Original subtitles

>> 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.

One of these mysteries

is centered here

in the Grand Canyon in Arizona.

Close to a billion tons of rock

have been carved out

of the ground.

The canyon left behind

could hold all the river water

on Earth and still be less

than half full.

For more than a century,

scientists have debated

how and when this vast chasm

was created.

And now geologists are

uncovering fresh evidence

of how the Grand Canyon fits

into the ever-evolving story

of "How the Earth Was Made." S02x01 Grand Canyon Original Air Date on November 24, 2009

-- Sync, corrected by elderman -- -- for www.MY-SUBS.com --

The Grand Canyon...

one of America's most

spectacular natural wonders...

A canyon 18 miles across

at its widest point...

277 miles long...

and more than a mile deep.

It is so vast that it can even

be seen from space.

Although Hell's Canyon in Idaho

is almost half a mile deeper,

and Australia's Capertee Valley

is nearly a mile wider,

the Grand Canyon remains

the most famous of them all.

And it also holds one of

geology's greatest mysteries--

Just how did the Colorado River,

only a tenth the size

of the Mississippi,

form such a large canyon?

The answer has eluded scientists

for more than a century,

because many of the clues

they normally rely on

have been swept away

by the river's water

over millions of years

or buried by landslides

or destroyed by volcanoes.

>> It seems like we should

understand perfectly

how the Grand Canyon formed.

The problem is, we've lost

a tremendous amount of evidence.

It's like a murder mystery where

most of the evidence is lost.

And so the best we can do

is piece together the evidence

that we have.

>> Even so, slowly but surely,

this geological icon is giving

up its most ancient secrets.

The canyon's richly colored

layers offer scientists

one of the most complete

geological records on Earth.

>> The first concept you have to

get your mind around as you're

thinking about the Grand Canyon

is that the stories told by

the rocks are exceedingly old,

millions and billions of years.

>> Karlstrom and his team

are setting out on a grueling

geology field trip

along the Colorado River.

It won't be an easy ride,

because this 1,450-mile-long

river packs a punch.

More than 800 million gallons

of water can flow down

the Colorado every hour...

more water every second

than the average U.S. household

uses in a year.

Karlstrom is investigating

the ancient history of the land

that was here before

the Grand Canyon even existed.

And for that, he needs

to identify its oldest rocks.

He is following in the footsteps

of pioneer explorer

John Wesley Powell.

In 1869, he was the first man to

successfully ride the Colorado

through the entire length

of the canyon.

>> All of us who work

in the canyon as scientists

admire John Wesley Powell

immensely for his pioneer

and scientific exploration

of the Grand Canyon,

And the questions that he framed

are still questions

that we work on today.

>> One of Powell's discoveries

was these intimidating black

rocks at the very base

of the canyon.

>> Well, we're deep in the Grand

Canyon, right by the Colorado

River. You can see these

spectacular black rocks.

Actually John Wesley Powell

called them ugly black rocks

because for him, these hard

rocks made bad rapids,

and that was harder on his trip.

But for those of us who are

interested in the early history

of Grand Canyon, these rocks

are the bonanza.

>> Powell had no way of dating

these rocks, now identified

as vishnu schist.

All he could conclude

from their appearance was that

they had once been molten

deep underground, but Karlstrom

has an advantage--

modern instruments that can

accurately date the rocks

by measuring radioactive decay.

And the first step

in figuring out what happened

here in the ancient past

is to record when these rocks

were created.

>> These rocks are about

1.7 billion years old.

It's less than half

of the age of the Earth.

So we have a great story here

in the Grand Canyon of the last

almost 2 billion years

of Earth's history.

>> But Karlstrom needs

more information,

and these ugly black rocks

hold another crucial clue

to what this land looked like

before the canyon was cut.

They can tell him not only

when they were formed,

but also precisely how deep

in the Earth's crust

they were made.

These tiny stones embedded

throughout the ancient boulders

are literally jewels,

garnets that only form

under immense pressure,

the sort of pressure that's

found when layers are crushed

by the weight of millions

of tons of rock on top of them.

>> The silver-bullet clue

is the garnet.

These garnets are the key

to understanding the amount

of rock above us.

