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>> Earth, a unique planet,

restless and dynamic.

Continents shift and clash,

volcanoes erupt,

glaciers grow and recede--

titanic forces that are

constantly at work,

leaving a trail of geological

mysteries behind.

This episode investigates

the deadliest and most

destructive volcanic event

in U.S. history.

Mount St. Helen's,

a pristine snow-capped mountain,

suddenly blew 600 feet

off her summit in a type

of eruption no one had ever

witnessed before.

Scientists trying to understand

what made this event so lethal

uncover evidence for one

of the biggest landslides

in history, a sideways-directed

blast that knocked over

230 square miles of forest

and mega mud flows that

thundered down the valleys

and destroyed everything

in their path.

What scientists have discovered

from this unique event brings

geologists one step closer

to understanding

"How the Earth Was Made." S02x10 Mt. St. Helens Original Air Date on February 2, 2010

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

In Washington State,

100 miles south of Seattle,

stands Mount St. Helen's,

one of 20 major volcanoes

that form part of the Cascade

Mountain Range at the

North American West Coast.

Before May 18, 1980,

she was at the center

of a thriving recreational

paradise and prosperous

timber industry.

>> The volcano had

a beautiful conical form.

It was called the Mount Fuji

of North America,

and the form was a bit concave.

>> But beneath her beauty

lay an ominous secret.

The mountain was brewing

something that had

a deadly potential.

The story began

on March 20, 1980,

when a 4.2-magnitude earthquake

woke Mount St. Helen's

from a slumber that had lasted

123 years.

The last known eruption

was witnessed in 1857

by local tribes.

In 1980, the earthquake

was an alarming sign because

warthquakes may be an indication

that an eruption is building up.

Earthquakes can happen

when magma rises from deep

inside the earth,

shifting and breaking the rock

on its journey up.

>> The very first signals

we had were earthquakes

at shallow depth

neneath the volcano,

and within a matter

of just a few days, it was clear

this was something unusual.

So the word went out to

scientists around the country,

and very quickly we started

to converge at St. Helen's

and started trying to understand

what was going on,

and what we saw was the volcano

was becoming more and more

and more restless.

>> Volcanoes are dangerous

because they are hard

to predict, and in 1980,

the science of predicting

volcanic eruptions was still

in its infancy.

The most recent explosive

eruption that occurred

on the continental U.S.A.

was Lassen Peak in California

in 1915. Since then,

most of the experience

volcanologists had gained

came from studying

quiet lava flows on Hawaii.

>> Volcanoes are incredibly

complicated natural systems,

and they're always full

of surprises.

St. Helen's surprises us

all the time.

Other volcanoes surprise us.

But we're learning.

>> Armed with the latest

scientific equipment,

volcanologists were anxious

to study a possible eruption.

Little did they know that they

were about to witness

the most deadly volcanic blast

in the U.S. in living memory.

On Coldwater Ridge, 6 miles

mortheast of the volcano,

they installed a trailer

with highly sophisticated

equipment and began to closely

monitor the mountain.

From there, they had a perfect

view over the volcano.

>> We had a front-row seat

to seeing the evolution

and reawakening of a major

Cascade volcano, and we were

fortunate in that we had people

here with a lot of energy.

People were working

15 to 20 hours a day.

>> In addition to earthquakes,

another key indicator

for volcanic activity

is gas emissions.

Magma made out of hot molten

rock contains gases that come

from deep inside the earth,

30 miles down.

As magma moves up from these

depths, there is less rock

weighing down on it from above,

so the pressure on the magma

decreases. Gases dissolved

in the magma escape and rise

to the surface.

But gases are not just

a telltale sign of rising magma.

They are also responsible

for the explosiveness

of the eruption.

The greater the buildup of gas

pressure within the volcano,

the more explosive

yhe eruption will be.

>> Well, this is a simple

experiment to demonstrate

the importance of gas pressure

in a magma.

The bottle is partially filled.

This is filled about 80%

with water. This will be

simulating a volcanic eruption.

So I'm going to carefully

place the bottle here.

We'll tilt it a little bit

away from me.

I'm going to put on

my safety goggles.

