All language subtitles for Mount St Helens 720p HDTV x264 AAC PBS NOVA 2010 EN SUB

(explosion)

NARRATOR:
Mount St. Helens-- the biggest
volcanic eruption

in North America
in nearly a century.

Virtually all life for 200
square miles is wiped out.

It seems impossible that life
could ever return

to this barren wasteland.

We found a lot of our
conventional wisdom

was just flat wrong.

NARRATOR:
In recent years
there are ominous signs

the volcano is awakening.

MAN:
These things were like
skyscrapers that were being

shoved out of the ground.

They were literally that big.

NARRATOR:
A 30-year quest to understand

one of the most complicated
volcanoes in the world

is revealing new mysteries
deep inside the mountain.

MAN:
We don't know whether it's going
to erupt explosively again

in two years or in 20 years
or in 200 years.

NARRATOR:
Is Mount St. Helens preparing
to erupt again?

Right now, on NOVA--

"Mount St. Helens:
Back from the Dead."

Major funding for NOVA
is provided by the following...

Supporting NOVA and promotingg
public understanding of science.

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for Public Broadcasting,

and by PBS viewers like you.

NARRATOR:
October 2004.

Mount St. Helens
comes back to life.

Steam and ash spew from the
crater on the mountain's summit.

We saw the boiling material
come out of the ground,

we saw that it was blasting up,
it was dark

and it was light
at the same time.

It made a plume that rose up
over the rim of the caldera.

It came up to above
our altitude,

to 10,000 or 12,000 feet.

NARRATOR:
It's a frightening development.

(explosion)

For years, Mount St. Helens
has been quiet.

The volcano went from quiet
to unrest to eruption

very, very rapidly.

NARRATOR:
It could be headed
for a massive explosion.

DZURISIN:
It seemed possible
that we were headed

toward an explosive eruption.

We didn't know.

That was a key question.

NARRATOR:
The effort to understand
what is happening

inside the mountain couldn't be
more urgent.

Is the volcano about
to repeat the events

of three decades earlier, when
it shattered the tranquility

of its peaceful surroundings?

(bird screeches)

NARRATOR:
Spring 1980.

Mount St. Helens is one
of the major peaks

in the Cascade Mountains.

It's an area of outstanding
beauty, rich in wildlife.

For over 120 years,
the volcano has been quiet.

But in recent weeks
it's been rumbling.

Nobody is sure what to expect.

Then, on May 18, 1980,

a 5.1 magnitude earthquake
rocks the mountain.

Within ten seconds,

the volcano's northern flank
collapses

in the largest landslide
in recorded history.

It releases millions of tons of
magma in a colossal explosion.

A cloud of searing gas and rock,
known as a pyroclastic flow,

races over the surrounding
countryside.

Forests are flattened.

Four miles below the summit,

an enormous lake is choked
with debris.

The eruption continues to shoot
poisonous steam and ash

miles into the air.

It was just, again,
astounding is the best word

to describe what happened in
1980 here in Mount St. Helens.

NARRATOR:
The northern slope of
the mountain is buried

in several feet of ash.

Virtually all life
is extinguished.

57 people are dead.

They include loggers, campers,
scientists and a reporter.

Some are up to 13 miles away
in areas considered safe.

The plume of steam and ash rises
miles into the sky

for the rest of the day.

The drifting ash cloud
disrupts air traffic

for hundreds of miles.

The scale of the destruction
is enormous.

NARRATOR:
Across more than
200 square miles,

the surge of ash and rock
incinerates trees.

Thousands of birds

from more than a hundred species
disappear.

Billions of insects are gone.

Deer and elk are wiped out.

This vast area of devastation
becomes known as the blast

or blow-down zone.

Nearer the crater, ash and rocks
from the landslide

litter the northern slope
of the mountain.

It looks like the moon.

It's called the pumice plain.

It's directly below the crater.

Four miles from the volcano,

the enormous Spirit Lake
is scarcely recognizable.

The avalanche has lifted its bed
more than 200 feet.

The surface is smothered
in dead trees.

Hundreds of species of aquatic
life, including insects,

amphibians and fish, are killed.

CHARLIE CRISAFULLI:
It was black water.

And it de-gassed and bubbled,

and there were hot springs
that were coming up.

If you were to put your fingers

in to your wrist and wiggle
them, you wouldn't even be able

to see your fingertips.