>> By analyzing the chemical

structure of the garnet,

in particular its calcium

content, investigators can

determine how much weight of

rock was crushing down upon it

at the moment it was made.

>> In simple terms,

if you analyze the garnet

and you see higher calcium

content of the garnet,

it means you're deeper

into a mountain belt,

more rocks above you.

So we take these garnets

back to the laboratory.

We cut a very thin section.

We put them under

an electron microprobe,

and the scientific result

after this analysis is that

we were 6 miles deep beneath

the surface of the peaks

which were above us,

and that's a long ways. Ha ha.

>> So, nearly 2 billion years

ago, before the canyon evolved,

ancient mountains 6 miles

above sea level stood here,

towering peaks as high

as the modern Himalayas.

Over the next 500 million years,

these mountains were worn away

by the relentless forces

of erosion. Over millennia,

the freezing and thawing of ice

cracked open the rock

of the mountain slopes.

Wind and water carried the rock

debris down towards the oceans,

leaving behind a flat

and featureless plain

with no sign at all of a canyon.

>> Geologists learn to visualize

the way that this place looked

in the past. Knowing how to read

the texture of the rock,

the kind of rock it is,

the fossils that are in it,

geologists can--

It's like a detective story.

You can uncover what

this place looked like

billions of years ago.

>> This is now desert country,

more than 300 miles inland.

And yet these shells

encased in solid rocks...

are ocean fossils.

>> In this one cliff,

you can find fossil shells

that look like you'd pick up

on the seashore today.

They die, they fall to

the bottom of the sea floor,

and they get trapped

and die in the mud

at the bottom of the ocean

at the time that they're

being deposited.

>> Shells like these come

from shallow, tropical waters,

an inland sea that first arrived

here half a billion years ago

and covered the flat,

low-lying plain.

But that did not happen

just once. Many different layers

in the walls of the Grand Canyon

tell Karlstrom that over

hundreds of millions of years,

this land has been submerged

by the sea not just once,

but at least 8 times.

The last time this part

of Arizona was under the sea

was around 80 million years ago.

>> As we go higher in the layers

in the Grand Canyon, we have

different-aged seas, which are

depositing different kinds of

rocks, different environments,

different fossils that live

at the different times.

This chapter of seas coming in

and seas going out is itself

hundreds of millions of years.

>> Each sea deposited

different types of material that

hardened to become solid rock.

Some sediment was sand that

gecame buff-colored sandstone,

some was mud that hardened

into darker shale,

while the calcified remains

of marine organisms were crushed

into light-colored limestone.

And yet the dominant color

is red.

That comes from iron

locked within all the rocks.

Over millions of years,

the iron rusts into

a distinctive red hue.

For the geology detectives,

descending into the Grand Canyon

is like traveling back itime.

The calcium content inside

garnet gemstones reveals

that nearly 2 billion years ago,

mountains the size

pf Mt. Everest stood

where the Grand Canyon is now.

Sea fossils exposed

in the cliffs show that as late

as 500 million years ago,

the land was the muddy bottom

of an ancient inland sea.

The next puzzle for geologists

is uncovering which awesome

forces transformed that

unremarkable flat land

into this breathtaking

natural wonder of the world.

>> From 1.7 billion years ago

to 70 million years ago,

the landscape of western Arizona

has undergone a series

of extraordinary changes.

Ancient mountains have given way

to prehistoric seas

which have withdrawn to reveal

a low-lying flat plain

stretching as far

as the eye can see.

The magnificent gorge

of the Grand Canyon

does not yet exist

but over the next 20 million

years, this landscape was

to undergo immense changes

that would create a unique

high plateau and set the scene

for the formation of the canyon.

Ancient fossils of sea animals

tell geologists that this land

was once under the waves

of an inland ocean, but that

leads to yet another mystery.

The investigation needs

to figure out why these undersea

rocks are now high in the air,

thousands of feet

above sea level.

>> It's surprising to go up

a mile above sea level,

and you find a clam shell

or what looks like a clam shell,

and you say, "that's what I see

"when I go down to the ocean."

"so why is it here,

a mile above sea level?"

>> It's clear that this region

underwent a type of geological

disturbance that pushed up

the entire seabed.