And now I'm about ready

to start pumping.

Here we go.

Oops. There's a little bit

of gas coming at the bottom.

A little bit more.

And the pressures are up to

about 60 pounds per square inch.

Whoa!

Did we get it?

We got it! Yay!

>> Like in the bottle,

the presence of gas

at the surface of a volcano

is a sure sign that an explosive

eruption is building.

In spring 1980, Casadevall's job

was to detect these gas

emissions. The gas he was

looking for was sulfur dioxide.

It's a gas that smells like

rotten eggs and is associated

with volcanic activity.

But the key to the measurements

wasn't the smell. The secret

was subtle differences

in the color of the light.

>> This instrument looks

at the light in the sky,

and it looks for the presence

of certain molecules,

like sulfur dioxide, which also

absorb light from the sky.

And it measures the difference

between the light absorbed

by sulfur dioxide and the light

available in the rest

of the sky.

>> In March and April 1980,

Casadevall and his team

were using this instrument

to detect the amount

of escaping gases.

They assumed that the gas levels

would increase prior

to the eruption as more

and more magma would rise

inside the volcano.

But in 1980, the assumption

proved wrong.

Gas emissions didn't change,

even though earthquakes

were getting stronger,

and hundreds of small tremors

were recorded.

>> There was really

no significant variation.

It was a very low level

of sulfur dioxide emissions.

And there was nothing in those

emission rates that really

indicated that an eruption

was just around the corner.

>> But by late April,

after 5 weeks of tremors,

the mountain gave them

another clue to what

was brewing inside.

On the north face, a huge bulge

was growing outwards.

>> A casual observer would look

at the volcano and say,

"well, it's not longer

a nice fuji-type shape."

But in fact, the north side now

is bulged out, and it was

obviously deformed in some way.

And we were trying to understand

what could have been causing

that deformation.

>> It was the first time

scientists were able to monitor

the deformation of a mountain.

On Coldwater Ridge,

they installed an instrument

that uses laser technology

to get precise measurements

as to how fast the bulge

was growing.

>> What we were trying to do

was to focus a laser beam

from this instrument

on a reflector in the volcano,

receive the returned

reflected signal, and then

measure that distance.

>> The reflectors they used

were small mirrors

about 3 inches across

they had fixed on the bulge.

>> As was the case in 1980

before May 18th,

the north flank of the volcano

was moving outward.

It was bulging outward.

And so the distance

was getting shorter.

And what we learned is that

from Coldwater 2, that distance

was getting shorter

about 5 feet a day.

>> By May 11th, the bulge

had expanded outward

a staggering 450 feet.

Dzurisin had a hunch

that it was caused

by rising magma.

We knew that the north flank

was deforming. We knew

that something had to be

causing it to deform.

What could that be?

Well, it could be magma forcing

its way up into the volcano,

shoving the north flank aside.

That was a possibility.

Well, what else could be

causing it? Well, maybe

the earthquakes were simply

causing the volcano to become

unstable as a result of gravity.

Maybe there was just

a slow landslide going on.

It was very difficult

to understand or to make

a measurement that would

tell you the difference.

The plausible story was

that magma was forcing its way

up under the volcano.

>> As the bulge grew,

the flank of the mountain

became increasingly unstable.

Still, scientists had no idea

how deadly the eruption

would turn out to be.

>> So we didn't know for sure

until... until it happened.

But as the bulge continued

to grow, we knew we were getting

closer to the final outcome.

We just didn't know

what that would be.

>> There were other signs

that an eruption was imminent.

Earthquakes became more frequent

and stronger as time went on.

Authorities feared the worst

and zoned off the area

around the volcano with limited

access for the public.

In spring 1980, geologists

were dealing with a number

of clues that Mount St. Helen's

was building to an eruption.

Earthquakes were an indication

that the volcano was waking up.

Despite low and unchanging

gas emissions, a growing bulge

on the north face suggested

that the magma was indeed

on the rise.

Mysteriously, after May 14th,

the volcano quieted down.

There was hardly any seismic

activity, and the bulge grew

at a slower rate.

