Yellowstone Park
transcript of radio talk show regarding the potential for
a super eruption
Supervolcanoes BBC2 9:30pm Thursday 3rd February
2000
NARRATOR (SINÉAD CUSACK): Yellowstone is America's first and
most famous National Park. Every year over 3 million tourists visit this
stunning
wilderness, but beneath its hot springs and lush forests lies
a monster of
which the public is ignorant.
PROF. ROBERT
CHRISTIANSEN (US Geological Survey): Millions of people come to
Yellowstone every year to see the marvellous scenery and the wildlife and
all and yet it's clear that, that very few of them really understand that
they're
here on a sleeping giant.
NARRATOR: If this giant
were to stir, the United States would be devastated and the world would be
plunged into a catastrophe which could push humanity to the brink of
extinction.
PROF. ROBERT SMITH (University of Utah): It would be
extremely devastating on a scale that we've probably never even thought
about.
PROF. BILL McGUIRE (Benfield Greig Centre, UCL): It would
mean absolute
catastrophe for North America and the problem is we know
so little about these
phenomena.
NARRATOR: In 1971 heavy
rain fell across much of east Nebraska. In the summer palaeontologist Mike
Voorhies travelled to the farmland around the mid-west town of Orchard.
What he was to discover exceeded his wildest dreams.
PROF. MIKE
VOORHIES (University of Nebraska): Well I was walking up this gully
looking for fossils, the way I'd walked up a thousand gullies before,
keeping my eye on the ground looking for pieces of fossils that might have
washed down in the rain the previous night and I scrambled up to the top
and I saw something that completely astounded me, a sight that no
palaeontologist has ever seen.
NARRATOR: It was a sight of
sudden, prehistoric disaster. Voorhies's digging
revealed the bones of
200 fossilized rhinos, together with the prehistoric
skeletons of
camels and lizards, horses and turtles. Dating showed they had all
died
abruptly 10 million years ago.
MIKE VOORHIES: It suddenly dawned on
me that this was a scene of a mass
catastrophe of a type that I'd
never, never encountered before.
NARRATOR: The cause of death,
however, remained a mystery. It was not from old age.
MIKE
VOORHIES: I could tell by looking at the teeth that these animals had died in
their prime. What was astounding was that here were young mothers and and their
babies, big bull rhinos in the prime of life and here they were
dead
for no, no apparent reason.
NARRATOR: For the animals
at Orchard death had come suddenly. There was another strange feature to
the skeletons, an oddity which offered a crucial clue about the cause of
the catastrophe.
MIKE VOORHIES: We saw that all of these skeletons
were covered with very
peculiar growth, soft material that I first
thought was a mineral deposit. Then
we noticed that it was cellular.
It's biological in origin so there was
something actually growing on
those bones. I had no idea what that stuff was,
never seen anything
like it.
NARRATOR: A palaeo-pathologist, Karl Reinhard, was sent a
sample of the bones.
PROF. KARL REINHARD (University of Nebraska):
This specimen is typical of the
rhino bones. You see this material, in
this case it's a whitish material that's
deposited on the surface of
the original bone. This is peculiar to me, but as I
thought back in my
experience I realised that this was similar to something
that turns up
in the veterinary world, a disease called Marie's
disease.
NARRATOR: Marie's is a symptom of deadly lung disease.
Every animal at Orchard
seemed to be infected.
KARL
REINHARD: One of the clues was that all of the animals had it. Now that
is
a very important observation for all the diseases, all the animals
to exhibit
this disease there had to be some universal
problem.
NARRATOR: Scientists discovered the universal problem was
ash. 10 million years
ago ash had choked them to
death.
KARL REINHARD: It may have been a bit like pneumonia with
the lungs filling
with fluid, except in this case the fluid would have
been blood for the ash is
very sharp. There'd be microscopic shards of
ash lacerating the lung tissue
and, and causing the bleeding. I would
imagine these animals as stumbling
around the thick ash, spitting up
blood through their mouths and gradually
dying in a most miserable
way.
NARRATOR: Only a volcano could have produced so much ash, yet
the wide flat
plains of Nebraska have no volcanoes.
MIKE
VOORHIES: I remember some of my students and I sitting around after
a
day's digging and just speculating where did this stuff come from?