Geologists discovered

in the 1960's that collisions

between separate plates

of the Earth's crust could

force land up into the air.

It happens all over the globe

and usually deforms the land

into tilted mountain ranges.

But this Arizona uplift

was unique.

>> After all the flat layers

are deposited at sea level,

there was a major uplift event

called the Laramide Orogeny,

which lifted these rocks

without tilting them,

still flat, lifted them up

to high elevation.

>> Because the land rose

straight up, like being

in an elevator, it formed

a high, smooth plateau.

The sea that had been there

drained back toward

the northeast, but as of yet,

there was no Grand Canyon.

The Colorado River, the force

that cut the canyon from

the rock, had yet to arrive.

>> Geologists, from the very

early days, from the late 1800's

are quite comfortable

with the knowledge

that the Colorado River

has carved the Grand Canyon.

[thunder]

>> The high plateau was

surrounded by even higher

mountain ranges.

New rivers began flowing

from the mountains

out across the plateau.

It's essential for

the investigation to establish

when the Colorado River,

in particular, arrived,

because only then could it

begin to carve the canyon.

Until a few decades ago,

some investigators thought

this ancient riverbed

called Hindu Canyon

provided the answer.

They believed that Hindu

Canyon's creation 50 million

years ago marked the arrival

of the Colorado River

and the beginnings

of the Grand Canyon.

But in 1969, the discovery

of these pebbles turned

everything that geologists

thought they knew about

the canyon on its head.

>> It turns out that to explain

how the Grand Canyon got there

is very much more complex

than people thought.

So the early geologists

thought it was simple,

but now we realize there's

a lot more to the story, and

it's kind of a detective story.

You start out with a few clues,

and you put the clues together,

and then finally you get

the satisfaction of saying,

"well, you know, I figured this

out before anybody else did."

>> Figuring it out

before anyone else was just

what Young did in 1969,

when he was a 24-year-old

geology graduate student

at Washington University.

His professor sent him

to investigate Hindu Canyon.

But when Toung arrived

at the dusty riverbed,

he discovered that it had

nothing to do with the Colorado

or the Grand Canyon itself.

His discovery flew in the face

of all the established

geological theories

and revolutionized thinking

about the canyon's history.

The evidence Young had uncovered

was the alignment of pebbles

in the bed of the river.

>> Look at these pebbles.

You can see that the pebbles

are flowing, or the pebbles

are arrayed in this direction,

which is a stable direction

for water flowing to my right.

If the pebbles had been

oriented this way, the water

would have flipped them over.

So when we find pebbles

that are oriented this way,

that tells us that the water

was flowing to my right.

>> It is a crucial clue.

The Colorado River could never

have flowed to Young's right.

It has always run

in the opposite direction,

towards the Pacific Ocean.

The river here, 50 million

years ago, was not the Colorado,

and it did not cut

the Grand Canyon.

Young's findings meant

scientists had to rethink

all their ideas about when

the Colorado had arrived

on the plateau and about

the age of the canyon.

They started examining evidence

from another less-ancient site.

This is Muddy Creek,

near Lake Mead, Arizona,

just a few miles downstream

from where the Colorado River

exits the Grand Canyon today.

The underlying rocks prove

that this was once the site

of a vast freshwater lake.

>> The upper part of the Muddy

Creek formation is this nice

limestone, which formed

in a freshwater lake.

The water would have been

very clean. There would have

been lots of plants and animals

living in the water.

And as they sank to the bottom,

the calcium carbonate

in their shells would form

this limestone, which is

typically what forms

the limestone rock.

>> The limestone

is the calcified remains

of the creatures that once

lived in this lake.

Then 5.5 million years ago,

the animals all disappeared.

There were no shells

to make fresh limestone.

The only explanation

is that the animals died

5.5 million years ago

because that was the date

when the Colorado River

arrived here.

The river would have been

carrying masses of dirt

and rock sediment

from the fledgling Grand Canyon.

>> The water would have been

too muddy and dirty,

and limestone does not form

in dirty, silty, muddy water.

It's just incompatible.

The animals and plants

that live in such a lake

can't exist if there's a lot

of silt and mud in the water.

