All language subtitles for How the Earth Was Made HC 720p HDTV S01E05 New York EN

Earth, a 4.5- Billion-year-old planet,
still evolving.

As continents shift and clash, volcanoes
erupt and glaciers grow and recede,

the earth's crust is carved
in numerous and fascinating ways,

leaving a trail
of geological mysteries behind.

In this episode,

the 450-million-year-old geological
history of New York City is explored.

A metropolis pockmarked
with strange rocks,

haunted by footprints
of ancient giant reptiles,

and lined with a vast curtain
of solidified lava.

Scientists investigate the evidence

for fiery volcanoes,
massive floods and ice sheets

four times as high
as the Empire State building.

The clues to understanding New York
City's geological past provides a window

into the formation of the earth itself.

The investigation into New York City's
geological history begins here,

with Manhattan's rocky outcrops.

These rocks are clues
to how the land was made

and how its geology helped it become
a dense, thriving, pulsating city.

They're scattered all over Manhattan,

poking through the surface of parks

and through the concrete
between the buildings.

Some, squashed between two apartment
blocks, are the size of a whale.

They are the extraordinary
survivors of ancient times.

Most importantly, they are
the surface tips of the bedrock

in which Manhattan's buildings
are anchored.

Gigantic skyscrapers stand in two
clusters, in downtown and midtown.

In the section between,
the buildings are lower.

The clues to the shape
of Manhattan's familiar skyline

are the rocks beneath the surface.

A leading expert on the rocks in New
York is geologist Charles Merguerian.

The entire history of the
development of the earth's crust

is emblazoned in the rocks beneath us.

The rocks here in New York City
harbour an ancestry

that dates back
over a billion years of time.

Merguerian is searching for evidence to
show how the city's bedrock was made.

At Inwood Hill Park in Upper Manhattan,
he's found an extremely hard piece

of the bedrock
known as Manhattan schist.

To the untrained eye,
it's just a piece of rock,

but to Merguerian, this is his first clue.

The rocks that we're looking at
right here

are rocks of the Manhattan schist
formation

and the... these rocks are very severely
deformed,

and the structures here in this rock
is a structure that comes up like this,

bends around and comes back down
on itself as such,

and in three-dimensional view, it's a
structure that looks something like this.

A very, very tight fold
with a plunge towards the south here.

These are rocks that were
very, very strongly deformed

over protracted periods of time.

And it's the same bedrock that occurs
over much of New York City.

This tight fold in the rock
suggests New York's bedrock

was formed under great pressure.

To confirm this hunch, Merguerian takes
a sample to the lab for detailed analysis.

Radiometric dating proves this rock
is about 450 million years old.

But the rock has even
greater secrets to tell.

It contains a kaleidoscope
of minerals,

which opens a window
into the ancient world.

To me, minerals are like
the instrument cluster in your car,

they tell you everything about
how your car is running.

Merguerian uses a microscope with
polarised light to view the minerals.

The examination tells us
the former depth regime,

how deep the rocks were,
they tell you the age of the rocks,

they tell you everything
you want to know

about the development
of the earth's crust.

What's striking about these samples is
that the minerals inside are elongated.

It is a clue that these rocks must once
have been crushed by massive forces.

And the colours support this theory.

Under the polarised light, the sample
from Inwood Hill Park shows up blue.

This comes from a mineral called
kyanite, which forms at great depths.

It's conclusive evidence that this rock

was compressed
deep under the surface.

Rocks forged at these depths
are much harder,

ideal for a city's foundations.

But what gigantic weight was on top?

Merguerian believes
there is only one answer.

The rock was once buried
under the crushing weight

of a chain of massive mountains.

The minerals that we find
in the bedrock units of New York City

tell us that the rocks of New York City
were formerly buried

when they were formed,
under very high pressures,

and that those high pressures indicate

that these rocks formerly were
produced at depths of 20 to 25 miles,

and probably the mountains were
as high as the Alps are today.

But even the most impressive mountain
chains can't survive the ravages of time.

The Rocky Mountains, for example.