There, there
are no volcanoes in Nebraska now. As far as we know there
never have been. We,
we obviously had to have volcano somewhere that,
that produced enough ash to
completely drown the landscape here, but
where that was really was anybody's
guess.
NARRATOR: One
geologist in Idaho realised there had been a volcanic eruption
which
coincided with the disaster at Orchard 10 million years ago, but the
site
was halfway across North America.
PROF. BILL
BONNICHSEN (Idaho Geological Survey): It seemed like a
really
fascinating story which made me think, because I had been
working on volcanic
rocks in south-western Idaho that potentially could
make lots of ash and, and
there was some age dates on that that were
around 10 million years and I began
to wonder wow, could this situation
in Nebraska have really been caused by some
of these large eruptions
that evidently had happened in south-western Idaho.
NARRATOR: The
extinct volcanic area, Bruneau Jarbridge, was 1600 kilometres
away, a
vast distance. How could this eruption have blasted so much ash so
far?
Bonnichsen was sceptical.
BILL BONNICHSEN: Volcanoes
will spew ash for a few tens or maybe a few hundreds
of miles. This
ash, and it's like two metres thick, in Nebraska is 1600
kilometres or
more away from its potential source, so that's an amazing thing.
There
really had been no previous documentation, to my knowledge, of
phenomenon
like that.
NARRATOR: Despite his doubts
Bonnichsen decided to compare the chemical content
of ash from the two
sites. He analysed samples from both Bruneau Jarbridge and
Orchard and
plotted their mineral composition on a graph looking
for
similarities.
BILL BONNICHSEN: if you have a group of
rocks that are very similar to one
another they should be a closely
spaced cluster of pods. We had these analyses
come out from the Orchard
site and I thought I'd try the clock again and see
how close they were
to one another. By golly, they fall right in the same
little trend as
the Bruneau Jarbridge samples.
NARRATOR: Bonnichsen's hunch had
proved correct. Bruneau Jarbridge was
responsible for the catastrophe
at Orchard. An eruption covering half of North
America with two metres
of ash was hundreds of times more powerful than any
normal volcano. It
seemed almost unbelievable, but then Bruneau Jarbridge was
that rarest
of phenomena which scientists barely understand and the public
knows
nothing about: a supervolcano.
ROBERT SMITH: Supervolcanoes are
eruptions and explosions of catastrophic
proportions.
BILL
McGUIRE: When you actually sit down and think about these things they
are
absolutely apocalyptic in scale.
PROF. MICHAEL RAMPINO
(New York University): It's difficult to conceive of a,
of an eruption
this big.
NARRATOR: Scientists have never witnessed a supervolcanic
eruption, but they
can calculate how vast they are.
BILL
McGUIRE: Super eruptions are often called VEI8 and this means that
they
sit at point 8 on what's known as a volcano explosivity index. Now
this runs
from zero up to 8. It's actually a measure of the violence of
a volcanic
eruption and each point on it represents an eruption 10
times more powerful
than the previous one, so if we take Mount St.
Helens, for example, which is a
VEI5, we can represent that eruption by
a cube of this sort of size, this
represents here the amount of
material ejected during that eruption. If you go
up step higher and
look at a VI6, something of the Santorini size for example,
then we can
represent the amount of material ejected in Santorini by a cube of
this
sort of size, but if we go up to VEI8 eruptions then we're dealing
with
something on an altogether different scale, a colossal eruption
and you can
represent a VI8, some of the biggest VI8 eruptions by a
cube of this, this sort
of size. It's absolutely
enormous.
NARRATOR: Normal volcanoes are formed by a column of
magma, molten rock, rising
from deep within the Earth, erupting on the
surface and hardening in layers
down the sides. This forms the familiar
dome or cone-shaped mountains.
BILL McGUIRE: Most people's idea of
a volcano is a lovely symmetrical cone and
this involves magma coming
up, reaching the surface, being extruded either as
lava or as explosive
eruptions as, as ash and these layers of ash and lava
gradually
accumulate until you're left with a, a classic cone
shape.
NARRATOR: Vulcanologists know this smooth flowing magma
contains huge
quantities of volcanic gases, like carbon dioxide and
sulphur dioxide. Because
this magma is so liquid these gases bubble to
the surface, easily escaping.
There are thousands of these normal
volcanoes throughout the world. Around 50
erupt every year, but
supervolcanoes are very different in almost every way.