On May 17th, officials gave in

to pressure and allowed some

people with property inside

the restricted area to gather up

whatever they could.

Another group was scheduled

to enter for 10 am

the next morning.

But on that day,

disaster struck.

>> On the morning of May 18th,

scientists were about to witness

one of the worst volcanic

disasters in modern history.

For more than 2 months,

a team of volcanologists

had been monitoring

Mount St. Helen's.

>> That's another reason

why May 18 was a surprise,

Because basically in all of this

data we collected before,

there wasn't anything that told

that, you know, May 18th

was going to be the day.

>> David Johnston, a young

volcanologist with the U.S.

Geological survey, was on duty

that day. He had spent the night

on Coldwater Ridge to carry out

measurements of the growing

bulge.

>> It's interesting that even

on the morning of May 18th,

the measurements that Dave made

indicated that that bulge

was still growing at about

the same rate.

>> The same morning, Dan Miller

was on his way to Cowater

Ridge to check on

the time-lapse camera they used

for filming the north face.

>> I was headed to the north

on interstate 5 out of

Vancouver, Washington,

and as I got a few miles

north of town, there's

an overlook point as you go

down the highway where you can

look off to the east

and see Mount St. Helen's,

and I looked over there, and it

was a beautiful clear day,

and there was Mount St. Helen's

with this giant mushroom cloud

going up above it.

At that point, I knew something

very serious was underway.

>> From a safe distance,

35 miles away, Miller witnessed

how the eruption unfolded.

>> The first thing I did

was went to our radio,

our communications radio,

and I made some calls up to Dave

to try to raise him and find out

what was happening.

And not only did I not reach

Dave, but even our repeater,

which was on a mwuntain peak

that was another few miles

to the north of Coldwater 2,

did not answer, indicating

that it had been destroyed.

And that was very scary.

At that point, I realized

that something bad had probably

happened to Dave.

>> Thick dark smoke was

billowing out of the crater,

obscuring the view.

Within hours, daylight

turned murky grey and reduced

visibility up to 300 miles

northeast.

It wasn't until 24 hours later

that the air was clear enough

for scientists to inspect

the devastation.

>> The entire landscape

was almost unrecognizable

to those of us who had spent

almost 2 months before

the big explosion on May 18

working up there every day.

>> Suddenly I realized

that there was complete

and utter silence.

There were no insects.

There were no small animals.

And there were no colors.

The only color was ash grey

for as far as I could see.

>> We made our way up along

the edge of the ridge,

and we found the small quarry

where Dave's trailer

and our vehicle had been parked

on the morning of May 18th,

and we could see

that it was gone.

>> Neither Johnston

nor the trailer were ever found.

A total of 57 people were killed

along with thousands of deer,

elk, bears, and other wildlife.

230 square miles of forests

were destroyed, and the eruption

had torn a 2,000-foot-wide

crater in the summit.

>> It was no longer

a beautiful fuji-type volcano.

And in fact, we could see

that the top of the volcano

appeared to be missing

or was obscured.

>> In the months to come,

scientists faced the difficult

task of finding out

what exactly happened

on that fateful day.

They searched the ground

for clues and methodically

pieced together the chain

of events that had led

to the devastation.

The first important clue

was an earthquake measuring 5.2

that shook Mount St. Helen's

at 8:32 that morning.

The second piece of evidence

came from photographs taken

by tourists who flew over

the mountain around

the same time.

>> They looked down, and they

were able to document,

by a series of photographs,

some shaking on the top

of the mountain,

and then right afterwards,

the whole front of that mountain

started to move sideways.

>> In a series of still

photographs, they documented

how the bulge collapsed.

>> A magnitude-5.1 earthquake

caused the north flank,

which was greatly weakened

by the deformation,

to break loose in the form

of a giant landslide.

>> The volcano just couldn't

take it anymore, and the north

flank became unstable

and slid away.

>> Within seconds,

1,300 feet of the mountain

slid down and destroyed

everything in its path.

What used to be a quiet

mountain valley with the Toutle

River running through it

was now filled with debris

up to 600 feet high,

forming a hilly terrain

known as the hummocks.

