>> Scientists were now on the
hunt for what was generating all
this molten rock.
The investigation led them to
Mount Gibbs, another unusually
colored mountain in the northern
reaches of the park.
It lies above a layer of granite
and is made from reddish-gray
rock.
Strangely, these rocks were
also once molten.
>> Here I have a rock from Mount
Gibbs, and I can see that most
of the crystals are very, very
small. Most of them I can't
even see.
>> These small crystals tell
an incredible story. When molten
rock cools quickly in the open
air, crystals don't have time
to grow.
>> What that tells me is that
this is a volcanic rock that was
spewed out at the surface.
>> It is 200 million years ago.
Volcanoes shatter the tranquil
West coast. The earth explodes
with molten rock. For the next
100 million years, lava pours
thick and fast onto the land,
covering the sandstone in
volcanic rock 10,000 feet high.
It stretches for 400 miles along
the coast.
North America's greatest
mountain chain--the mighty
Sierra Nevada--is forming, and
the area that would become
Yosemite is caught right in the
middle of it.
But some of this rising, molten
rock never makes it to the
surface. Trapped beneath the
blanket of mountains, it cools
slowly, creating a giant chamber
full of solidifying granite
two miles below the surface.
400 miles long, 60 miles wide,
and 5 miles deep, it stretches
along the entire spine of the
Sierra Nevada mountain range.
Yosemite's immense granite
monoliths were forged in a fiery
furnace two miles below ground.
>> A hundred million years ago,
I would have been standing in a
vast chamber of molten rock,
with rock rising thousands of
feet above me.
>> Scientists investigating how
the rocks at Yosemite formed
have found...
bands of granite--evidence
Yosemite's rocks were once
molten...
and volcanic rocks--proof that
the granite formed deep beneath
the earth.
But scientists were still
mystified. Yosemite granite is
so strong, it's unlike granite
anywhere else on earth.
For hundreds of years, they
tried to solve the mystery,
while the answer was actually
staring them in the face.
>> 250 million years ago,
Yosemite's landscape was a
peaceful coastal plain.
Now the investigation moves to
100 million years ago.
The land that would become
Yosemite is in turmoil.
Volcanoes dot the skyline,
spilling mountains of lava onto
the land.
And entombed beneath two miles
of volcanic rock lies Yosemite's
molten granite.
The next ten million years is a
critical period for the rocks
of Yosemite.
Something unique is happening to
the granite, making it tough
enough to hold up cliffs 3,000
feet high.
The search for the secret of
Yosemite granite's immense
strength led scientists to the
biggest steep-sided granite
block in the world--the mighty
El Capitan.
Twice the height of the Empire
State building, over 3 billion
cubic feet of rock rises into
the air.
>> El Capitan is the largest
Granite monolith in Yosemite
National Park--3,000 feet of
pure granite. It's one of the
biggest cliffs in the world.
When I look up at something like
this, I really want to hang onto
the rock because I feel like I'm
going to fall backwards with
vertigo. Amazing.
>> El Capitan is the ultimate
"big wall" climb. Once
considered impossible to
conquer, it's a treacherous
ascent up a vertical, near
featureless rock face, with only
a handful of routes to the
summit.
Sheer granite walls are normally
unstable and over time get
pulled down by erosion, so it
was a mystery how rocks could
hold up cliffs this big.
[rhythmic tapping]
Scientists are sampling the
rocks to find out, but the
Yosemite granite doesn't give in
without a fight.
>> Whew! These Yosemite granites
are really hard. You can work up
quite a sweat trying to collect
a bagful of this stuff.
>> Buried within this rock is
the secret to Yosemite granite's
success.
>> Oh, finally.
Well, one clue to why these
rocks are so tough is given by
this sample right here.
These are really large crystals.
Large crystals make a rock
really strong and they kind of
weld together and they're
flawless and they give a rock
a lot of strength.
>> But it was a mystery how
these tough crystals got so
large and created such flawless
rocks.
This is in contrast to normal
granite, which, when it cools,
forms a hard rock with a fatal
flaw.
Riddled with cracks, these rocks
are vulnerable to erosion.
>> If you take anything and it's
very hot and you cool it
quickly, it'll shatter.
Granite bodies that cool
quickly form a lot of cracks
that weaken the rock body and
make it unable to support
3,000-foot cliffs.