First, they
look different. Rather than being volcanic mountains,
supervolcanoes
form depressions in the ground. Despite never having seen
a
supervolcano erupt, by studying the surrounding rock scientists have
pieced
together how supervolcanoes are formed. Like normal volcanoes
they begin when a
column of magma rises from deep within the Earth.
Under certain conditions,
rather than breaking through the surface, the
magma pools and melts the Earth's
crust turning the rock itself into
more thick magma.
Scientists don't know why, but in the case of
supervolcanoes a vast reservoir
of molten rock eventually forms. The
magma here is so thick and viscous that it
traps the volcanic gases
building up colossal pressures over thousands of
years. When the magma
chamber eventually does erupt its blast is hundreds of
times more
powerful than normal draining the underground reservoir. This
causes
the roof of this chamber to collapse forming an enormous crater.
All
supervolcano eruptions form these subsided craters. They are called
calderas.
BILL McGUIRE: The main factor governing the size of
eruptions is really the
amount of available magma. If you've
accumulated an enormous volume of magma in
the crust then you have at
least a potential for a very, very large eruption.
NARRATOR: The
exact geological conditions needed to create a vast magma chamber
exist
in very few places, so there are only a handful of supervolcanoes in
the
world. The last one to erupt was Toba 74,000 years ago. No modern
human has
ever witnessed an eruption. We're not even sure where all the
supervolcanoes
are. Yellowstone National Park, North America. Ever
since people began to
explore Yellowstone the area was known to be
hydrothermal. It was assumed these
hot springs and geysers were
perfectly harmless, but all that was to change.
ROBERT
CHRISTIANSEN: I first came to Yellowstone in the mid-1960s to be a
part
of a major restudy of the geology of Yellowstone National Park,
but at that
point I had no idea of what we were to
find.
NARRATOR: When geologist Bob Christiansen first began
examining Yellowstone
rocks he noticed many were made of compacted ash.
But he could see no extinct
volcano or caldera crater, there was no
give-away depression.
ROBERT CHRISTIANSEN: We realised that
Yellowstone had been an ancient volcanic
system. We suspected that it
had been a caldera volcano, but we didn't know
where the caldera was or
specifically how large it was.
NARRATOR: As he searched throughout
the Park looking for the volcanic caldera
Christiansen began to wonder
if he was mistaken. Then he had a stroke of luck.
NASA decided to
survey Yellowstone from the air. The Space Agency had designed
infrared
photography equipment for the moon shot and wanted to test it over
the
Earth. NASA's test flight took the most revealing photographs of
Yellowstone
ever seen.
ROBERT CHRISTIANSEN: What was so
exciting about looking at the remote sensing
imagery was the sense that
showed it in one, one sweeping view of what this
truly
was.
NARRATOR: Christiansen hadn't been able to see the ancient
caldera from the
ground because it was so huge. It encompassed almost
the entire Park.
ROBERT CHRISTIANSEN: An enormous feature. 70
kilometres across, 30 kilometres
wide. This had been a colossal
supervolcano. Certainly one of the largest known
anywhere on
earth.
NARRATOR: Bob Christiansen was determined to find out when
Yellowstone had last
erupted. He began examining the sheets of hardened
ash, dozens of metres thick
blasted from the ground during the
eruption. What he found was 3 separate
layers. This meant there had
been 3 different eruptions. When Christiansen and
his team dated the
Yellowstone ash he found something unexpected. The oldest
caldera was
formed by a vast eruption 2 million years ago. The second eruption
was
1.2 million years old and when he dated the third and most recent
eruption
he found it occurred just 600,000 years ago. The eruptions
were regularly
spaced.
ROBERT CHRISTIANSEN: Quite
amazingly we realised that there was a cycle of
caldera-forming
eruptions, these huge volcanic eruptions about every
600,000
years.
NARRATOR: Yellowstone was on a 600,000 year
cycle and the last eruption was
just 600,000 years ago. Yet there was
no evidence of volcanic activity now. The
volcano seemed extinct. That
reassuring thought was about to change. There was
another geologist who
was fascinated by Yellowstone's volcanic history. Like
Bob
Christiansen, Professor Bob Smith has been studying the Park for much
of
his career. In 1973 he was doing field work, camping at one end of
Yellowstone
Lake.