Millions of years ago, they soared
nearly six miles into the sky.

Today, erosion has halved their size.

The same process
happened in New York.

Rain, wind and ice
wore the ancient mountains almost flat.

But the microscopic crystals
found in the rock in Manhattan

testify that they existed in the past.

How did the mountains form?

The answer lies in the way
the earth's crust moves.

A network of interlocking individual
pieces makes up the Earth's surface.

Geologists call them tectonic plates.

Over millions of years, they collide and
break apart to form different continents.

450 million years ago, the Earth's
surface looked completely different.

North America was much further

to the south.

MERGUERIAN: North America
was tilted 90 degrees clockwise

from its present orientation

and it was straddling the equator.

As such, the climate was tropical,

the east coast of North America was
really experiencing Club Med conditions.

The weather
may have been awesome,

but the ancient East Coast
was heading for trouble.

The plate beneath it was moving.

The East Coast was on a collision
course with ancient West Africa.

450 million years ago, they collided.

The impact unleashed geological chaos.

Under intense compression,
the land was forced upwards

to form a soaring range of mountains.

The collision that took place
is the most fundamental

and impressive mountain-building event

to affect the east coast
of North America.

Today, all that remains
are their stumps,

stumps that form the bedrock
of modern-day New York.

The collision that built up the ancient
mountains also folded the bedrock

into dips and rises.

These folds are responsible for
the shape of Manhattan's skyline.

The city boasts two clusters of
skyscrapers in downtown and midtown.

Here, the hard bedrock that formed
deep underground was forced up.

It is now close to the surface
and provides solid anchorage

for the high-rise buildings.

In the dip in the middle
the rock was folded down.

The area is filled with loose sediments,
less suitable for skyscrapers.

MERGUERIAN: When the bedrock
is at the Earth's surface

where it's actually exposed,

then it's pretty easy
to build tall buildings

'cause you can root them
directly into solid rock.

However, in areas where the bedrock
is deep and covered by glacial sediment,

in those cases, it's very difficult
to build tall buildings

because you need to root those
buildings either into solid rock

or build concrete abutments called
caissons that can support tall buildings.

New York's deep history
is beginning to take shape.

Building up from tiny crystals
in the rock,

scientists revealed how
New York's bedrock was formed

under the crushing weight of
a massive ancient mountain range.

The result was hard Manhattan schist,

a perfect foundation
for the city's skyscrapers.

But New York City
still had a long way to go.

The colliding plates created
an enormous landmass -

the last great supercontinent,
called Pangaea.

New York was now trapped
in the centre...

...but somehow it made it back
to the coast.

Hidden beyond the city's streets is
evidence of huge volcanic eruptions,

mass extinctions and continents
torn into pieces.

Clues that could explain

how New York became one of
the world's great maritime cities.

Investigators are piecing together

how New York City's unique geology
was formed.

Much of its early success
as a trade and commerce centre

is owed to its deep-water harbour
and its location at the coast.

But 450 million years ago,
things were different.

The area of New York City
was landlocked,

embedded in the heart
of a huge supercontinent.

How did it get to the coast?

The investigation fast-forwards
250 million years.

In a quarry in New Jersey,
25 miles northeast of Manhattan,

paleontologist Paul Olson
unearths the first of a string of clues

that could explain how
New York City reached the coast.

A giant fossilised footprint.

This is the footprint, actually
the mud that filled in the footprint,

of a four-footed crocodile relative
that was the dominant carnivore

during the late Triassic.

You can see the toes here
have little pads on them

and here's the handprint,
and these animals would have been,

in this case, about the size
of a modest crocodile,

but some of them became much,
much larger, the size even of a T. Rex.

(ROARS)

The footprints are from
a huge crocodile-like creature

called Postosuchus.

It first appeared on the Earth
around 230 million years ago.

Then, some 30 million years later,
its footprints suddenly vanished.

But Postosuchus wasn't alone.

Half of all land animals
perished at the same time.

The fossil evidence proves it to be

one of the biggest mass extinctions
ever recorded.