>> So the muddy death

of the lake gave geologists

a confirmed date for when

the Colorado arrived in Arizona

and commenced its excavations.

The Grand Canyon was born

a mere 5.5 million years ago.

The investigation has reached

a significant milestone.

It has discovered the age

of the canyon.

The angle at which pebbles lie

in ancient riverbeds reveal

that the Grand Canyon

is far younger than geologists

had previously ever suspected.

The limestone discovered

at Muddy Creek reveals the date

that the Colorado River

arrived on the plateau

and the true age of the canyon--

5.5 million years old.

Now geologists had

a new mystery to solve--

discovering why the Colorado

River took the path it did

some 5.5 million years ago

and why it carved a canyon

of such remarkable dimensions.

>> The investigation into

the history of the Grand Canyon

has uncovered a 1.7 billion

year old landscape that has

evolved from ancient mountains

through to the Colorado plateau.

5.5 million years ago,

the Colorado River began

carving out the Grand Canyon

from this plateau.

The question scientists

now had to answer was

what happened at that time

yo cause the river to dig deep

and carve out the canyon.

It is a debate that has been

going on for more than a century

and which continues today.

>> I got interested

in the Grand Canyon

when I went to school

and started studying,

and I heard about

the different ideas associated

with the Grand Canyon and was

just blown away when I found out

we did not understand

how the Grand Canyon formed.

That something as iconic

as the Grand Canyon wasn't

understood just seemed crazy

to me, and so I basically

decided to dedicate

a large portion of my life

to trying to figure it out.

>> One theory is that several

ancient rivers merged,

and their combined cutting power

started digging out the canyon.

Another assumes that the river

cut down into the plateau

as the land uplifted around it.

But John Douglass has

his own theory, one that has

gained respect among many

leading geologists since

he first published his ideas

in 2000. What Douglass calls

his "spillover theory"

seems to work well on paper.

>> Spillover is incredibly easy.

All it means is the Colorado

River poured into a basin.

When it poured into that basin,

it had to form a lake,

and this lake was huge.

All that lake had to do was rise

and spill across the plateau.

It poured down, cutting rapidly,

and over time, you would have

ended up the beginnings

of Grand Canyon very quickly.

>> At his college campus

in Phoenix, Douglass is building

a scale model experiment to see

if his spillover theory

actually works in real life.

He sculpts tons of dirt into

a model of the Colorado plateau.

Running faucets represent

the flow of the Colorado River

into the ancient lake.

>> Now we have our large lake.

The water's getting higher.

It's getting ready to spill

across. We have a tiny little

trickle of water pouring down

off the lake.

That little tiny trickle

of water doesn't seem like much,

but over time, that little bit

of water flowing down that steep

slope is going to gain energy,

it's going to start cutting,

making waterfalls that work

their way back. One waterfall

has now reached the lake.

You can see that we have

just released a significant

amount of water, much more water

than was previously

pouring down.

Now we have huge canyon-cutting.

Landslides are sloughing off

the side of the canyon walls

into the water, flushing it

downstream. The lake,

you can see that it's starting

to shrink in size.

That lake is getting lower.

And right there, you can see

that we have cut our own

small-scale version

of the Grand Canyon.

>> Douglass' experiment proves

that the spillover theory works

in miniature, but he needs

evidence to show that it could

have happened on an infinitely

bigger scale.

Douglass sets out in search

of a lake large enough

and old enough to be the source

of his spillover flood.

He has a prime suspect in mind.

This is the site of the ancient

Lake Bidahochi, 100 miles

to the east of the Grand Canyon.

And a clue here on the old

lakebed reveals how deep

this lake once was.

>> These green clays,

which indicate deep lake water,

this is the classic evidence

for the giant lake necessary

for the overflow explanation

of Grand Canyon.

>> These green deposits

are only created

in one specific environment.

>> To have green lake clays,

you need deeper water,

where there is little oxidation.

I think that's an indication

that the Colorado River

has arrived in this basin,

that it's made its way

from the Rocky Mountains

to this location, and this is

its basically stopover point

before it eventually spills

across to form the Grand Canyon.