That's how grossly modified
the water was.

NARRATOR:
Mount St. Helens is now

a lifeless jumble of shattered
forest, rock and ash.

It's hard to imagine life
will ever return.

The eruption was so powerful,

it altered the shape
of the mountain.

Mount St. Helens was a typical
cone-shaped volcano

known as a stratovolcano.

But the landslide has torn
1,300 feet off the summit,

leaving a gaping crater
a mile wide and 2,000 feet deep.

It's the largest volcanic
eruption in North America

in nearly a century.

(helicopter blades whirring)

Weeks after the eruption,
scientists arrive at the crater.

The volcano is still steaming
and rumbling.

It's a new and unfamiliar world.

One of the first to arrive
is Dan Dzurisin.

DZURISIN:
There was a tremendous amount
of steam

and you could see
that it was very hot.

You didn't see red lava oozing
out of the ground.

You didn't see fantastic
fire fountains.

There was this constant
background roar of rocks

cascading down the crater walls.

Occasionally a very large rock
the size of the helicopter

would come bouncing down
and you could watch it

and it was almost slow motion
because the crater was so large.

NARRATOR:
Mount St. Helens has a long
history of eruptions.

More than 500 years ago, two
massive explosions took place

within two years of each other.

They were nearly four times
larger than May 1980.

The mountain sits on one of
the most active seismic zones

in the world,
the Pacific Ring of Fire--

a vast arc of volcanoes running
for thousands of miles.

It's home to some of the biggest
and most dangerous volcanoes

active today.

Here, the enormous plates
making up the earth's crust

are being squeezed together.

Along the coast, the
plate below the Pacific

is sliding un the
North American plate.

60 miles down
pressure and friction melt the rock.

Magma wells up.

When it reaches the surface,
it bursts out.

(explosion)

But there are still many
unanswered questions.

Scientists' understanding
of what triggers an eruption

this massive is incomplete.

And given the scale
of destruction,

they need to find a way
to predict

when it might happen again
before it's too late.

Mount St. Helens is
about to become

one of the most intensely
studied volcanoes in the world.

The mysteries are not
just geological.

Biologists want to know
if any life has survived

and what its future will be.

Charlie Crisafulli,
one of the leading experts

on the mountain's ecology,
arrives soon after the eruption.

CRISAFULLI:
Nothing could have prepared me
for the sights and sounds

that I saw when I got here.

It was complete
and utter barrenness

and there was no sign of life
whatsoever.

NARRATOR:
His job is to survey
the mountain,

looking for any living things.

CRISAFULLI:
It was just intriguing to think
about how would life come back

to this landscape.

What would the pattern be?

How would the rate be?

NARRATOR:
Much of the mountain is
still inaccessible.

So he starts work
in the blow-down zone

in an area some eight miles
downhill from the crater.

CRISAFULLI:
We flew over in a helicopter
very close to the ground.

We would have these bumping,
twisting flights

across the landscape
following a contour.

NARRATOR:
In the first three months,

there's nothing but
dead and uprooted trees.

Then he notices something...

Signs of freshly
disturbed earth.

CRISAFULLI:
Lo and behold,
in many locations,

brown earth on top
of the gray volcanic ash.

NARRATOR:
Is there something down there?

Crisafulli returns on foot
to investigate.

There, emerging from the ash,
is a tiny burrowing animal.

It's a northern pocket gopher.

CRISAFULLI:
It was very thrilling.

NARRATOR:
How can it possibly have
survived when nothing else has?

CRISAFULLI:
This tiny animal lives entirely
beneath the ground.

And so when the blast occurred,

it would have been
safely protected

beneath a mantle of soil

and may very well have survived
in many locations.

NARRATOR:
Over the following months,
he finds more gophers.

It appears that life is
returning to the mountain

just three months
after the eruption.

NARRATOR:
By fall, the volcano
is still active.

Plumes of steam and ash

continue to shoot thousands
of feet into the air.

But something else
is happening, too.

The crater floor
appears to be moving.

Is the mountain preparing
for another major eruption?

DZURISIN:
We would sometimes
notice a crack

that hadn't been there
the day before.

And by the end of the day
the crack was larger.

And you realized that the ground
was moving beneath your feet.

NARRATOR:
It's an unsettling experience

to stand on the floor of a
volcano that's visibly moving.