The evidence
for this mass extinction

is that we have lots
and lots of fossils right in this area.

And what you see is especially
in the... in the reptile footprints,

you see one group of forms,

the forms that are related
to crocodilians, disappear.

Whatever caused the mass extinction
must have been a catastrophic event.

Olsen had a hunch that the
mass extinction was somehow related

to New York's return to the coast.

The ancient area of New York
sat on a line of great weakness,

the plate boundary
where two continents joined

to form the supercontinent Pangaea.

And it was unstable,
prone to earthquakes and volcanoes.

Olsen's quest - to find the evidence
for the natural disaster

that finished off Postosuchus
200 million years ago.

A band of dark rock
above the footprints

in the New Jersey quarry caught his eye.

It was basalt, the smoking gun
Olson was looking for.

Basalt is a volcanic rock.

It forms when hot lava erupts
onto the surface and cools.

Did the volcanoes that forged
this basalt trigger the mass extinction

and also rip Pangaea apart?

On its own, the evidence
at the quarry was unconvincing.

The layer of basalt
is only a few feet thick.

To prove mass lava flows
caused this global catastrophe,

scientists needed
corroborative evidence.

High above the Hudson River,

geologist Matt Gorring
follows another lead.

He's studying the Palisades,

a dramatic geologic feature
that hugs the Hudson River,

beginning across mid Manhattan
and running into northeast New Jersey.

They too are made of basaltic rock,

the same rock implicated
in the mass extinction of land animals.

But the Palisades are on
an altogether different scale.

The Palisades are a sheet of basaltic
magma, about 1,000 feet thick,

it's about 40 miles long,

so it's a very prominent set of cliffs

that run all the way up the west side
of the Hudson River.

Here is proof of
massive volcanic activity.

Hot lava flooded out of ruptures
in the Earth's crust

and covered ancient North America
in a mile-deep sheet.

The lava cracked as it cooled.

The vertical ruptures formed
regular pencil-shaped columns.

These distinctive rock formations
have been known to geologists

since the 19th century.

Intriguingly, they appear
on both sides of the Atlantic,

in North and South America,
Europe and Africa.

Geologists suspected that
their presence pointed to the spot

where Africa and Europe
separated from America.

But that was just an unproven
theory, until the 1950s,

when scientists developed
a revolutionary technique

called paleomagnetism.

Now, they could study
the magnetic properties of rocks.

Many rocks, including basalt,
have a distinctive magnetic signature,

formed as the rock is born.

Tiny crystals inside the rock
act like compass needles.

When the magma that
forms the rock is fluid,

the crystals align to the Earth's
magnetic field, pointing north.

As the rock solidifies,
the crystals freeze,

forever locked
in that fixed magnetic alignment.

As continents move
and the rocks travel,

the crystals end up pointing
in a different direction than north.

Gorring and his team investigate the
magnetic signature of the Palisades.

To get a sample, they bore
into the rock with a water-cooled drill.

They measure the exact orientation
of the crystals today with a compass.

When they offset this reading
with magnetic north,

they can calculate
the original location of the rock.

Uh... 14.

One of the useful things
that you can do with this rock

is you can take it back in the lab
and measure its magnetic orientation,

and that magnetic orientation
will be when this rock crystallised

or solidified 200 million years ago.

So this rock would have minerals
that would be pointing

in some other direction
other than north today.

When scientists compared the
magnetic orientation of the Palisades

with the other basalt outcrops
around the Atlantic,

they discovered they formed at
approximately the same latitude.

Not only did the rocks
have the same age,

they were also born
at the same location.

For example, 200 million years ago, New
York and Morocco were neighbours.

The geologists had
all the proof they needed.

They could now confidently
piece together what happened.

It began with a global volcanic disaster.

About 200
million years ago,

North America and Africa
began to pull apart from each other.

There were gigantic lava outpourings.

These lava flows erupted along
very long cracks in the Earth's crust,

that would have produced
fountains of lava

extending thousands and thousands
of feet into the atmosphere.