If you walk up to a typical
granite and look at it, you'll
find cracks a foot apart, two
feet apart. If you walk up to
El Capitan, you might have to
go a hundred feet or more
between cracks in the rock.
>> Clearly, something different
happened here 100 million years
ago.
>> El Capitan is really just one
large, uniform, essentially
faultless piece of granite, and
that makes it very difficult for
erosion to deal with.
>> For many years, it was
a puzzle what had caused
Yosemite granite to form so
differently. But the secret had
been staring scientists in the
face.
>> One of the things you can see
is a clue as to how this
landscape formed in that dark,
diagonal splotch.
That's called the North American
wall because that dark blob
looks like, with some
imagination, a map of North
America and you can kind of see
Baja California sticking down on
the left side there and Alaska
up to the northwest.
And it's made up of a much
darker, finer-grained rock
like this.
>> This dark granite is not just
a map of North America.
It's a window into the unique
events that happened inside the
Yosemite granite, when it was
buried deep beneath the earth.
>> And it's a younger body of
granitic rock that filled a
crack within the main body of
El Capitan granite.
>> 100 million years ago, the
main body of granite cools and
begins to harden beneath the
earth.
But then fresh, molten rock
invades a crack and injects it
with a blast of heat.
The granite now takes ten times
longer to cool. Huge, robust
crystals form and reseal the
defective cracks. Time after
Time, fresh molten rock invades
old, re-melting and welding
the Yosemite granite, creating
gigantic, shatterproof blocks
which now tower thousands of
feet into the air.
>> Given that these are
obviously very strong rocks,
it puts the Whitney-Muir
controversy in an interesting
light, because Whitney wanted
the rocks to have broken and for
the valley to have fallen down
in between a couple of major
cracks. Muir wanted the whole
thing to be carved by glaciers.
And both of those seem really
difficult, given how strong
these rocks are, and yet behind
me here is a 3,000-foot-deep
valley. Something carved it.
>> In the search to understand
why Yosemite granite is so
strong, scientists have found...
large granite crystals welded
together to form a tough,
flawless rock, and the North
America wall--evidence that
Yosemite granite was reheated
time after time, creating huge,
faultless blocks.
100 million years ago,
Yosemite's granite lies buried
beneath a volcanic mountain
range two miles high.
Rivers slice into this soft
volcanic roof, gouging and
washing it away.
Then, 60 million years ago, when
Dinosaurs are no more, the
featureless lump of Yosemite
granite is finally exposed to
the elements.
But Yosemite's ultra-tough
granite defies attack.
For another 50 million years,
it remains unscathed.
It would take a catastrophic
event to chisel the 3,000-foot
Canyon out of one of the
toughest rocks on earth.
>> Yosemite's Merced River
meanders in a 3,000-foot canyon.
But 250 million years ago,
no vast canyon stood here.
Yosemite is a flat coastal
plain. It is blasted by
volcanoes, and its granite is
buried beneath two miles of
volcanic rock.
When the Yosemite granite is
finally exposed, it is
a shallow, run-of-the-mill
river valley.
For a staggering 50 million
years, it defies the destructive
powers of erosion.
If scientists were going to
solve the mystery of how
Yosemite valley formed, they'd
have to investigate the pivotal
moment 7 million years ago when
a 3,000-foot canyon was suddenly
chiseled into the granite, one
of the toughest rocks on earth.
A clue to what almighty force
overpowered the Yosemite granite
lies high up on the exposed
cliffs--strange, deep cracks
running for hundreds of feet
through the rock.
>> Yosemite's granites are
strong, but they're not always
strong. I'm standing on a fin of
rock between two deep fissures.
These cracks run right across
the Yosemite landscape, and they
clearly formed after the
granite formed.
And if I look at their
orientation...
they all go to the northeast.
>> Strangely, Yosemite valley is
also oriented along the same
northeast-southwesterly
direction.
Perhaps these cracks were the
remnants of a cataclysmic
rock-splitting event, as
Josiah Whitney had predicted.
A respected, Yale-educated
scientist, he passionately
believed titanic forces beneath
Yosemite had pulled the land
apart, causing it to collapse
along gigantic fault-line
cracks.
Could it be that the argument
was swinging in the bombastic
Whitney's favor, and Muir's
glacial theory was the rambling
of an ignorant shepherd after
all? Scientists searching for
the answers as to how these
giant cracks formed used modern
technology to take a closer
look at the ground beneath the
Sierra Nevada.