If you stood and looked very,
very, very carefully,

with a reference point
in the background,

sometimes you could see it move,
but just barely.

NARRATOR:
The scientists set up a time-
lapse camera on a nearby ridge.

Over several days,
the pictures show a dome

rising in the middle
of the crater floor.

The volcano is oozing
a sticky gray lava,

cooling as it reaches
the surface.

Over several months,
the dome grows larger.

The geologists are puzzled.

What is going on
inside the mountain?

Is Mount St. Helens simply
rebuilding its summit

or is it about to blow up?

DZURISIN:
We didn't know
what might come next,

whether the lava might continue
to grow for many years--

or even decades or centuries--

and we didn't know
if there might be

explosive eruptions in store.

NARRATOR:
Then winter closes in,

restricting access
to the mountain.

The scientists' work is limited.

Answers will have to wait
until spring.

(bird singing)

Spring 1981, nearly a year
after the initial eruption.

Life returns to the hills and
valleys of the Cascade Range.

But on Mount St. Helens,

the devastation of the previous
year is still obvious.

Despite the danger,

Crisafulli moves closer
to the active volcano's core.

The pumice plain is buried
in several feet of coarse ash.

It's a dusty, barren wilderness.

Life seems impossible.

CRISAFULLI:
This is an area where super-hot
incandescent flows came down

and killed all life
that was here.

NARRATOR:
Helicopter is the only way in.

CRISAFULLI:
We were flying back and forth,
very low,

just above the ground surface,
looking for any form of life.

NARRATOR:
He crisscrosses the area
but there's nothing to see.

Then suddenly,
amidst the acres of barren rock,

there's an unexpected flash
of color.

CRISAFULLI:
So we set the helicopter down
and we walked up.

Right out in the center of
the pumice plain we saw a plant.

NARRATOR:
At first, Crisafulli can hardly
believe his eyes.

It was a prairie lupine, a
species that typically grows

high in the slopes
of Mount St. Helens.

NARRATOR:
It's not only growing,
it's flourishing.

CRISAFULLI:
Not only had the plant
established,

but at that point
was in full flower.

And it was quite remarkable.

When we saw the first one
we were very surprised.

NARRATOR:
It's only four miles
from the volcano's crater.

It's the first sign of life

in an area where everything has
been extinguished.

But how has the plant managed
to grow in such a barren area?

The answer is a special
root structure

that provides its own
fertilizer.

These are little factories where
a bacterium works with the plant

and provides nitrogen
to the plant.

In return the plant provides
the bacterium with simple sugars

that it fixes through
photosynthesis.

And so this is a great
relationship

where you scratch my back,
I'll scratch yours.

NARRATOR:
This special process means
lupines can grow

in even the most
inhospitable terrain.

The lupine, like the gopher
Charlie found earlier,

is a pioneering species.

CRISAFULLI:
It's really important

in landscapes
like Mount St. Helens,

because the volcanic material
that fell on the ground

tends to be really
nutrient poor.

NARRATOR:
The conditions are difficult,

but can the lupine pave the way
for other life to follow?

NARRATOR:
In the spring, geologists also
return to the mountain.

During the winter months,

lava has continued to ooze out
of the crater floor.

The lava dome has grown several
hundred feet taller

and doubled in diameter.

It's still a hazardous place.

DZURISIN:
When I stepped out of
the helicopter in 1981

on the crater floor,

steam was actively rising
off the growing lava dome.

There was still a tremendous
amount of noise.

Rock falls were constant...

and 2,000 feet above your head

used to be where the summit
of the volcano was.

You were now standing
in a crater with a lava dome

that had not been there a few
months previously

or a year previously.

It was actively steaming.

That was a very...
very exciting thought.

NARRATOR:
It's a rare opportunity to watch
the process of dome building

unfold in front of their eyes.

The geologists set up
instruments to monitor

what's going on,

including seismometers that can
detect tremors set off by lava

as it forces its way
through the rocks.

They place a series of these
as close to the lava dome

as possible.

(birds chirping)

At the Cascades Volcano
Observatory

in southern Washington,
the seismic data pours in.

DZURISIN:
The seismic record like this

records any vibration
of the ground...

so we can see real
rock-breaking earthquakes,

we can see rock falls.