They covered an almost
inconceivably large area,

roughly four million square miles,

from southwestern France
to southwestern Brazil,

from New York to central Mali in Africa.

This area was covered
in ponded lava flows

that in some places ended up
being nearly a mile thick.

Volcanic eruptions led to
soaring temperatures.

Half of the plants and animals died.
Postosuchus didn't stand a chance.

As enormous forces tore Pangaea apart,

a giant sea formed between
the separating land masses -

the Atlantic Ocean.

The city of New York
was now at the coast.

200-million-year-old footprints
beneath a layer of basalt

and the Palisades
towering above the Hudson

provide evidence
that Pangaea split apart

to create the east coast
of North America.

But the story of New York City
was far from over.

After being built by fire, the region
was about to be overcome

by another destructive force.

Scientists are piecing together the story
of New York's violent geological past.

250 million years ago,
the Atlantic Ocean opened up,

leaving the area of New York
on the coast.

But the maritime city
still had a long way to go.

There was no deep
Hudson River channel.

It was nothing but a small stream.

What forces transformed it
into the wide river

capable of carrying heavy freighters
far inland?

A clue to how the Hudson Valley
was created is the strange boulders

that are scattered throughout
Manhattan's Central Park.

Some of them weigh several tons.

But they're strangers to these parts,
totally unlike the surrounding rocks.

Geologist Charles Merguerian
investigates where they came from.

This boulder is a boulder
from the Palisade sheet

on the other side of the Hudson
in New Jersey.

You can see it's very nicely polished.

Compositionally, it's totally different
than the surrounding bedrock,

which is Manhattan schist,

and the Manhattan schist here
is very rich in mica.

This rock has no light-coloured mica
in it whatsoever.

The Palisade sheet is located
to the west and north of us.

The Palisades run for 40 miles
along the Hudson River.

Something immensely powerful
must have moved the boulders

such a great distance.

Merguerian knows the answer is ice.

Scientists have noticed a similar
phenomenon 4,000 miles away

in the Swiss Alps.

As huge glaciers grind their way
across the landscape,

they gouge out lumps of rock and
carry them along in the base of the ice.

These rocks act like sandpaper
and carve out deep scratches.

When the ice melts,
it leaves the boulders behind.

Merguerian is convinced the same thing
happened in Central Park.

Ice moved the Palisade boulders

and carved out grooves
in the bedrock underneath.

This bedrock exposure in Central Park
shows the profound effects of glaciation

in the form of these
spectacular glacial grooves

that move up the outcrop
and show this pattern

where glaciers grabbed huge boulders

and those huge boulders acted
like tools to produce these scratches.

To Merguerian, the rocks in Central Park
are compelling evidence

that New York once was covered in ice.

Over millions of years,
growing and receding ice

has repeatedly turned North America
into a frozen wilderness.

But the grooves on the rocks
in New York don't tell the full story.

The destruction caused by the ice
points to a gigantic glacial event

that would dwarf the future metropolis.

To find out the extent of the ice,

Merguerian travelled to Bear Mountain,

some 50 miles north of New York City.

Once again, the clue was in the rocks.

He found glacial marks
similar to those in Central Park.

What we're looking at here
are chatter marks.

Chatter marks are very
diagnostic features of glacial erosion,

they're produced by boulders embedded
in the base of a thick sheet of glacial ice.

Those boulders impinged on
this bedrock surface,

polishing it, smoothing it off
and then plucking pieces of rock off

as the glacial ice moved over with
the boulders embedded in the base.

The gouges in the rocks
could mean just one thing.

The glacier must have been
thousands of feet thick.

In this case, although
we're standing at an elevation

of about 1,280 feet above sea level,

the glacial ice sheet covered Bear
Mountain as if it weren't even there.

Scientists have found identical
chatter marks on nearby peaks

up to a mile above sea level.

It was unmistakable proof.

A glacier at least one mile thick
ground its way across these mountains.

Did this same ice sheet also
plough through Central Park?

Another nearby rock face
provided the answer.