They did not find evidence of
Whitney's cataclysmic rifting
valley, but discovered
a catastrophic event which had
ramifications on a far greater
scale.
Seismic evidence has revealed
that the earth's crust is made
of layers. The lighter outer
shell forms the land, but
fastened beneath is a dense
anchor of sturdy rock.
And beneath the Sierra Nevada,
scientists discovered something
extraordinary--a huge section of
this lower crust, 200 miles long
and 40 miles wide, is missing.
>> This model represents the
Sierra Nevada as it looked ten
million years ago.
So the top here represents the
mountain range. The bottom
there, it represents the lower
crust.
Sometime in the past ten million
years, part of that lower crust
disconnected from the mountain
range and dropped away.
The end result is that there was
uplift of the Sierra Nevada
because they were being held
down by those dense rocks. So
there was uplift of the Sierra
Nevada, but because most of the
root fell away to the east, the
whole mountain range, rather
than bobbing straight up, tilted
to the west.
>> 7 million years ago, a huge
chunk of the lower crust
suddenly drops away.
The entire mountain chain snaps
along its spine and tilts
upward. Finally, here is a force
capable of overcoming Yosemite's
ultra-tough granite.
As it snaps, the enormous
pressure cracks the solid
granite along the entire length
of the mountain range.
But it was a mystery how these
cracks could give rise to
a 3,000-foot-deep canyon.
Scientists realized some other
force must have attacked the
weakened, vulnerable granite.
The hunt was on for an
accomplice...
and 5 miles downstream of
Yosemite, at El Portal, they
find it.
Rapids.
Here, Yosemite's meandering
Merced River has transformed
Into a raging torrent.
At times, 1,000 cubic feet of
water gush through this narrow
gorge every second, creating
treacherous waves and turbulent
eddies.
>> So this is the Merced River,
about 5 miles downstream of
Yosemite valley.
It's extremely powerful. It's
a series of rapids and cascades
over these boulders, and there's
a lot of energy being expended
by this river right now.
>> Scientists realized this
ferocious force, combined with
the rock-weakening cracks, even
had the power to cut into
Yosemite's granite.
>> The erosion of the river is
focused right along the river
bed, and so it will cut down
like a saw blade into the rock,
about the width of the river.
So as it does that, it will come
down and steepen the valley
walls. Those walls will become
unstable, and there will be
landslides into the river.
The river will wash away the
material, and you'll be back to
that "V" shape.
>> Scientists realized, if the
Merced River created this deep,
V-shaped canyon, then it could
also have cut a deep Yosemite
Valley.
7 million years ago, as the
Sierra Nevada tilts upwards, the
mountain slope gets 3 times
steeper, and the western-flowing
Merced River surges with
a torrent of water.
Like a saw blade, it zigzags
along the path of weakened,
cracked granite.
In less than 5 million years,
it cuts a 3,000-foot, v-shaped
canyon running straight through
the heart of Yosemite.
Scientists investigating how
Yosemite's deep canyon formed
have discovered...
strange cracks in the rock--
proof that the up-lifting Sierra
Nevada weakened the granite...
and rapids downstream--evidence
that the Merced River cut a
deep, V-shaped Yosemite canyon.
But one part of the riddle
remained unsolved...
Yosemite's unique, box-like
shape.
Some other extraordinary force
had caused the canyon walls to
collapse and the valley floor
to flatten.
The controversy about how this
strange canyon formed raged on.
>> Yosemite has had a turbulent
past. 200 million years ago, its
coastal plain is shattered by
volcanoes. And then, 7 million
years ago, the Merced River
suddenly carves a deep,
V-shaped Yosemite valley.
For the next 4 million years,
the river runs in this narrow
canyon. But then, Yosemite is
radically transformed again.
>> About two and a half million
years ago, when our ancestors
Were beginning to evolve in
Africa, the v-shaped valley that
was here began to evolve into
the present valley floor, which
Is flat, and is bounded by these
vertical, sheer rock walls
on its side.
>> It was during this second
extraordinary stage of cliff
formation that the most iconic
rock in America formed--
Half Dome.
This natural wonder's unique
shape has intrigued people for
centuries.
Its enormous granite dome is
over 3 billion cubic feet in
size, as large as 1,000 football
stadiums, and its striking
northwest face is a 2,000-foot
vertical drop.