ROBERT SMITH: I was working at the south
end of this lake at a place called
Peal Island. I was standing on the
island one day and I noticed a couple of
unusual things. The, the boat
dock that we normally would use at this place
seemed to be underwater.
That evening as I was looking over the expanse of the
south end of the
lake I could see trees that were being inundated by water. I
took a
look at these trees and they were be, being inundated with water a
few
inches, maybe a foot deep and it was very unusual for me to see
that because
nowhere else in the lake would the lake level have really
changed. What did it
mean? We did not know.
NARRATOR:
Smith commissioned a survey to try to find out what was happening
at
Yellowstone. The Park had last been surveyed in the 1920s when the
elevation,
the height above sea-level, was measured at various points
across Yellowstone.
50 years later, Smith surveyed the same
points.
ROBERT SMITH: The idea was to survey their elevations and
to compare the
elevations in the mid-70s to what they were in 1923 and
the type of thing that
we did is to make recordings at a precision
level of, of a few millimetres.
NARRATOR: The two sets of figures
should have been similar, but as the survey
team moved across the Park,
they noticed something unexpected: the ground
seemed to be heaving
upwards.
ROBERT SMITH: The surveyor said to me there's something
wrong and he said it's
not me, it's got to be something else, so we
went through all the measurements
again trying to be very careful and
the conclusion kind of hit me in the face
and said this caldera has
uplifted at that time 740 millimetres in the middle
of the
caldera.
NARRATOR: As the measuring continued, an explanation for
the submerged trees
began to emerge. The ground beneath the north of
Yellowstone was bulging up,
tilting the rest of the Park downwards.
This was tipping out the sound end of
the lake inundating the shoreside
trees with water. The vulcanologist realised
only one thing could make
the Earth heave in this way: a vast living magma
chamber. The
Yellowstone supervolcano was alive and if the calculations of the
cycle
were correct, the next eruption was already overdue.
ROBERT
CHRISTIANSEN: Well this gave us a real shiver of nervousness if you
will
about the fact that we have been through this 600,000 year cycle
and that the
last eruption was about 600,000 years
ago.
ROBERT SMITH: I felt like telling people, that is we basically
have on our
hands a giant.
NARRATOR: The scientists had
found the largest single active volcanic system
yet discovered. There
were many things they needed to find out. How big was the
magma chamber
deep underground, how widespread would the effects of an eruption
be
and crucially, when would it happen? To answer any of these
questions
vulcanologists knew they first had to understand
Yellowstone's mysterious magma
chamber.
ROBERT SMITH: It's
incredibly important to understand what's happening inside
of the magma
chamber because that pressure and that heat, the fluid is
what's
triggering the final eruption. It's like understanding the
primer in a bullet.
NARRATOR: Understanding the magma chamber would
be very difficult. Smith and
his team needed to discover the size of
something 8 kilometres below the
ground. They began harnessing
information from an ingenious source:
earthquakes.
ROBERT
SMITH: Well, what we have here is a seismometer. This is the working
end
of a seismograph, the device that's used to record earthquakes. It
is able to
pick up the smallest of earthquakes in, in Yellowstone plus
it picks up
moderate to large earthquakes around the world, it is so
sensitive. This forms
one of a network of 22 seismograph stations in
Yellowstone that is used for
monitoring and all the data are
transmitted to a central recording facility at
the University of
Utah.
NARRATOR: Like many thermal areas, Yellowstone has hundreds
of tiny earth
tremors each year. They are harmless, but in his
seismographic lab Smith has
been using them to trace the size of the
magma chamber.
ROBERT SMITH: Earthquakes are essentially telling
you the pulse. They tell you
the real time pulse of how the caldera is
deforming, of how faults are
fracturing.
NARRATOR: Bob
Smith's 22 permanent seismographs are spread across the Park.
They
detect the sound-waves which come from earthquakes deep underground.
These
waves travel at different speeds depending on the texture of what
they pass
through. Soundwaves passing through solid rock go faster than
those travelling
through molten rock or magma. By measuring the time
they take to reach the
seismographs Smith can tell what they've passed
through. Eventually this builds
up a picture of what lies beneath the
Park.