The feature
that we're looking at here

are a series of sub-parallel
glacial scratches and grooves

and these are, again, are produced
by the glacial ice sheet

dragging boulders across
this very durable granite surface,

it's kind of polished the surface.

And, in addition, it's produced these
rather subtle but... but... but obvious,

when the lighting is right,
striae or grooves in the bedrock.

Now if we measure
the orientation of these...

...these... these come out about
north 20 degrees west,

just about identical in orientation to the
striae that we measured at Central Park.

It's significant evidence that the ice
sheet that covered Bear Mountain

also flowed over the surface
of Central Park.

Proof that New York City was covered
by a glacier four times higher

than the Empire State Building.

MERGUERIAN:
Just imagine glacial ice,

a huge thick ice sheet over a mile thick,

exerting tremendous pressure
on the surface

and sculpting the surface into
the landscape that we see today.

The ice sheet's crushing weight
bulldozed everything in its path

and cut through the remains
of the ancient mountains.

Before the ice arrived,

the waters of the Hudson River had
gently cut down through the landscape

to form a V-shaped valley.

But a mile-thick glacier
takes no prisoners.

It gouged out the sides
and the bottom of the river valley

and turned it into a U-shaped riverbed.

The Hudson River was now
navigable for big ships.

The picture of modern-day
New York was almost complete.

Long grooves in the rocks in
Central Park showed scientists

that a vast ice sheet flowed over the
eroded remains of these mountains.

And marks and glacial striations
on Bear Mountain

proved this ice sheet was
at least four times higher

than the Empire State Building.

When the ice melted,

it left a vast ridge of debris blocking
the Hudson River from the Atlantic.

The final challenge for geologists was
to find out how the ridge was destroyed

and how New York's harbour
opened to the oceans.

New York today boasts one of
the largest natural harbours in the world.

But it wasn't always that way.

Towards the end of the last Ice Age,
the port's wide entrance was blocked.

16,000 years ago, the melting glaciers
left behind a 220-foot-high wall of debris.

The ridge stretched from Long Island
to Staten Island

and forced the Hudson River
through a narrow,

more westerly course to the ocean.

What powerful forces
destroyed this rock jam?

The prime suspect was a flash flood.

But scientists needed evidence
to prove that such a flood had happened.

In the 1960s, fishermen
made an unexpected find

at the mouth of the Hudson River.

They dredged up a giant mammoth tusk
from the depths of the sea floor.

(TRUMPETS)

Herds of these giant beasts
roamed the plains of North America

before they became extinct
at the end of the last Ice Age.

Finding the odd mammoth tusk here
and there is not so surprising.

But since the initial discovery,

scientists have found hundreds
more tusks and bones

in the mouth of the Hudson River.

It was as though a violent torrent
swept the mammoths away

and dumped their remains off the coast.

And there were more clues nearby.

Huge boulders resting
on the sandy sea floor,

some of them as big as cars.

The boulders must have been part
of the ancient moraine

that once ran
between Long Island and Staten Island.

Geologist David Franzi knows that only
a raging torrent could have shifted them.

Based on the size of the boulders
that we see here,

we know that that flood
must have discharged

on the order of 1.5 million cubic feet
per second.

That's three times larger than the largest
Mississippi River flood ever recorded.

And all that water had to
come from somewhere.

Scientists began looking
for the source of this flood,

a flood powerful enough to transport
huge boulders all the way to the sea.

Rocks to a geologist are
like pages in a history book.

For us, erosion oftentimes rips some
of the pages out of our history book,

so it's the job of the geologist
to put together a fragmentary record

into a coherent history of the events
that happened in the past.

300 miles north of the city,
in upstate New York,

Franzi tracked what might
have been the flood's path

to an unusual grove of trees

on Covey Hill

in the Adirondack
Mountains.

The trees are jack pines.

They are rare in this area, where
the soil is usually fertile and deep.

But on Covey Hill, their presence shows

that there is no more than a few inches
of soil on top of the bedrock.

The jack pine
is essentially rooted

right on the top
of a rock's surface here.

This is a bare sandstone surface,
very little mineral soil,

and it's subject to prolonged periods
of dryness during the summertime.