From the amount of debris

from the landslide,

scientists calculated

that 2/3 of a cubic mile

of rock slid down the mountain,

enough to bury Washington, D.C.

under 50 feet of rubble.

It was one of the biggest

landslides ever recorded

in history.

It tore a gaping hole

in the side of the mountain

almost 2 miles wide

and over 2,000 feet deep.

To scientists, it was a mystery

why suddenly the entire northern

flank of the volcano collapsed.

Geologists today can still

follow the trail of destruction.

>> What we're looking at here

is a part of a large debris

avalanche. We're looking at

one of the hummocks.

And this is a rock outcrop

that's actually outcropping

in the hummocks,

and the coloration that you see,

first of all is telling you

that this has been altered.

>> This color provides

an important clue

to why the landslide

became so big.

Instead of the usual black

of volcanic lava rock,

it is yellow.

This is a sign that the rock

came in touch with hot water,

turning some of the minerals

in the rock yellow.

Geologists found the same

yellow rock in the crater wall,

suggesting the discoloration

must have happened

on the mountain

before the eruption.

>> So you have rainwater

or glacial water that seeps

down into the mountain,

gets close to the magma,

heats back up, and as it comes

out of the mountain,

then it starts to change

and chemically alter these rocks

and turning them, you know,

into these different colors.

>> But the hot water

didn't just change the color.

More importantly, it also

weakened the rock.

>> So you can see how crumbly

these rocks are just by kind of

digging your hammer through

here, and the reason

they're so crumbly is because

when the hot water moves through

and alters them, they sometimes

altered the clay and some really

soft materials. So if you took

any other rock, it would be

much stronger.

>> This yellow crumbly rock

isn't just found here,

but stretches over 17 miles

throughout the valley,

suggesting that large parts

of the mountain were rotten,

weakened before the eruption.

>> So probably what had been

happening over thousands

of years is magma

down on the volcano

heating up water,

and this acidic water

was gradually rotting out

the center of the volcano.

It couldn't be seen

anywhere at the surface.

>> When the bulge on the north

side finally collapsed,

it pulled along big parts

of the rotten volcano

and left behind

the horseshoe-shaped crater

St. Helen's is famous for today.

>> This volcano is just

a sand pile, if you will.

It had very little internal

integrity or strength,

so that when failure did occur,

literally part of the mountain

slid away.

>> But more destruction

was to come.

Within minutes of the landslide,

Mount St. Helen's channeled

her fury into another

deadly force.

>> The air turned absolutely

black, so nobody could see

anything, but a few of these

witnesses that managed

to survive out in the very edge,

the ground shook intensely,

and they sensed that all

the trees were coming down

at once, even though

they couldn't see it.

>> The landslide had spread

east and west, but this force

surged in a northerly direction.

Within 3 or 4 minutes,

it destroyed everything

on Coldwater Ridge

and ripped out trees

in a 230-square-mile

fan-shaped area.

>> Ao even where we're sitting

here, 8 miles from the volcano,

tou can see this ridge

behind me. This ridge

is 3,000 feet tall,

and all the texture

that you see on this ridge

are the trees, the old-growth

forest trees that were blown

down by the surge that went up

and over this thing

like it wasn't even there.

And so for the surge to have

gone up and over this,

it had to still have been going

roughly 300 miles per hour,

and then it continued

9 miles farther than that

before it finally stopped.

>> To geologists,

this was a surprise.

They had never witnessed

a sideways eruption before.

>> Scientists were intrigued

as to what had caused

this devastating surge.

So they began searching

the ground for evidence.

5 miles northeast of the crater,

Waitt is revisiting

the old deposits.

Mixed with burned wood

from the shredded trees,

he also finds rock.

To the untrained eye, there is

nothing unusual about it,

but not so for the experts.

>> It's clearly young rock.

It's relatively light,

and there's lots

of little voids in here.

In other words, little bubbles

that are frozen in the rock.

So this was the rock that was

growing--there was a hot liquid,

almost solid liquid, that was

growing inside Mount St. Helen's

and causing the bulge

before May 18th.

>> This small rock was

a crucial piece of evidence for

scientists trying to figure out

the chain of events.