>> Establishing the depth

that Lake Bidahochi once spread

over 20,000 square miles

and contained more than 3,000

cubic miles of water.

That makes it bigger

than Lake Michigan.

Douglas needs to date

the age of this lake.

For his spillover theory

to work, it has to be older

than the Grand Canyon,

more than 5.5 million years.

He unearths the proof he needs

in these deep-water fossils.

>> Ok. These fossil shells

are freshwater mollusks

maybe as young as 6 million

years old.

>> The dates match up.

The lake was here at the right

time to have spilled over

and begun cutting

the Grand Canyon.

Dating the fossils helped

confirm Douglass' belief.

But his search for more

evidence continues.

>> In reality, we're never

going to know how it formed

to 100% certainty, unless

someone builds a time machine.

By doing this kind of work,

all you're trying to do

is increase your level

of confidence on your ideas,

build up your case,

build up your evidence.

>> Building up his evidence

is exactly what Joel Pederson

is doing. He is using

the very latest technology

to prove exactly how fast

the Grand Canyon was carved.

He has found the evidence

he needs right here

at the very start

of the Grand Canyon.

>> Here at Lee's Ferry,

there are all of these gravels

that are evidence of where

the river has been in the past

and the path it has taken

during incision,

and amongst the gravels,

sometimes you see

these great lenses of sand,

and we can use the sand

to get an absolute date

on these deposits.

>> As the Colorado River

carved out the canyon,

it deposited more and more

of the gravel and sand debris

at this spot. Newer layers

buried older layers

over millions of years.

And geologists now have

instruments that can measure how

light has affected individual

atoms within the sand.

That reveals precisely

when each sand layer

was originally buried,

away from the light of the sun.

For the technique to be

accurate, it is essential

that the sample is not exposed

to daylight.

>> So here we can take

a metal tube, and we can

hammer it into the sand outcrop,

and in the metal tube then,

we'll get a sample of the sand,

and it'll stay out of sunlight,

and then we take it back

to a darkroom laboratory

and remove the sand,

still sheltered from light,

and then we can analyze

the optical properties of it

to get an absolute age.

In this case, the absolute age

would tell us when the river

was at this point

in the landscape.

>> Back at the lab, Pederson

compares multiple sand samples,

each from a different depth

in the canyon deposits.

Discovering the age of each

individual layer of sand

lets him estimate how rapidly

the river has been cutting

down through the rocks.

>> Here at Lee's Ferry,

we can use this last

half-a-million-year history

along with our absolute dates

and all the information we get,

and the rate of canyon-cutting

here is about 1,000 feet

per million years.

>> That's one foot

in every thousand years,

a little more than one inch

every century. It proves

that the entire 5,300-foot-deep

Grand Canyon could have

been cut in a little more

than 5 million years.

In geological terms,

the mere blink of an eye.

The investigation is assembling

evidence on how the Grand Canyon

was created.

Green clay deposits support

the theory that an ancient lake

was big enough to spill over and

trigger the canyon's creation.

Deep-water fossils prove

that the lake existed

6 million years ago, the right

time to have overflowed

and cut the Grand Canyon.

But proving how

the canyon-cutting began is

only part of the investigation.

This is one of the widest

and deepest canyons

in the entire world,

and to discover how it grew

so large, geologists will

have to examine some of the most

dramatic and dangerous features

the canyon has to offer.

>> The landscape of western

Arizona has transformed

from ancient mountains

to prehistoric seas

to a flat uplifted plain.

Just 5.5 million years ago,

the Grand Canyon was cut

through this plateau.

It happened so fast that

geologists had to think again

about the awesome power

of the Colorado River.

>> It cuts through rock not by

yhe water wearing it away.

You could pour water over rock

for a long time, and nothing

would happen. It's the tools

that the river carries.

The river carries boulders

and sand, and those bump

against each other,

and they eat away at the rock.

>> Every day, the Colorado

can carry almost 500,000 tons

of rock and debris,

enough material to fill more

than 100 olympic swimming pools.

That is 5 tons every second.

So investigating river erosion

is never an easy task,

because the powerful flow

of the Colorado River

has scoured and washed away

many of the clues that

the rock detectives need.

They looked for evidence

in the hundreds of rapids that

disrupt the river's progress.