In 1870, amateur geologist John
Muir had a radical theory about
how Half Dome and Yosemite's
other vast cliffs formed.
He believed that thousands of
years ago, the earth's
temperature had plummeted and
the valley had been filled with
ice.
>> You know, one thing that's
important to keep in mind about
John Muir, he didn't come up
with this idea overnight. He was
trekking, hiking, walking, going
up to the tops of mountains. And
the entire time he was doing
this, he was listening, he was
seeing, he was feeling, he was
touching. And so he was slowly,
letter by letter, word by word,
learning the language of
Yosemite.
>> Muir knew from research in
the european Alps that glaciers
were capable of gouging out
solid rock.
As these great rivers of ice
flow downhill, they press down
on the canyon walls with a
weight equal to 200 trucks per
square yard, ripping out chunks
of rock and causing the valley
walls to get steeper.
>> In general, glaciers
transform a v-shaped valley that
we see as characteristic of a
river into a u-shaped valley
by focusing their erosion on the
valley walls, rather than on the
valley bottom, until it has
reached this "u" shape, at which
point it can then continue to
erode as that "u" shape.
>> Muir knew that as a glacier
flows, it grinds and polishes
the bedrock with rough shards
of rock lodged at its base.
As they melt and disappear,
glaciers leave behind u-shaped
valleys and distinctiv
scratch marks.
Muir proposed that giant rivers
of ice also once flowed through
Yosemite valley.
He hunted high and low for
evidence of glacial scratch
marks and eventually found a
small, 20-foot square patch of
rock 30 feet from the valley
floor.
>> The granite has scratches on
its surface. Each one of these
scratches is associated with
a rock embedded in the sole
of a glacier. I can imagine Muir
claiming that this is
irrefutable evidence of glacial
occupation of the valley floor.
>> But if glaciers ploughed down
the valley, why weren't there
millions of scratch marks on
the cliffs?
If Muir was correct, this is
a picture of what happened.
Two and a half million years
ago, the temperature plummets.
Yosemite valley fills to the
brim with a gigantic glacier.
Only the tallest mountains peek
above the sea of ice.
For thousands of years, the
glacier grinds away at the
granite walls, scratching the
cliffs and undercutting a peak
that will become Half Dome.
When it retreats, the glacier
leaves precarious slabs of
overhanging rock.
The unstable rock face crashes
to the valley floor...
cleaving the great northwest
face of Half Dome.
In the same way, subsequent
smaller glaciers cut away at
the base of the valley for two
million years, cleaving sheer,
vertical cliffs and removing all
traces of glacial scratch marks.
But there was still one
glaringly obvious problem with
Muir's theory, and arch enemy
geologist Josiah Whitney
pounced on it.
If glaciers had carved the
valley, why was Yosemite
uniquely box-shaped and not
U-shaped, like a classic
glacial valley?
Whitney attacked Muir's theory
and stubbornly declared this
square, flat-bottomed valley
could only have formed if the
bottom had fallen out.
>> Whitney hypothesized that
instead of being carved by
glaciers that this was a
fault-bounded valley with the
valley walls being faults down
which the block in between the
valley walls has dropped.
In Whitney's theory, the flat
valley floor would be the top
of the down-dropped block.
>> Iin the quest to unravel the
mystery of Yosemite's
flat-bottomed canyon, scientists
are investigating the valley
floor.
>> The kind of sediment that I
see in this cut bank are coarse
grains, pebbles, and sand--the
same kind of rocks and sediment
that the present stream can
carry. In contrast to that, the
deposit I see at my feet...
is very fine-grained.
>> Light, fine grained sediment
like this is easily carried by
flowing water. But when a river
hits a body of still water, its
energy levels slump, and it
dumps its load.
>> This the kind of sediment
that we would expect to see on
the floor of a lake.
>> Further research has found
similar strange sediments all
over Yosemite.
It's proof that 10,000 years
ago, an ancient lake drowned
this entire valley.
Starting at the head of the
canyon, it stretched for 5 miles
through the landscape.
But it was a mystery how Lake
Yosemite had formed.
Scientists scoured the valley
looking for answers, and near
the base of El Capitan at the
farthest end of the valley,
they stumbled upon an
insignificant-looking ridge.
It's an important clue to how
Lake Yosemite and the valley
formed. This mound is made of an
unusual collection of rocks,
And it runs from one side of
the valley to the other.