After the landslide

tore a gaping hole into

the north flank of the mountain,

it exposed the magma underneath.

Without a cap of earth

to keep it sealed,

the magma suddenly expanded

and surged outwards,

pulling along rock

from inside the mountain.

>> You can envision it

as sort of a colossal-sized

ash hurricane. It was a cloud

of rocks and ash and hot gases

that was maybe several thousand

feet thick that was moving

across the countryside at speeds

of several hundred miles

an hour, a very turbulent

mixture with blocks as large

as 3 or 4 feet in diameter

flying through the air.

It was incredibly destructive.

>> After the initial surge,

Mount St. Helen's turned quiet.

For half an hour, there was

calm and tranquility.

Then the volcano began

hurling its fury skywards.

[explosions]

An enormous mushroom cloud

formed high above the volcano,

3 times as tall

as Mount Everest.

Ferocious explosions

went on for hours, releasing

huge amounts of energy

equal to 27,000 Hiroshima-sized

atomic bombs.

What happened on May 18, 1980,

became a landmark event

for scientists.

It provided them with

an unprecedented chance

to study a lateral eruption.

Discolored rock in the valley

and in the crater wall

were evidence that the mountain

was rotten before the eruption.

Grey volcanic rock

9 miles north of the volcano

was evidence that the landslide

uncorked a massive

lateral blast.

At 5:30 in the evening,

the volcano began to slow down

until it finally quit.

But there was more impending

danger, as the eruption

had caused another

cataclysmic effect.

>> On May 18, 1980,

Mount St. Helen's exploded

in a type of eruption

never witnessed before.

one of the largest landslides

in history triggered a powerful

sideways-directed blast.

Ferocious explosions followed

and formed an immense

mushroom cloud.

Late that afternoon,

the volcano slowed, but more

chaos was about to strike,

as the eruption had triggered

another destructive process.

Hot volcanic ash had melted

the snowfields on top

of Mount St. Helen's.

Hundreds of tons of meltwater

mixed with soil and formed

a series of mud flows

that cascaded down the slopes.

>> Most people don't realize

that this hazard can affect

people living so far downstream

because up to 100 miles

from a volcano can still be

a hazard zone for these kind

of volcanic mud flows.

>> The biggest of the mud flows

came down the Toutle River

Valley. On its way there,

it reached record speeds

of 90 miles per hour

and raced over hills

as high as 20-story buildings.

It destroyed a total

of 27 bridges, nearly 200 homes,

and more than 185 miles

of highway and roads.

Today, its remains are still

preserved on the banks

of the Toutle River.

>> This deposit is an excellent

example of what we scientists

call a lahar, but other people

refer to as a mud flow--

A big massive flow of material

that's much like wet concrete

that was coming down the river

as a huge wall of material,

pushing logs and debris

in front of it.

>> At first, it was a mystery

how this mud flow had become

so big, but the rocks

provided a clue.

>> What we see are rocks of

different types that have come

down from Mount St. Helen's.

Some of these are actually

from the original cone

of the volcano that collapsed

as the debris avalanche.

>> Geologists could now

piece together what caused

these destructive mud flows.

These rocks from high up

in the volcano came from

the landslide that had been

thrown in the valley

in the first minutes

of the eruption.

When meltwater mixed with

the landslide debris,

it formed a gigantic mud flow.

This incredible torrent

lasted till late that night

and dumped more than 65 cubic

miles of mud along the way.

Even 30 years later,

proof of its destructive power

is still in the field.

>> These lahars came down

and totally buried this forest.

We see these standing stumps

of trees that are the remains

of what is often referred to

as a ghost forest.

This was buried by this lahar.

The trees were killed,

and we only see them now because

of the erosion of the river

which has come back in

and eroded into the bank

and uncovered them.

So this is a fantastic clue

of the power and the destruction

of this lahar, which completely

inundated this lower valley.

>> But one deposit farther

upstream was causing confusion

in the investigation.

It looked like a lahar deposit,

but instead of volcanic rock,

it was full of rounded

river pebbles.

>> The scientist who first

studied this wasn't sure

this was a lahar deposit

because it contained so much

of this rounded river rock,

which is characteristic

of streambed deposits

and not lahars.