(rumbling)

It's our job to try
to understand

what all those signals mean

in terms of what
the volcano might do.

NARRATOR:
The first traces reflect
extreme activity.

DZURISIN:
Here you see the record
is almost continuous--

one earthquake after the other--
bang, bang, bang.

The seismic signal is
essentially continuous.

NARRATOR:
The lava is breaking
through rocks

and flowing across
the crater floor.

Then, the seismic record reveals
a cyclical pattern.

For periods of weeks to months,

earthquake activity in the
crater would be pretty quiet.

NARRATOR:
The lava is no longer flowing.

DZURISIN:
And then a few days later, we
might see a pattern like this,

more and more of these
very sharp earthquakes.

NARRATOR:
It's the sign of lava
on the move again,

forcing its way
through the round.

DZURISIN:
Eventually lava would make it
on to the surface,

maybe in just a couple of days,

and we're seeing a continuous
record of ground shaking,

both earthquakes and rock falls.

Then, after a period
of dome growth

that might last a few days or a
few weeks, it goes quiet again.

That episode has ended and
the pattern begins itself over.

NARRATOR:
This cycle of dome building
continues

for the next five years.

The pattern is so regular
that when the cycle begins,

the scientists can accurately
predict

what the volcano will do next.

When the first rock-breaking
earthquakes occur,

they know it's only a matter
of days or weeks

before the lava starts
to flow again.

At one point the dome reaches
nearly 1,000 feet--

almost as high as
the Empire State Building.

Then, in late 1986,
the seismographs go quiet.

DZURISIN:
It was pretty clear

that that period of dome
building had ended.

NARRATOR:
But for how long?

Has the mountain gone
back to sleep?

JON MAJOR:
It wasn't clear whether the
mountain had gone back to sleep

now for centuries

or whether it was going
to just go back to sleep

for a couple of years.

NARRATOR:
It seems the pattern
has changed.

For six years the scientists
have been able to predict

what the mountain will do next.

Now they are back to guessing
if and when it will erupt again.

But even if the volcano
has gone to sleep,

the wildlife continues
to bounce back.

More and more gophers
are spreading across

the blow-down zone.

Lupines are colonizing
the pumice plain.

And what's happening
at Spirit Lake is remarkable.

The May 1980 eruption
obliterated all visible life

in the lake.

The surface was smothered
in a blanket of debris.

In the murky water there was
an explosion of bacteria.

CRISAFULLI:
There were a couple of species
of pneumonia

that were described,

and also the disease...

the bacteria that causes
Legionnaires Disease,

legionella.

And so, many of us working
in the lakes in the early days

came down with a fever.

NARRATOR:
The bacteria rapidly consumed
the oxygen,

making life impossible for
any air-breathing organisms,

including fish, amphibians
and insects.

We said it's going to be
decades and decades

before this resembles anything
like a typical lake

in the Cascade Mountain Range.

Well, we were surprised,

because that's not exactly
what happened.

NARRATOR:
Scientists begin
routine water sampling.

It's a unique opportunity to see
if and when life will return

from the dead.

At first there's nothing.

But as the debris settles,
the water clears.

Light levels improve.

Then, three years
after the eruption,

there's a crucial discovery...

microscopic plants.

They're phytoplankton-- plants
that turn sunlight into oxygen.

They've been brought in by birds
or blown in by the wind.

They are the basic building
block of aquatic life.

Over the following months,

as light levels continue
to improve,

the plankton population grows.

CRISAFULLI:
In fact, between 1983 and 1986,

135 different species
of these tiny plants

had colonized the lake.

They provide the oxygen and
also the prey for the food web.

NARRATOR:
Sunlight, oxygen and food.

Several years after
its complete destruction,

Spirit Lake is coming
back to life.

Four miles away,
the volcano remains quiet.

The lava dome has stopped
growing.

Many geologists think
the show is over,

at least in their lifetime.

We had the feeling that we had
probably seen our last eruption

of Mount St. Helens.

We knew there was a chance
it would erupt again.

But none of us were
betting on it.

NARRATOR:
As the mountain sleeps,
wildlife bounces back...

even in the most
unexpected places.

In one of the most devastated
areas of the mountain--

the pumice plain--
a gopher is seen.

It's surviving by eating lupine.

Lupines provide the food.

Gophers enrich the pumice

by burrowing their way
through the ash.