ROBERT SMITH: The magma chamber we found extends basically
beneath the entire
caldera. It's maybe 40-50 kilometres long, maybe 20
kilometres wide and it has
a thickness of about 10 kilometres. So it's
a giant in volume and essentially
encompasses a half or a third of the
area beneath Yellowstone National Park.
NARRATOR: The magma chamber was
enormous. If it erupted it would be
devastating. To discover the extent
of the devastation scientists had to
understand the force of the
eruption. The clues to this could be found in a
much smaller volcano
halfway across the world: the Greek island of Santorini.
The eruption
here 3,500 years ago, although not VEI8 in scale, did have a
small
magma chamber. Professor Steve Sparks has spent much of his
career studying
Santorini.
PROF. STEVE SPARKS (University
of Bristol): When I first came to Santorini and
started to look at the
pumice deposits from these caldera forming eruptions I
found evidence
of a dramatic change in the power and violence of the
eruption.
NARRATION: By examining the layers of Santorini pumice
Sparks discovered magma
chambers could erupt with almost unimaginable
force and spread their
devastation widely.
STEVE SPARKS:
There's dramatic evidence of a sudden increase in the power.
Huge
blocks about 2 metres in diameter were hurled out of the volcano
reaching 7
kilometres and smashing into the ground and to do that the
blocks must have
been thrown from the volcano at hundreds of metres per
second, about the speed
of Concorde and you can imagine this enormous
red rock crashing in and breaking
up on impact.
NARRATOR:
To understand why caldera volcanoes could erupt with such power
Sparks
replicated their violence at one trillionth of the scale.
STEVE
SPARKS: OK, so we need this…
NARRATOR: In the lab he modelled a
reaction which occurs in the magma chamber
of an erupting
caldera.
STEVE SPARKS: The problem is we can't go into a magma
chamber so the next best
thing to do is to go to the laboratory and try
and simulate what happens in the
magma chamber and in the pathway to
the surface.
NARRATOR: Sparks believed escaping volcanic gas
trapped in the magma might be
responsible for the violence of the
eruptions. Into a glass flask - the magma
chamber - he poured a mixture
of pine resin and acetone. the pine resin
mimicked the magma, the
acetone modelled trapped volcanic gases like carbon
dioxide and sulphur
dioxide.
STEVE SPARKS: Pine resin is a very sticky, stiff material
so it has some
properties which are rather like magma and we thought
that if we could get a, a
gas which dissolved in pine resin, like
acetone, then we could get a, a
laboratory system which would represent
the, the natural case.
NARRATOR: Sparks then created a vacuum above
the flask to mimic the
depressurisation that occurs in the magma
chamber when a supervolcano begins
its eruption and the dissolved
volcanic gas can expand. When the vacuum reached
the liquid it caused a
dramatic change. The dissolved acetone suddenly became a
gas. This made
the resin expand causing violent frothing and blasting the
contents out
of the chamber.
STEVE SPARKS: These experiments give us tremendous
insight into the tremendous
power of gases coming out of solution and
enabled to drive these very dramatic
explosive
flows.
NARRATOR: Unlike supervolcanoes, normal volcanoes don't have
this vast
reservoir of magma and trapped volcanic gases and don't have
the potential for
such powerful eruptions. But experiments in the
laboratory cannot answer the
biggest question of all surrounding
Yellowstone: when will it next erupt?
Scientists face a problem. They
have never seen a supervolcano erupt. Until a
VEI8 eruption is observed
and analysed no-one knows what the telltale
precursors would be to a
Yellowstone eruption.
BILL McGUIRE: We can actually model volcanoes
and their activity. We can do it
in the laboratory on computer, but we
need observational data in order to make
those models
realistic.
ROBERT SMITH: What the precursors might be for a giant
volcanic eruptions
they've never been observed scientifically and
they've never been documented,
so we don't know what to look
for.
ROBERT CHRISTIANSEN: Nobody wants to see a global disaster of
course and yet
we'll never really fully understand the processes
involved in these
supervolcanic eruptions until one of them
happens.
NARRATOR: A terrible truth underlies all mankind's efforts
to understand the
vast mechanisms which drive VEI8 eruptions.
Ultimately trying to find out what
makes supervolcanoes work may be
pointless. Consider the last one. 74,000 years
ago a supervolcano
erupted here in Sumatra. It would have been the loudest
noise ever
heard by man. It would have blasted vast clouds of ash across
the
world.