>> Given this arc of a ridge is
just down from El Cap, one might
expect that it could have been
caused from a rock fall.
But if you look around...
that's not the case. Clearly,
even in this local area, we have
rocks that came from at least
3 different places, no doubt, up
the valley. This pink rock came
from one portion of the valley,
this gray granite came from
another place. And this
particular one, we can see the
distinctive feldspars associated
with the cathedral peaks
granite.
>> This strange distribution of
rocks means that the ridge could
not have formed from a
catastrophic rock fall.
Something else just as epic
must have created it.
This is a glacial moraine, the
remnants of a glacier which once
filled the valley. It's the
conclusive evidence Muir had
hoped for.
Moraines form as glacial ice
rips fragments of rock from the
valley walls, carries them like
a conveyor belt several miles
downstream, and dumps the rubble
at the mouth of the glacier.
Scientists realized this ridge
was also key to solving the
mystery of how Lake Yosemite and
the flat-bottomed valley floor
formed.
It wasn't just a moraine, it was
a giant dam.
It's two and a half million
years ago, and glaciers grind
through the valley. 10,000 years
ago, the temperature rises and
the glacier retreats.
A huge moraine is dumped at the
valley mouth, damming back the
icy melt water.
The entire valley floods,
creating an enormous Lake
Yosemite.
Mountain streams spill into this
still water and dump ton after
ton of fine sediment.
The lake chokes with silt,
creating Yosemite's distinctive
flat-bottomed valley floor.
>> We know from seismic evidence
collected in the 1930s that it's
2,000 feet thick. That's enough
sediment to cover New York City
by a foot.
>> And beneath this lake of
sediment, they found the
evidence that had always eluded
Muir--a u-shaped basin, the
hallmark of a classic glacial
valley. It was indisputable
evidence that Muir's intuitive
Observations and seemingly
far-fetched theory were
correct, and the final nail in
the coffin for Whitney's theory
of a cataclysmic rifting valley.
Scientists trying to understand
how the steep cliffs and
flat-bottomed valley formed
have found...
scratch marks--evidence that the
sheer cliffs were carved by
glaciers,
and a glacial moraine--proof
that an ancient Lake Yosemite
filled with sediment and
created the flat valley floor.
For over two million years,
the glaciers whittle away at
Yosemite. Then 10,000 years ago,
the glaciers retreat for good,
Leaving the one-mile-wide
Canyon we know today.
But strange, dangerous forces
are still exerting their power
on Yosemite's landscape, with
catastrophic consequences.
>> Yosemite valley has been
sculpted by torrential water and
thick slabs of ice, but today
another hidden force is shaping
this magnificent landscape.
[loud rumbling]
It's 6:52 pm, July 10, 1996.
Yosemite is hit by
a catastrophic rock fall.
Within minutes, a huge dust
cloud of pulverized rock engulfs
the valley. News spreads that
Happy isles, the busiest trail
In the valley, has been
decimated. Yosemite is in
a state of emergency.
>> When I got here, the
devastation was really in full
force. I saw trees that were
just everywhere. There were
ambulances, and chaos just was
everywhere. People were running
around screaming, and the whole
area looked like a bomb had
gone off.
>> 80,000 tons of rock had
suddenly dislodged from the
cliff face.
>> The weight of 1,600 trucks
basically fell down. It hit and
then exploded, and it created
a windblast.
>> As strong as a tornado,
the wind blast uprooted trees a
half-mile away from the impact
zone.
>> We can have about 3,000
people go up and down that trail
in one day, and so we did not
know how many people were
trapped under the trees. And,
tragically, a young man was
pinned by a tree and was killed,
and a young woman was trapped
under a tree and is paralyzed.
>> A deadly, mysterious force is
continually at work in Yosemite,
causing the surface layers of
the ultra-tough Yosemite granite
to peel away in huge chunks.
One large rock fall occurs every
week in the park, and yet up to
4 million people visit Yosemite
each year, getting perilously
close to these potentially
unstable cliffs.
To discover what's causing
Yosemite's incredibly tough
granite to fall down,
researchers have dotted
vibration sensors and
solar-powered seismic stations
all over Yosemite's cliffs,
enabling them to listen to the
rocks.
>> This computer allows me to
see what's going on all the
time.
For example, if I throw this
rock at the cliff over there...
I just created a mini-rock fall,
and take a look. And these
spikes are the rock hitting the
cliff and then tumbling down.