They mix in fresh soil and
help new plants to spread.

CRISAFULLI:
When you walked around the
landscape, it was those islands

created by gopher-turned soils
that were very green

and full of flower and seeds.

NARRATOR:
The gophers also play another
role in helping wildlife spread.

Crisafulli finds a salamander
in a gopher's tunnel.

CRISAFULLI:
What's interesting about
the gopher is they create

kilometers of underground
tunnel systems.

NARRATOR:
Elk are returning
to the area,

helping to expand this amazing
web of life.

CRISAFULLI:
When elk move across
the landscape,

they collapse the tunnels,
creating entranceways

that salamanders and other
amphibians can get access to.

And once they get beneath
the ground,

these are very cool and moist
sites that enable them

to survive in an otherwise
inhospitable area.

And the importance of that
is that it allows them to use

these underground burrows
as stepping stones

during hot, dry weather and
eventually to colonize

new patches of terrestrial
habitat

as well as ponds and lakes.

(frog croaking)

NARRATOR:
Spirit Lake now teems
with amphibians.

Fish, brought to the lake
by fishermen, are thriving,

a clear indication
that the water quality

is returning to normal.

CRISAFULLI:
What's happened with the fish
was actually remarkable.

While we don't have
a good handle

on the total number of fish,
we know from our snorkeling

and surveys that
the population is enormous.

NARRATOR:
Spirit Lake is beginning
to resemble

a typical mountain lake.

Just over a decade
after the eruption,

life is flooding back to
the slopes of Mount St. Helens.

The rate of recovery
is far faster

than anybody had expected.

CRISAFULLI:
Clearly our understanding of
the ability of these organisms

to disperse was greatly
underappreciated.

We found a lot of our
conventional wisdom

was just flat wrong.

(birds chirping)

NARRATOR:
Then, as life recovers,
new threats emerge.

In September 2004,

the seismographs at the Cascades
Volcano Observatory

pick up a new series
of tremors

deep below Mount St. Helens.

The volcano has woken up.

John Pallister takes a
helicopter to investigate.

PALLISTER:
You could see the absolute
beginning of the eruptions,

unusual-- really unusual--
to just happen to be there,

in a helicopter,

the crater rim,
on the upwind side,

so the plume was going away
from us.

NARRATOR:
Pallister has no idea how big
this eruption will be.

PALLISTER:
We saw the boiling material
come out of the ground.

We saw that it was blasting up.

It was dark ash coming out
and light steam coming out

at the same time.

It made a plume that rose up
over the rim of the caldera

and drifted downwind.

NARRATOR:
The speed and suddenness
of the eruption

catches everybody by surprise.

The volcano went from quiet
to unrest to eruption

very, very rapidly.

(explosions)

NARRATOR:
During the next two weeks,
there are three more eruptions

of steam and ash.

No one knows what will
happen next.

DZURISIN:
It seemed possible

that we were headed toward
an explosive eruption.

We didn't know.

That was a key question.

NARRATOR:
Then, after 14 days,
the seismographs quiet down.

Almost as quickly as it started,
the eruption stops.

But then something strange
happens.

Over the next few weeks,
the seismographs pick up

a new pattern of tremors

the geologists have never
seen before.

Could they be linked
to a gigantic lump of lava

growing out of the crater floor?

PALLISTER:
It was a huge kind
of recumbent spine,

this big mass lying
in the crater floor

some 300 meters or so high.

NARRATOR:
The spine of lava is as long
as the Eiffel Tower.

MAJOR:
Everybody was just awestruck.

To have this large spine just
shoving up out of the ground

was completely different
and outside the experience

of any of us here in the staff.

NARRATOR:
Despite the risk, John Pallister
goes in to take samples.

PALLISTER:
We landed right next to it.

And I was able to get out,
helicopter helmet on,

rapidly run up to the edge
of the spine.

NARRATOR:
It's an unbelievable sight.

DZURISIN:
Had someone suggested to me
that we make a movie

of a lava dome growing that way,
I think I would have said

it's a little too fantastic,
let's make it more realistic.

NARRATOR:
At the observatory,

where geologists
have been puzzling over

the strange seismic traces,
they now realize what they are.

They're the unique autograph
of the giant spines

as they push their way
out of the ground.