Jack pine's adaptations
make it able to survive here

where no other tree species can.

What happened to the soil?

The jack pines continue to grow

at the entrance of a long gorge
over 300 feet wide.

Usually, gorges like this are cut down
over thousands of years,

but in this case, the missing topsoil
points to a sudden flood event.

A raging torrent must have ripped away
the soil and cut deep into the rock.

In a helicopter,
Franzi follows the gorge west.

Eventually, it opens
into a vast, empty basin

located next to one of
the Great Lakes, Lake Ontario.

It doesn't take much of a stretch
of the imagination here

to imagine this valley filled with water

and then with these hills
poking up through as islands.

16,000 years ago, this basin was filled
with two billion cubic miles of water -

a huge lake geologists
call Lake Iroquois.

It formed at the end of the last Ice Age.

As glaciers receded, the melt
waters slowly filled up the lake.

The ice dams holding
the waters weakened.

Eventually, the dams collapsed, causing
sudden and devastating flash floods.

Lake level dropped
on the order of 70 feet

and about 160 cubic miles of water

were released into the
Champlain Valley, catastrophically.

That floodwater would have
coursed down the Champlain Valley,

through the Hudson Valley and
ultimately out into the Atlantic Ocean.

The torrent raced towards
New York City, 300 miles to the south,

then took the straightest course
to the sea.

The floodwaters smashed into
the ancient moraine,

the huge pile of debris blocking
the direct exit of the Hudson River.

The bridge we see behind me
spans the channel that was cut

by the flood event.

When the flood wave came through,
it was of sufficient intensity

to over-top the dam
and very rapidly cut the channel.

The gap that was created
by the flood still exists.

It's now a tidal strait called the Narrows.

Today, the gap is spanned
by the Verrazano Bridge.

The channel is deep enough
for even the biggest ocean-going ships.

It's the most important entrance
to New York City's harbour.

Mammoth tusks and huge boulders
at the mouth of the river

showed scientists that there was
a torrent big enough to blast a hole

through the ancient moraine.

And a channel leading towards the Great
Lakes revealed the source of the flood.

It was this flood that created
the Narrows

and gave New York a wide entrance
to its port.

A unique geology laid down
the foundations for New York City.

But the same forces that constructed it
may also have sown the seeds

for New York's destruction.

Scientists have pieced together the
half-billion-year history of New York City.

Huge mountains, volcanic eruptions
and glacial ice shaped the area.

But New York's story doesn't end here.

The geology that created one
of the greatest cities on Earth

also has the potential to destroy it.

Experts have been studying
the potential threat to the city.

We're standing here
in Lower Manhattan

on one of our major thoroughfares,

Canal Street.

And it's important, because in 1821,

a Category 2 hurricane raised the water
level at the Battery 13 feet in one hour

and, literally, the Hudson River
met the East River

and Canal Street was covered by water

and Manhattan was actually two islands
for three hours, until the water receded.

New York City is vulnerable
because of its position on the coast.

Long Island stretches northeast at a
right angle from the New Jersey shore.

New York City is nestled behind
the western end of Long Island.

Normally, the island protects the city

from the sea,

but when hurricanes threaten,
the opposite is true.

Long Island becomes
a dangerous liability.

Hurricanes racing north along
the beaches of the Atlantic coast

pile up huge bulges of water
in front of them.

They're called storm surges.

Hitting the right-angled junction
at Long Island,

the winds funnel the storm surge in
through the Narrows,

the gap between Long Island
and New Jersey.

This is the place, at the actual apex
of the right angle in New York

where all the water being pushed
by a hurricane would be concentrated.

And in the distance
is the Verrazano Bridge,

and all that water is gonna go through
the passage we call the Narrows

and it's gonna be accelerated
towards New York City,

where it will rise to abnormal heights.

Experts believe that in the United States,

New York is the third most vulnerable
city after Miami and New Orleans

to a hurricane disaster.

If it was hit today, the consequences
would be serious.

New York City is hit by hurricanes
only infrequently.

