Volcanic Odysseys

In Mexico, millions of people live in the shadow of one of the Earth's most dangerous forces,

where volcanoes have buried villages...

This is all that is left of the village of San Juan.

...and influenced culture.

This is the plant that's being used to produce tequila.

Where we get a bird's eye view at danger...

I don't know if I want it to erupt right now or not.

...and witness the raw power of Volcano de Colima at night.

Look

ed like a fire fountain probably 500 metres high.

The Earth's continents are moving.

Between North and South America,

Mexico is being torn apart, bent,

and thrust upwards on a slow-motion tectonic

roller coaster.

One of the by-products of this movement is volcanism,

and Mexico has two of North America's most active

and dangerous volcanoes.

Millions of people live in their shadows.

They largely rely on faith

and Mexico's civil protection to keep them safe

from the dangers posed by these volatile giants.

This is Puebla, Mexico's fourth largest city.

1.4 million people live beside the country's biggest,

Popocatepetl, which translates into "smoky mountain."

It's an active volcano, with an extremely violent past.

Volcanoes like Popo are of major interest

to volcanologists like Dr. Tom Pfeiffer.

Po

pocatepetl is a massive stratovolcano. In fact, it is not only one of the largest

and most active volcanoes in volume and by activity,

but it also is the second highest active volcano

in the whole of North America.

Popocatepetl is a highly explosive volcano.

It has had 15 major eruptions

since the arrival of the Spanish in the 16th century...

and five cataclysmic eruptions in the last 10,000 years.

Eruptions so massive that the volcano blew itself apart.

For sixty years, Popocatepetl remained relatively quiet, but re-awoke violently in December of 1994.

The first eruptions were quite powerful and sent bombs to three, four kilometres' distance.

We just saw one of these explosions that occur

maybe on average 10 times a day at the moment.

You see an ash plume that has risen to approximately

I'd guess one kilometre above the crater.

So nothing really that small.

If you had been very close it would be quite impressive.

Since 2013 it has been erupting consistently,

and lately activity has been on the rise

raising new fears that a major eruption

might be on the horizon.

This volcano has clearly the potential to threaten

the area where we are, but also beyond.

Parts of Mexico City are built on deposits

from Popocatepetl, showing that in the past

eruptions have been able to affect that area.

So it is a volcano that is on the top priority list

of potential natural disasters to monitor.

Puebla was founded nearly 500 years ago

by Spanish colonialists.

They had no way of knowing the extreme danger

the volcano posed...

that buried beneath their feet

lay the remnants of the ancient centre of Cholula,

a historic city that was wiped out

by an eruption of Popocatepetl.

The event was rated as a Plinian eruption similar to the colossal event that buried Pompeii.

Plinian Eruptions generate massive ash clouds

and pyroclastic flows:

hot avalanches of debris and ash

that destroy everything in their path.

When the Spanish colonials started to build

the Our Lady of the Virgin Sanctuary

they did not realize that the hill they chose to build on

was the ruins of Mexico's largest pre-Hispanic pyramid

buried beneath ash and debris.

I think here's a very interesting spot

because we see the actual deposit of the ash flow

that invaded the city.

You see it's full of pottery fragments and debris

from building material.

This came racing down and destroyed anything

that was rising higher than a certain level.

Because of the significance of the 16th century church,

only part of the pyramid has been excavated.

To a

ccess the structure, archaeologists cut tunnelsinto the volcanic debris.

This archaeological site is one of several around Popo

that were buried in this eruption.

In places the debris is two stories deep.

I get this claustrophobic feeling

with these little alleys going here and there.

I think I have the sense of orientation.

We came from there, so we get back onto

the main tunnel over there, yeah.

An interesting, eerie feeling.

Most of the tunnels are bricked up for support.

What Tom is looking for is evidence that the pyramid

was buried by ash fall or a pyroclastic flow.

Look, this is part of the old structure.

It's clearly built.

And this is the loose deposit, the ignimbrite.NARRATOR: Ignimbrite is formed from compacted hot ash

and material that has flowed off the volcano

during an eruption.

You can see the marks where the deposit was removed.

The remnants of these ancient cities

are a potent reminder of what could be

if Popo erupts again.

The worst-case scenario would be the collapse

of the cone during a major eruption.

But this is not the only hazard they pose.

A volcano's hydrothermal system effect

runs deep underground and often extends

beyond the volcano's structure.

Popo's underground system is so massive

that parts of Mexico City have been built on it.

This means that there can be far-reaching effects

beyond the normal eruption behaviour.

Buried within the crowded city,

a geological feature said to be a volcano

has fascinated people for a thousand years.

If it was a volcano you would expect lava,

but this is definitely not lava.

So, if you look at this dented surface

that's almost exactly what you would find

in limestone areas where caustic processes

chemically erode the limestone.

Basically it's the same stuff you find in the tubes

of your washing machine when you have

hard mineral-rich water; it's deposits from water.

The cone does have a crater on the top

that descends below ground level.

It may not be from a volcano,

but it is likely the by-product of one.

As magma rises, it comes into contact

with the area's hydrological system.

Underground water is converted to steam

and it can erupt as a geyser.

It's said that it appeared in 1063 AD

during an eruption of Popocatepetl.

So from far maybe you would see steam,

hot steam coming out from there

that people would associate with volcanic activity.

Here you see it very nicely, how this cone is built up

by individual layers, some very thin like here,

and you see those typical cinder structures:

deposits left from water that was rich

in mainly carbonites.

So since its formation the hydrothermal system

that once fed it has apparently changed again.

But remember, it's a thousand years ago.

It's rare you can go inside.

The probability of a geyser or a volcano

opening in downtown Cholula or Mexico City

is not very high.

But it is possible.

It happened eight hours away in Michocan State.

In 1943, a fissure opened up in a Mexican corn field

near the town of San Juan.

A farmer was there planting his field

and he witnessed the moment the volcano was born.

Fissures opened, lava came out,

and he probably probably witnessed rocks

being blown out and eventually saw fire underneath.

That must have been an amazing event.

The volcano is called Paricutin

and it erupted for nine years

before it went extinct.

The event was a field day for scientists.

It was the first time that a volcano had been witnessed

from its birth to its death.

CNN listed it as one of

the Seven Natural Wonders of the World.

Paricutin's lava flow was slow enough

that most of the people in the area were able to move away

before the lava and ash destroyed their homes.

With the exception of the stone church,

Paricutin completely destroyed the town of San Juan.

Today marks the 72nd anniversary

of the start of the eruption.

As they do every year, the locals mark that day

with celebrations.

And the ruins of San Juan's church

play a major part in that.

To get to the church you can either walk for two hours

or go by horseback.

For me it's an

adventure. I'm not very familiar with horses.

Are you going to be okay?

You're not going to throw me into the lava?

Go, go... Go, yes.

This is all what is left of the village of San Juan.

The front part of the main church, with one tower,