The swirling rapids are created

when flash floods sweep boulders

into the river from the many

smaller side canyons.

> The river has to focus

a lot of energy at these points

to deal with all of the boulders

that are coming in it.

The more coarse boulders

and more resistant material that

the river has to fight against

to accomplish its incision,

the steeper it gets.

>> As the water flows

over the rapids, it cuts deep

into the bedrock below.

At this set of rapids alone,

yhe river drops 10 feet,

and there are a lot of rapids

on this section of the Colorado.

>> So put all these rapids

together in a string

through Grand Canyon,

and that gives you the overall

sort of unusually steep

Grand Canyon profile

of the river going through it.

>> Gravity and simple physics

are at the heart of how

the Colorado has carved

so much rock so quickly.

5.5 million years ago,

the Colorado River was flowing

over the steep edge

of the plateau that had been

pushed up thousands of feet

above sea level.

The river ran rapidly

over the steep downward slope

and swept rough, rocky debris

along in its wake.

An incision formed, digging back

into the edge of the plateau.

>> The power of the river

to incise as it dropped off

that huge escarpment must have

been really great.

And so the river quickly incised

there, and that very steep

drop-off of incision would have

worked its way back upstream

through the Grand Canyon region.

It's sort of a waterfall that,

a few million years later,

has spread itself out

through the length of the river.

>> It is this steep drop-off

between the Colorado plateau

and the land beneath it

that fuels the Colorado's

incredible erosive power.

The river begins its life

high in the Rocky Mountains

in Colorado. For every mile

that it travels, the river

falls 10 feet. By contrast,

the Mississippi River, a river

moving 10 times as much water

as the Colorado, meanders

across a flat landscape.

With no steep slope to drive it,

the Mississippi can't carve

any canyons.

But the Grand Canyon

is not just steep.

It's also wide.

And here on the south rim,

at the heart of the National

Park, the true majesty

of the Grand Canyon is revealed.

This is where the canyon

is at its widest, as much

as 18 miles from rim to rim.

This landscape appears serene

now, but its unique beauty

was forged by violent forces.

>> Grand Canyon, oftentimes

we just associate it

with the Colorado River.

The Colorado River is what cut

the Grand Canyon. It formed

the initial gash to allow

the river to flow across.

But what makes Grand Canyon

grand is really its width.

And all the layers of rock

that are exposed, and that

isn't only solely tied

to the Colorado River.

What's happening is this rock

that's exposed, it's being

beaten on by rain, and the rain

gets in there, and it weathers

the rock, and it weakens it.

And then because this is

so incredibly steep, gravity

will act on that material,

transporting it deeper

down into the river,

flushing it back out.

And that process just repeats

over and over again

to allow the canyon

yo get wider over time.

We have classic rockfalls

that are cascading down

onto the black rock

in the far distance.

Those events are indications

that this is actively ongoing

canyon-widening and retreating

from these processes.

>> The fall of these rocks

is not a gradual process.

This is erosion

at its most violent.

>> Very few people are ever

going to see Grand canyon

actually change. I've spent

numerous nights in Grand Canyon.

I've only heard one or two rocks

ever fall, but change will

happen, and when it does happen,

it happens very rapidly.

>> The rocks fall because both

harder and softer rocks

are layered, one on top of

the other, in the canyon walls.

The harder layers are made

of limestone and sandstone.

These rocks don't weather

easily, but the softer shale

beneath is made of mud that

expands when it rains, causing

the shale to crumble away.

>> Those weaker rocks,

yhey weather and retreat back,

and they undermine

the resistant cliff rocks above

that will then fail as dramatic

rockfall, landslide events,

allowing the canyon

to increase its width.

>> The rockfalls are merely

the first step towards

increasing the Grand Canyon's

enormous width. Without the help

of an accomplice,

the entire canyon would

fill up with debris.

>> Without the Colorado River,

you could not have the Grand

Canyon as wide as it is.

By flushing that material

downstream, it like...

it wipes it all clean

to allow a whole new material

to build up again.

And once you repeat that

over and over again,

it allows the canyon walls

to retreat back, and the entire

canyon just grows, as these

guys continue to march and push

and move all that material

downstream.

