The Formation of the Grand Canyon | How the Earth Was Made (S2, E1) | Full Episode | History

[music playing]

NARRATOR: Earth, a unique planet, restless and dynamic.

Continents shift and clash.

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.

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 Hells 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 geologist 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.

JOHN DOUGLASS: 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.

NARRATOR: 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.

NARRATOR: 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 US 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.

KARL KARLSTROM: All of us who work in the canyon

as scientists admire John Wesley Powell immensely

for his pioneer in scientific exploration of the Grand

Canyon.

And the questions that he framed are still questions

that we work on today.

NARRATOR: One of Powell's discoveries

was these intimidating black rocks

at the very base of the canyon.

KARL KARLSTROM: 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.

NARRATOR: 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.

KARL KARLSTROM: 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 two billion years of Earth's history.

NARRATOR: 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.

KARL KARLSTROM: The silver bullet clue is the garnet.

These garnets are the key to understanding

the amount of rock above us.

NARRATOR: 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 micro probe, and the scientific result

after this analysis is that we were six miles deep

beneath the surface of the peaks, which were above us.

And that's a long ways.

NARRATOR: 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.

KARL KARLSTROM: Geologists learned

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.

NARRATOR: This is now desert country, more than 300 miles

inland.

And yet, these shells encased in solid rocks are ocean fossils.

KARL KARLSTROM: In this one cliff,

you can find fossil shells that look like you 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.

NARRATOR: 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 eight times.

The last time this part of Arizona was under the sea

was around 80 million years ago.

KARL KARLSTROM: As we go higher in layers in the Grand Canyon,

we have different age seas which are depositing different kinds

of rocks, different environments, different fossils

that live at different times.

And this chapter of seas coming in and seas going out

is itself hundreds of millions of years.

NARRATOR: Each sea deposited different types of material

that hardened to become solid rock.

Some sediment was sand that became 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 in time.

The calcium content inside garnet gemstones

reveals that nearly 2 billion years ago mountains

the size of Mount 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 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.

KARL KARLSTROM: It's surprising to go up a mile above sea level

and you find a clam shell or what looks like a climb

clamshell.

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?

NARRATOR: It's clear that this region underwent

a type of geological disturbance that pushed up

the entire seabed.

Geologists discovered in the 1960s

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.

KARL KARLSTROM: After all the flat layers

are deposited in that 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.

NARRATOR: Because the land rows 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.

KARL KARLSTROM: Geologists from the very early days,

from the late 1800s, are quite comfortable with the knowledge

that the Colorado river has carved the Grand Canyon.

NARRATOR: 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, I figured this out before anybody else did.

NARRATOR: 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 professors sent him to investigate Hindu Canyon.

But when young 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.

RICHARD YOUNG: If you look at these pebbles,

you can see that the pebbles are flowing--

or the pebbles are oriented 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.

NARRATOR: 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.

RICHARD YOUNG: 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 a limestone rock.

NARRATOR: 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.

RICHARD YOUNG: The water would've

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.

NARRATOR: 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 angles at which pebbles lie in ancient riverbeds

revealed 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 have 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 to 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.

JOHN DOUGLASS: I got interested in the Grand Canyon

when I went to school and started studying.

And I heard about the different ideas

especially the Grand Canyon and was just blown

away when I found out we did not understand how the Grand

Canyon formed.

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 my life to trying

to figure it out.

NARRATOR: 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 Douglas calls his spillover theory

seems to work well on paper.

Spillover's 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 is rise and then

spill across the plateau.

It poured down, cutting rapidly, and over time you

would've ended up with the beginnings of Grand Canyon

very quickly.

NARRATOR: At his college campus in Phoenix,

Douglass is building 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.

JOHN DOUGLASS: 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.

A 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 a huge canyon cutting.

Landslides are sloughing off the side of the canyon walls,

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've

cut our own small scale version of the Grand Canyon.

NARRATOR: Douglass's 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 spill over 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 lake bed

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.

NARRATOR: These green deposits are only created

in one specific environment.

JOHN DOUGLASS: To have green lake clays,

you need deeper water where there is little oxidation.

I think that's 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.

NARRATOR: Establishing the depth that this point lets Douglass

work out how large an area was once covered by water.

He compares the depth of the water with the contour height

lines of the surrounding countryside,

and that shows 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.

Douglass 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.

NARRATOR: 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's belief,

but his search for more evidence continues.

JOHN DOUGLASS: 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.

NARRATOR: 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.

JOEL PEDERSON: 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 as you get an absolute date

on these deposits.

NARRATOR: 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's essential that the sample is not exposed to daylight.

JOEL PEDERSON: 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.

[music playing]

NARRATOR: 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.

JOEL PEDERSON: 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.

NARRATOR: That's one foot in every 1,000 years,

a little more than 1 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 plane.

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.

KARL KARLSTROM: It cuts through rock not by the 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.

NARRATOR: 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 five 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.

JOEL PEDERSON: 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 of coarse boulders, the more resistant material

that the river has to fight against to accomplish

its incision, the steeper it gets.

NARRATOR: As the water flows over the rapids,

it cuts deep into the bedrock below.

At this set of rapids alone, the river drops 10 feet.

And there are a lot of rapids on this section of the Colorado.

JOEL PEDERSON: 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.

NARRATOR: 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.

JOEL PEDERSON: 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.

NARRATOR: It is this steep dropoff 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 canyon.

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.

JOHN DOUGLASS: Grand Canyon oftentimes is associated

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 to get wider over time.

We have classic rock falls 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.

NARRATOR: The fall of these rocks is not a gradual process.

This is erosion at its most violent.

JOHN DOUGLASS: Very few people are ever

going to see Grand Canyon actually change.

I've spent numerous nights in Grand Canyon,

and I've only heard one or two rocks ever fall.

But change will happen.

And when it does happen, it happens very rapidly.

NARRATOR: 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, they 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.

[landslide]

NARRATOR: The rock falls 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.

JOHN DOUGLASS: Without the Colorado River,

you could not have the Grand Canyon as wide as it is.

By flushing the material downstream,

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.

These guys continue to march and push and move all that material

downstream.

NARRATOR: 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.

Rock falls on the canyon walls reveal how weak rocks rapidly

widen the canyon across the plains of Arizona.

But this is far from the end of the Grand Canyon 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 canyons 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 piece of the canyon was shattered by volcanoes.

[music playing]

This is Toroweap Point, in a remote area known

as the Arizona Strip, one of the most isolated places

in the continental US.

Few people other than geologists ever

see this area, although it boasts some of the canyons 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 immediately create just giant clouds of steam.

NARRATOR: 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 eight occasions

the volcanic eruptions created huge lava dams that

block 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.

NARRATOR: 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.

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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, I think, a good chance that in the future

there may be eruptions here as well.

NARRATOR: 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 rock falls 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 1800s.

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.

NARRATOR: 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 turbulent geological history.