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>> 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 MY-SUBS.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 MY-SUBS.com ---