DZURISIN:
This is the seismic signature
of solid blocks of rock

grinding their way
through the volcano,

coming out onto the surface.

As they do, they make these
small seismic signals,

one just like the other, just
like the other, very repetitive.

We came to call them
"drumbeats."

NARRATOR:
The drumbeats continue
for several years.

Spine after spine of solid lava
emerges from the crater floor.

It's unlike anything geologists
have seen on Mount St. Helens.

PALLISTER:
Now, spine doesn't do justice
to these things.

These things were
like skyscrapers

that were being shoved out
of the ground.

They were literally that big.

NARRATOR:
Sometimes the blocks grow
at a rate of 16 feet a day.

Then they collapse.

Seen through a time-lapse
camera,

one solid lump of lava
after another

pushes up through
the crater floor.

The process is mystifying.

What do the spines mean?

Why was the eruption in 2004
so different

than the style of eruption
in the 1980s?

Why in the 1980s did you have
this more fluid lava

that created the sort of short,
stubby lava flows

that came out and built
the lava dome?

Whereas in 2004, you basically
had solid rock being pushed up

in the ground.

NARRATOR:
There's one urgent question.

Is the volcano building up
to another major eruption?

MAJOR:
Trying to make sense of what was
going on was a challenge.

Trying to understand how the
eruption was going to progress

was a challenge.

We had lots of discussions
about whether or not

it was going to be
an explosive eruption,

whether it was going to be
another dome building eruption.

NARRATOR:
There is one way to find out.

Analyzing samples of the lava
might explain

the mysterious solid blocks and
what they mean for the future.

At the volcano observatory,
John Pallister compares lava

from the spines with samples
taken from previous eruptions.

Could there be something
in their composition

that explains why the mountain
sometimes pushes up spines...

sometimes oozes lava...

and sometimes explodes?

(rumbling)

Pallister begins with
a sample of the lava

that erupted so explosively
in May 1980.

He's immediately struck
by the large areas of blue.

PALLISTER:
Okay, so what's important
about this 1980 rock

is the abundance
of this blue area, which...

and that's basically bubbles.

Now, that's not a mineral;

that's just open space
in the thin section.

That's where gas bubbles were.

This sample would float in
water, it had so much gas in it.

NARRATOR:
The gas comes from water,
a component of the magma.

As magma rises,

changes in pressure turn
the water into gas.

The gas pressurizes the magma.

It's what gives volcanoes
their explosive force.

(explosions)

PALLISTER:
1980 had a lot of gas in it.

So it exploded,
tore itself apart

in a tremendous explosive
eruption.

NARRATOR:
But when he looks at lava taken
from the 1983 period

of dome building,
it's different.

PALLISTER: There is much less
of this open space,

of the gas filling space
in the rock.

NARRATOR:
The 1983 lava behaves like
a bottle of soda going flat.

Finally he looks at
a sample of lava

from one of the spines in 2005.

PALLISTER:
I don't see any blue space,

any of that--
oh, there's just a little bit--

but dominantly it is...
it is lacking in space.

It's a gas-poor magma...
in fact almost none.

NARRATOR:
There's just enough gas
to push it to the surface.

But by the time it gets there,
there's nothing left.

PALLISTER:
So this one came up slow

and it made sticky, solidified
spines instead of making

either lava flows or
an explosive eruption.

NARRATOR:
It's a crucial insight.

The amount of gas determines
the nature of the lava

and the force of the eruption.

PALLISTER:
It all comes down to the gas
budget for the eruption.

Is it going to fizzle
or is it going to explode?

NARRATOR:
Suddenly the mountain's behavior
makes sense.

The spines are a sign the magma
under Mount St. Helens

is running low on gas.

Then, in 2007, as if to confirm
this new insight,

the familiar drumbeat
seismic traces...

vanish completely.

No more spines appear.

The lava below the mountain
has finally run out of gas.

How long will it take
the mountain to build up

enough gas pressure
for another eruption?

I think that's the most
important question

we have to answer.

How long does it take to build
up the gas necessary

to drive an explosive eruption?

NARRATOR:
That's now the question
they need to answer.

Geologists go back to the
mountain to look for clues.

The eruption in 1980 took
the top off Mount St. Helens,

leaving its history exposed in
the rock walls of the crater.

Most of the important previous
eruptions are marked

by different colored bands.