Like, in 1821 and in 1893 and in 1938.

However, the point is that the hurricane

that will eventually hit New York City
again will be catastrophic,

and what is going to happen
when the utilities are knocked out?

What is going to happen when
salt water reaches into the subways

and ruins the electrical system?

We're talking about unbelievable
amounts of money

to restore the infrastructure.

We're talking about setbacks
and delays in commerce

and banking and transportation,
a catastrophe that's never been seen.

Storm surges are not the only threat
to New York's future.

Earthquakes are also part of the vast
geological forces that shape this area.

They are still at work today.

Some could change the city
in an instant.

November 4th 1884.

New York City was shaken by an
earthquake that lasted ten seconds.

The Brooklyn Bridge swayed
and people panicked.

The earthquake showed 5.5
on the Richter scale.

January 17th 2001,
New York City was struck again.

This time the quake was
relatively small, only 2.4,

but it struck right under 125th Street.

The earthquake in 2001
is the first earthquake

that we could confidently locate
in Manhattan,

that's its claim to fame, it was felt widely.

It's impossible to study
the cause of the quakes at the surface.

The evidence is buried beneath the city.

Deep within New York's bedrock,

seismologist Leonardo Seeber
studies the cause of these quakes.

(HORN BLARES)

In a subway tunnel 100 feet beneath
the bedrock under the East River,

there is a ready-made laboratory.

Here, Seeber can study the rocks
up close and personal.

It is the same bedrock
Manhattan is built on.

But Seeber fears it isn't as
solid as once was thought.

New York area is considered
a seismic zone,

meaning there is a cluster
of known earthquakes

that have occurred in this area.

So we are, as geologists, very eager
to discover which faults are responsible

for these earthquakes.

The majority of earthquakes
occur at the boundaries

between separate sections
of the Earth's crust,

the tectonic plates
on which the continents sit.

But New York's earthquakes
are different.

The city is firmly in the middle
of a tectonic plate,

halfway between the
mid-Atlantic ridge to the east

and the San Andreas Fault
to the west.

Seeber is anxious to discover
what's going on.

As he examines the walls of the tunnel,
he comes across a possible clue.

The long fractures in the rock
are fault lines that formed

when pressure built up.

As the tension was released,
the rock cracked and shifted.

This is felt on the surface
as an earthquake.

SEEBER: This is an example
of a very small fault,

but it's a fault that probably did
generate some small earthquakes.

When one of these faults
generates an earthquake,

we think that perhaps other faults of the
same family can generate earthquakes.

These faults in New York's bedrock
are evidence

that the area was hit by earthquakes
in the past.

But Seeber has no way of knowing if
the faults are still active and dangerous.

If a large earthquake
hit New York today,

the consequences
would be catastrophic.

A large proportion
of New York City buildings

are simply not built to withstand
earthquake shaking.

We worry about transportation tunnels,

in particular, tunnels that traverse rivers

where parts of the tunnels
are rooted in solid rock

and other parts
are resting on soft sediment.

The oscillation of these
two different materials

could cause severe
cracking and fracturing.

The infrastructure
would be severely damaged,

it would take tens of years to repair the
damage caused by such a large event.

With the evidence geologists
have collected,

the story of the creation of
New York City can now be told.

The city's bedrock was formed under
a chain of mountains over a mile high.

Volcanoes and lava fields
over millions of square miles

split up the ancient supercontinent and
created the east coast of North America.

Glacial ice, four times as high
as the Empire State building,

carved out the deep Hudson River.

A catastrophic flash flood
broke through the moraine

to form the Narrows and opened up
New York City's harbour to the oceans.

Looking ahead to the distant future,

geologists see more
challenging times for the city.

In 40,000 years,

they predict this region will
be engulfed by another ice sheet.

And in 250 million years,
the Atlantic will start to shrink again.

Europe and Africa will eventually
crash back into the American coast.

The fossilised remains of the
once great city of New York

will become just another layer of rock
in a vast new mountain range.

A footnote in the immense,
ever-changing story of planet Earth.