But then he realized

that it has this very, very hard

compact matrix in it and it was

not bedded or layered in any way

and came to the conclusion

this had to be a huge lahar

that was probably something

on the order of the flow

of the Amazon River.

>> Scientists were stumped.

Not only did this deposit

have rounded river rock,

it was also much bigger than

the deposit from the mud flow

that tore through the valley

on May 18th.

If it wasn't part

of the 1980 lahar,

where did it come from?

Geologists decided

to investigate further and took

samples back to the lab.

Radio-carbon dating showed

that this deposit swept

down the valley 3,000 years ago.

>> If we had been standing on

this spot about 3,000 years ago,

we would have first heard

a very low rumble that would

have gotten louder and louder.

And if we hadn't heeded

that warning, we would have seen

a huge wall of broken trees

and debris coming around

the river bend, probably

at 30 or 40 miles an hour

that would have been

hundreds of feet high.

And that wall of debris

and mud and rock would have

then just swept through here

like a huge freight train,

literally wiping the valley

clean of anything in its path.

>> But where did this ancient

monster flood come from?

The only place that could have

stored that amount of water

and rounded rock was Spirit Lake

35 miles upstream.

This discovery was crucial

because there was

an impending danger that nature

would repeat itself.

Not only did the landslide

that initiated the 1980 eruption

flow west into the Toutle

Valley, it also went east

into Spirit Lake and blocked

its exit. As rivers

and meltwater kept flowing in,

water rose to dangerous levels.

>> It became a very big

wake-up call for the hazards

community, because if this sort

of flood and lahar had happened

in the past, it could happen

again, and the 1980 deposits

dammed Spirit Lake once again

with the same type of weak,

unstable dam that had existed

in the past.

>> Authorities had to act

quickly. Within a couple

of years, Spirit Lake would

have filled up again, and had it

been allowed to overtop,

it would have caused

a catastrophic flood just like

the one 3,000 years ago.

>> The Corps of Engineers

came in. They immediately

devised a plan which involved

pumping water out of the lake

to keep the lake level stable

for the short term.

Their long-term solution

was to drill a boring

through a mountain ridge,

creating a permanent drain

so that Spirit Lake could never

get above that height,

and the danger for

an overtopping flood

was then eliminated.

>> Scientists now understood

what happened on May 18th.

Volcanic rock in mud deposits

along the banks of the Toutle

River valley is evidence that

gigantic mud flows thundered

down Mount St. Helen's,

and rounded river pebbles

in a 3,000-year-old mud deposit

became a warning sign

that Spirit Lake was able

to spill over and cause

an even bigger lahar.

In the summer of 1980,

scientists thought the May

eruption was their chance

of a lifetime, because

major volcanic eruptions

in the Cascades happen only once

or twice every hundred years.

But they were soon to be

proven wrong. After 2 decades

of inactivity, the mountain

began to stir again.

>> The explosion of Mount

St. Helen's in May 1980

scarred the mountain

with a massive crater

on its north side,

but in the summer

after the eruption, the volcano

began to rebuild itself.

Thick magma slowly rose

to the surface and formed

a dome inside the crater.

Had the activities continued

at the same rate, it would have

taken about 200 years

to rebuild the mountain

to its pre-1980 size.

But in 1986, magma flows ceased,

and the volcano died down.

Life returned to normal and

adapted to the new landscape.

Plants and trees took hold

in the fertile volcanic soil.

Elk and other animals

migrated back to the mountain.

Then on September 23, 2004,

the ominous rumbling began again

and put volcanologists

on alert.

>> The entire Cascade range

in the western U.S.

produces on average

about 2 eruptions every century.

So you think, well,

that's one eruption per career.

and St. Helen's in the 1980's

qas ours, and we all assumed

that that was it.

But we got a second chance.

>> Mount St. Helen's

qas cooking up another mystery.

Small earthquakes became

stronger and more frequent.

GPS measurements detected that

the area around the mountain

was sinking.

>> There was one continuously

recording GPS instrument

around the volcano,

and it's code name was JRO-1.