The lower part of the walls
where you see gray

and some yellows and some pinks

are all part of the older
edifice of Mount St. Helens.

NARRATOR:
These rocks, which make up the
bottom half of the rock face,

are around 16,000 years old.

MAJOR:
Then, if you look higher
on the wall, near the top,

you see darker colors.

And those are rocks
that began erupting

about 3,000 to 2,500 years ago.

So, by looking at what we call
the stratigraphy

in this magnificent exposure
of the rock types

in the crater walls, we can
piece back the puzzle

and understand the history
as far as eruptive activity

of Mount St. Helens.

NARRATOR:
Do the rocks give any indication
how long it takes to build up

enough gas between eruptions

for the sleepy mountain
to awake again?

John Pallister sorts
and categorizes rocks

from earlier eruptions.

He checks notes and photos
to try and determine

how often the mountain has
erupted violently.

Drawing on previous records,

he builds up a picture
of Mount St. Helens' past.

For much of the last
4,000 years,

there seems to be
a fairly clear pattern.

If we look at the number
of big eruptions

over the length of time
the volcano's been active,

you might say that there's one
roughly every thousand years,

a big eruption.

So from our context here
we could say

that it takes on the order
of a thousand years

to build up enough gas
to get a really large eruption.

NARRATOR:
The record suggests some of
these eruptions have been huge,

more than ten times
larger than 1980,

potentially enveloping vast
areas of Washington and Oregon.

But that would imply that the
next really big one isn't due

for about another century.

Except for one little detail
around 500 years ago.

PALLISTER:
In 1479 A.D. and 1482

there were two very large
eruptions.

(explosions)

So the volcano is capable
of surprising us and producing

two highly explosive eruptions

in a span of less than
three years.

NARRATOR:
Both these eruptions were
much bigger than May 1980.

There is no straightforward
pattern.

Mount St. Helens can pause
for a thousand years

between big explosive eruptions,
or it can pause for three.

These results have left
geologists with one certainty

and a number of questions.

First of all, we expect
this volcano to erupt again

as repeatedly in the past;

there's no reason to think
it's gone to sleep forever now.

NARRATOR:
There will be another eruption,

but nobody can determine when
or just how big it will be.

PALLISTER:
We don't know whether it's going
to erupt explosively again

n two years or in 20 years
or in 200 years.

That's an area that needs
a lot more work,

a lot more research
to understand

and it is of fundamental
importance to being able

to forecast and to save lives
and to save property.

(bird screeches)

NARRATOR:
For 30 years, Mount St. Helens
has led scientists

on an extraordinary journey
of surprise and discovery.

When they surveyed the
destruction in the early 1980s,

nobody could have predicted
the speed with which

life has returned.

CRISAFULLI:
It was another form
of an eruption,

it was an eruption of nature.

Nature marched back
with a vengeance.

NARRATOR:
Mount St. Helens has revealed
a rich and complex web of life

that has never been
documented before.

Today the slopes of the mountain
are a living testimony

to the miraculous ability of
nature to return from the dead.

CRISAFULLI:
Each time you would
come out here

and there would be a surprise,
something would be unveiled,

something that you hadn't
seen before.

Perhaps it would be
a new species of spider

or a new species of beetle.

Nature is very resilient,

and that is the take-home
message from 30 years

of ecological work on
the Mount St. Helens volcano.

NARRATOR:
But as nature bounces back, the
mountain still broods overhead.

It's like a ticking time bomb
waiting to destroy life

all over again.

MAJOR:
Based on the history
of this volcano,

we know it's been
extremely active

and it's not a matter of
whether, if it will erupt again,

it's a matter of when
it will erupt again,

when will it reactivate,
when will it reawaken.

NARRATOR:
These are questions scientists
are still wrestling with.

DZURISIN:
We have yet to find
a silver bullet,

a magic thing that we can
measure that tells us

when the volcano is going
to turn on.

NARRATOR:
Mount St. Helens
will erupt again.

The only questions are when and
how big that eruption will be.

On NOVA's "Mount St. Helens"
Web site,

go behind the scenes
with director Daniel Hissen.

See stunning images

of the landscape's remarkable
return to life, and more.

Find it on pbs.org.

Captioned by
Media Access Group at WGBH
access.wgbh.org

This NOVA program is available
on DVD at shopPBS.org,

or call 1-800-play-PBS.