This is a mini rock fall that
happened right here.
So the station will pick up
rock falls that happen that are
small right here, but it'll pick
up larger ones in other places.
>> This seismic listening device
has pinpointed a recent rock
fall behind Half Dome.
It's a chance for Valerie Zimmer
to investigate why Yosemite's
rocks are falling down, and all
the evidence indicates another
cataclysmic event has occurred.
>> Oh, look at this.
All these trees have been
knocked over like they're
toothpicks.
These rocks range from the size
of a house to just dust.
>> Analysis of the seismic data
shows this was a massive rock
fall, one of the biggest in the
last 20 years. It shook the
ground so hard, it was the
equivalent of a magnitude 2.4
earthquake.
>> If you look up on that
mountain, the amount of rocks
that came down, you know, you
look up high, it doesn't look
like a big area, but it's
deceivingly large. It's probably
the size of maybe 40 houses
all together.
>> Strangely, immediately above
the scarred rock face, a new
dome is forming.
Mysterious domes are forming all
over the valley, the most
famous--Half Dome.
Glaciers cleaved its vertical
face, but its domed top has been
shrouded in mystery.
A strange underlying force is
sculpting these peaks into
domes, causing the rocks to
tumble down.
North of Yosemite, on the dome
at Olmstead Point, it's possible
to get close enough to
investigate why the surface
layers of granite are
shattering.
>> These domes are all over the
place. You see these layers
running parallel to the surface.
They're almost like the layers
on an onion, and the layers are
starting to peel off. In fact,
look right here. This one is
coming apart.
>> The outer surface of the rock
has fractured into layers,
running over the summit and down
the sides of the dome.
>> These are weaknesses in the
rock. It's obvious that there
must be some sort of force
that's causing the rock to break
in this way and peel apart
like an onion.
>> The only force capable of
fracturing the ultra-tough
Yosemite granite like this
originates deep within the
Earth.
100 million years ago, the
Yosemite granite is beginning
to form. Submerged beneath two
miles of volcanic rock, it's
squashed and squeezed from every
direction.
Over the next 40 million years,
the volcanic roof erodes away.
The immense downward pressure is
removed, leaving these titanic
forces out of balance.
The surface of the exposed
Yosemite granite is now an
avenue through which this
pent-up pressure can be
released.
>> The granite is still being
squeezed from the sides and from
the bottom, but at the surface
it's free to move.
What happens is these layers
start to open up parallel to
the surface.
>> The release of this ancient
pressure causes the surface of
the granite to fracture into
onion layers, which then peel
away.
>> And so, when the tops of
those mountains fall off, you're
left with rounded domes.
>> But it's not just the granite
peaks that are affected.
It's the cliffs, too.
>> These layers are also formed
if the ground surface is
vertical like a cliff, creating
vertical fractures alongside the
cliff. These vertical fractures
allow the rock to then slide out
and create rock falls, and this
is one of the major contributing
factors to rock falls
in Yosemite.
>> Yosemite's catastrophic rock
falls are the legacy of its
prehistoric birth beneath the
Earth, 100 million years ago.
The gradual release of this
ancient, pent-up pressure has
created an untamed and dynamic
landscape.
>> Rock fall is a natural
process and is part of the
ongoing evolution of Yosemite
National Park. I always get
a little bit tickled when people
ask me questions like, "what are
You going to do to prevent
future rock falls?"
It's a constant reminder that
this is a wild place and that
there's nothing we can do or
should do to tame it in any way,
shape, or form. But it's
something that, after I'm long
gone, will continue to happen.
>> For the last 150 years,
scientists have been figuring
out how this magnificent valley
formed.
They found...
volcanic rocks--proof that the
molten granite cooled deep
beneath the earth;
karge crystals--evidence that
Yosemite granite cooled slowly,
creating tough, faultless rock;
Rapids downstream show that
the Merced River cut a deep,
V-shaped canyon;
And rocks carried by a glacier--
conclusive evidence that ice
carved Yosemite's steep cliffs
and flattened its valley floor.
Yosemite, a valley of giants,
is a geological masterpiece.
The unique strength of its
near-perfect granite has created
some of the most imposing and
iconic landscapes on planet
Earth, one where the deep earth
forces that created it are
continuing to shape its future,
Living proof that the earth is
never at rest.
-- Sync, corrected by elderman -- -- for addic7ed.com ---