JRO-1 had not moved

in any unusual way

right up until the day

the earthquakes started.

And then on that very day,

it started to move.

It moved toward the volcano

and downward, as if that entire

area of the crust was sagging

down toward the volcano.

>> The only plausible

explanation for the sinking land

was that the magma reservoir

deep underground was shrinking.

In earlier surveys, scientists

had detected a vast pool

of molten rock 8 miles

under the volcano.

If it was getting smaller,

magma had to be on its way up

towards the throat

of the volcano.

The renewed activities

caused widespread concern.

Scientists feared another

eruption was building up.

And they were puzzled what kind

of eruption it would be.

In search of an answer,

scientists turned to St. Helen's

early days.

>> Volcanoes are all

very individual. They have

individual types of eruptions

and traits, and what they've

done in the past is what they're

going to do in the future.

>> The key to past eruptions

is ancient volcanic rock.

Mike Clynne has specialized

in mapping these old deposits.

Southwest of St. Helen's,

he investigates an area covered

with large dark boulders.

A close look reveals the type

of eruption they formed in.

>> We know that this rock

came from a lava flow

because it has big crystals

set in a much finer grained

ground mass of little crystals.

The big crystals grew

in the magma chamber

while the magma was deep

under the volcano.

And the fine-grained ground

mass, which is tiny crystals,

grew when the lava erupted

at the surface and froze.

>> Radio-carbon dating

established the rock was born

2,500 years ago.

The nature of the eruption

it formed in was slow and quiet.

>> This kind of lava flow

erupts from the mountain

as a liquid, and it flows down

the mountainside under gravity.

As it flows away

from the mountain, it cools

until it becomes so viscous

that it can't flow anymore.

So that's where it stops,

and that's what you see here,

is the end result of emplacement

of this kind of lava flow.

So they're not dangerous.

You can stand and watch it

come down at you.

From deposits like these,

scientists could tell

that Mount St. Helen's

had produced a number

of quiet lava outpourings

in the last 300,000 years.

They slowly built up

the mountain from a small

cluster of rock

to a conical-shaped volcano.

But northeast of the mountain,

Clynne finds a different deposit

which tells a story

of a much more dangerous episode

in St. Helen's past.

>> We look at this deposit,

and there's a couple

of characteristics

that are important.

First of all,

that it's very loose,

and that it's composed

of rock fragments that are

all about the same size.

Another important characteristic

is that the rock fragments

are touching each other.

There's no material

in between them.

Well, that tells us

that these rock fragments

came here by falling

out of the air.

It's a big explosive eruption

that sends the material

very high into the sky,

and when the wind dies down,

they start to fall,

and they pile up here.

>> The rock fragments are

very light pumice that formed

during a violent eruption

similar to the one that produced

the huge mushroom cloud

in May 1980. But age-dating

revealed that this deposit

was much older.

This event happened

3,500 years ago, and compared

to the 1980 eruption,

scientists found evidence

that it was much more dangerous

and spewed out 4 times more

rock and ash.

>> This is the biggest eruption

in Mount St. Helen's history,

and it was about a cubic mile

of material that was erupted

at this time. And we know that

because we trace out

the deposit, measure

its thickness and its distance,

and you add that all up

together, and you get

the volume of the eruption.

This deposit can be traced

all the way to central Canada.

>> Studying Mount St. Helen's

past has revealed

that she has an unpredictable

eruptive nature.

>> Mount St. Helen's had

everything from relatively

benign lava flows to quite

violent eruptions in the past.

So it's very hard,

when a volcano starts acting up,

to know which of these

possibilities is going

to happen, and of course,

the various scientists,

we discuss and argue

and all that kind of stuff

what we think is going on,

and nobody truly knows

what's going on.

>> Scientists investigating

Mount St. Helen's have found

clues that show different

eruptive behaviors in her past.

Large dark boulders are evidence

that she is able to produce

slow and quiet eruptions.

A thick deposit of white pumice

is evidence for an ancient

dangerous eruption 4 times

larger than the one in 1980.

Fortunately, the events that

began in 2004 took a lucky turn.

The magma did reach the surface,

but it had lost

its explosive power.