>> The mystery of how the Grand

Canyon grew so deep and so wide

is being solved.

The Colorado rapids demonstrate

how the steepness of the

riverbed helps carve

the canyon so quickly.

Rockfalls on the canyon walls

reveal how weaker rocks

rapidly widen the canyon

across the plains of Arizona.

But this is far from the end

of the Grand Canyon's story.

In just the last million years,

the canyon has been transformed

by other overwhelmingly powerful

natural forces.

>> Geologists have established

that over 1.7 billion years,

the Grand Canyon emerged

from ancient mountains

and prehistoric seas to become

one of North America's

geological icons.

This is a rare look at one

of the most remote and secret

parts of the Grand Canyon.

A series of small cone-shaped

mountains line the canyon's

edge.

And there are flows

of black rock running down

from each rim.

They come from a remarkable era

just 725,000 years ago,

when the peace of the canyon

was shattered...

[rumbling]

by volcanoes.

This is Toroweap Point,

in a remote area known

as the Arizona strip...

one of the most isolated places

in the continental U.S.

Few people, other than

geologists, ever see this area,

although it boasts some of the

canyon's most stunning views.

The rock detectives come to see

how explosive volcanic

eruptions have changed

the canyon in the comparatively

recent geological past.

This black rock that seems

to have spilled over the rim

of the canyon is an ancient

lava flow, what was once

boiling-hot rock,

Forever frozen in time.

>> Powell talked about a river

of molten magma pouring down

into a river of melted snow.

And he talked about

how dramatic it must have been,

the boiling and seething

and the steam, and...

it must have been amazing.

You would picture red-hot lava

like you would see in Hawaii

pouring down the canyon walls

and coating them, and then

once it reached the river,

it would... you know, it would

immediately create just giant

clouds of steam.

>> The extensive lava flows

erupting from as many as 100

cinder-cone volcanoes

had a dramatic effect

on the Colorado River

running below.

Crow believes that on at least

8 occasions, the volcanic

eruptions created huge

lava dams that blocked

the river completely.

>> Well, behind me here

is one of many basalt remnants.

They're the remains of lava

flows that poured down the

canyon, partially filling it.

And then subsequently,

the Colorado River has removed

all but a few little chunks.

>> The lava dams brought

even the powerful Colorado River

to a halt...

For a while.

In time, the dams were no match

for the Colorado.

The rising pressure

of the dammed river behind them

eventually became too much,

and they shattered.

This explosive episode

has left its mark

on the canyon's walls.

Today, the cones appear

to be extinct and lifeless,

although some geologists believe

that the volcanoes might not

be finished quite yet.

>> The last eruption that sent

lava pouring into Grand Canyon

probably occurred about

100,000 years ago. There is

evidence for an eruption

on the rim that didn't actually

make it into Grand Canyon

that's 1,000 years old.

So there's... there's, you know,

I think, a good chance

that in the future, there may

be eruptions here as well.

>> The Grand Canyon's future

has yet to be written,

but investigators now understand

the story of its past.

The calcium in the garnet

discovered at the base

of the canyon reveals

the ancient beginnings

of this landscape--

an immense mountain range.

Limestone rocks show

that the canyon was only formed

5.5 million years ago.

Green clays that can only form

in deep water prove that

a huge lake, bigger than

Lake Michigan, could have been

the trigger for this

canyon-carving.

And rockfalls from

the crumbling cliff faces

of the canyon rim are evidence

of how the canyon grew

to the shape it is today.

Geologists have been studying

the canyon since the mid-1800's.

Yet even after more than

a century of investigation,

the story is still

far from finished.

>> The landscape is evolving,

and it's going to be changing

through the geological future.

And so the story about

the geology and the fascinating

questions here is not one

that's over, and it's going to

continue to evolve as scientists

continue to do work here.

>> The dynamic geological

phenomenon of the Grand Canyon

is a place where the vast

fiery forces within the Earth's

crust do battle with

the inexorable power of water.

The result--a natural wonder

whose walls record

nearly 2 billion years

of our planet's turblent

geological history.

-- Sync, corrected by elderman -- -- for www.MY-SUBS.com --