It flowed out like toothpaste,

in a dome-building style

of eruption. As geologists

carried on studying

Mount St. Helen's,

a volcano 4,000 miles away

began to stir. Because of

their experience in 1980,

scientists were convinced

a major catastrophe was

about to unfold, and this time,

12,000 lives were at risk.

>> The eruption of Mount

St. Helen's in May 1980

took scientists by surprise.

It was the first time

they witnessed the failure

of a massive bulge,

a huge landslide,

and a powerful lateral blast.

>> Prior to 1980, we just

didn't have the knowledge

to make those kinds

of specific predictions.

We started learning

in the 1980's at St. Helen's.

We've continued to learn

at volcanoes around the world,

and we've had some successes.

>> Forecasting volcanic

eruptions is difficult because

there is no strict pattern

to the buildup.

But as scientists are getting

more experienced in observing

volcanic behavior,

they are getting better

at their predictions.

In 1995, Soufriere Hills volcano

on the Caribbean Island

of Montserrat became restless.

It had been quiet for 350 years

until earthquakes rumbled it

to life again.

Residents were used to

a gently steaming mountain

and simply hoped

it would die down, but when

the earthquakes got stronger,

officials called for help.

A team of U.S. Volcanologists

Flew to the Caribbean

to monitor the reawakening.

>> There developed a situation

there whereby there was a region

of high seismicity occurred

just as St. Helen's.

If you went up on the mountain,

as we did, they just about

knocked you to your knees.

They were very strong events.

>> Strong earthquakes weren't

the only warning signs.

On the south side

of the mountain, they observed

how a monstrous bulge

began to form.

>> There were cracks

that were occurring.

You could see the cracks

were moving every day.

It looked like the whole side

could fall apart, so we could

get a slope collapse there,

a major slope collapse.

>> By October 1997, the bulge

was growing at a staggering rate

of 280 cubic feet per second.

Scientists were alarmed.

Because of their experience

on Mount St. Helen's,

they knew that a collapse

of the bulge was imminent.

People living in the proximities

of the volcano were in danger.

So they advised the authorities

to evacuate immediately.

>> 8,000 people had to leave

the island. Over 4,000 were

forced to move to a safer

location to the north.

On the 26th of December, 1997,

the volcano struck.

After an intense swarm

of earthquakes, a huge part

of the bulge broke loose

and roared down the valley.

Like at Mount St. Helen's,

the sudden removal of rock

released the pressure

on the magma below.

A lateral blast surged south

and spawned a vertical

ash column 36,000 feet high.

Within 15 minutes, the eruption

destroyed 4 square miles

of the island and completely

buried the island's capital,

Plymouth, under 39 feet of mud.

>> Montserrat is a prime example

where lessons learned

from a big catastrophe

have prevented another one.

>> Almost everything that

occurred at St. Helen's

did occur at Montserrat.

It replicated St. Helen's

not only in the lateral blast

and so forth, but it did

everything that St. Helen's did

on a smaller scale.

>> Scientists studying the

eruption of Mount St. Helen's

on May 18, 1980, have uncovered

a sequence of events

they had never seen before.

A growing bulge on the north

flank of the mountain

was an alarming sign that a pool

of magma was building up

within the volcano.

Grey volcanic rock

from the bulge 5 miles away

was evidence for a powerful

sideways eruption.

A 3,000-year-old mud deposit

became a warning sign that

the valleys around the mountain

have been repeatedly swamped

by huge mud flows.

A deposit of white pumice rock

found all the way up to Canada

showed that Mount St. Helen's

is able to produce eruptions

4 times larger than the one

in 1980.

As the investigation has shown,

Mount St. Helen's is full

of surprises.

>> St. Helen's, I think of it

as a teenager among the Cascade

volcanoes. It's young,

it's vigorously active,

it's explosive,

it's very energetic.

Even though to our eye,

as we look at it,

it appears to be sleeping,

it's active. It's doing

what it's been doing

for tens of thousands of years.

>> Mount s helen's looks

set to continue her erratic

and at times violent outbursts.

Her deadly potential

is a stark reminder

the earth is never at rest.

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