The resultant caldera formed Lake Toba, 100
kilometres long, 60 kilometres
wide. it was, in short, colossal.
Scientists are only now beginning to
understand the effects of so much
ash on the planet's climate. This is the
ocean core repository at
Columbia University in America. It contains thousands
of drill samples
from seabeds round the world, a historical keyhole through
which
scientists, like Michael Rampino can view volcanic history.
MICHAEL
RAMPINO: The size of the Toba eruption was enormous. We're
talking
about, about 3,000 cubic kilometres of material coming out of
that volcano.
That's about 10,000 times the size of the 1980 Mount St.
Helens eruption which
people think of as a large eruption, a truly
super eruption.
This is an ocean drilling core from the central
Indian Ocean. It's about 2,500
kilometres from the Toba volcano and
here are 35 centimetres of ash deposited
after the Toba eruption. It
shows that this Toba eruption was a supervolcanic
event, it was much,
much bigger than any other volcanic eruption we see in the
geological
record. Chemical analysis of the ash tells us that this eruption
was
rich in sulphur, would have released a tremendous amount of sulphur
dioxide and
other gases into the stratosphere which would have turned
into sulphuric acid
aerosols and affected the climate of the Earth for
years.
NARRATOR: For a long time scientists have known that
volcanic ash can affect
the global climate. The fine ash and sulphur
dioxide blasted into the
stratosphere reflects solar radiation back
into space and stops sunlight
reaching the planet. This has a cooling
effect on the Earth. In the year
following the 1991 eruption of Mount
Pinatubo for instance the average global
temperature fell by half a
degree Celsius. By comparing the amount of ash
ejected by past
volcanoes with their effect on the Earth's temperature, Rampino
has
estimated the impact of the Toba eruption on the global climate
74,000
years ago.
MICHAEL RAMPINO: I'm plotting a simple
graph where one side there's sulphur
released in millions of tons by
volcanic eruptions and on the other side
there's a cooling in degree
Celsius that we saw after these volcanic eruptions.
I'm plotting as
points the historical eruptions like Mount St. Helens,
Krakatoa,
Pinatubo, Tambora. There's a nice correlation between the
sulphur
released into the atmosphere and the
cooling.
NARRATOR: Because of this relationship between the sulphur
released by large
volcanoes and global cooling, Rampino can calculate
the drop in temperature
caused by the Toba
eruption.
MICHAEL RAMPINO: We can see this kind of plot predicts
that the Toba eruption
was so large that the temperature change after
Toba in degrees Celsius would
have been about a 5 degree global
temperature drop, very significant, very
severe global cooling.
NARRATOR: Five degrees Celsius average drop in global
temperature would
have been devastating causing Europe's summers to freeze and
triggering
a volcanic winter.
MICHAEL RAMPINO: Five degrees globally would
translate into 15 degrees or so of
summer cooling in the temperate to
high latitudes. The effects on agriculture,
on the growth of plants, on
life in the oceans would be catastrophic.
NARRATOR: This global
catastrophe would have continued for years, dramatically
affecting life
on Earth, but what impact did it have on humans? The answer may
be
buried not inside the ancient rocks, but deep within us all. Lynn Jorde
and
Henry Harpending are scientists specialising in human genetics.
Since the early
1990s they have been studying mitochondrial DNA using
the information to
investigate mankind's past. Most of our genetic
information is stored in the
nuclei of our cells, but a small, separate
quantity exists in another
component, the part which produces the
cells' energy, the mitochondria.
PROF. LYNN JORDE (University of
Utah): Mitochondria have their own genes. It's
a small number of genes,
a small amount of DNA, but it's distinct from the rest
of the DNA in
the cell and because of the way mitochondria are transmitted from
one
generation to the next, they're, they're inherited only from the mother
so
they give us a record of the maternal lineage of a
population.
NARRATOR: Mitochondrial DNA is inherited only by the
mother. All mutations are
passed on from mother to child, generation
after generation at a regular rate.
Over time, the number of these
mutations accumulate in a population.
LYNN JORDE: Every event that
takes place in our past, every major event, a
population increase, a
population decrease, or the exchange of people from one
population to
another changes the composition of the mitochondrial DNA in
that
population, so what happens is that we have a record of our past
written in our
mitochondrial genes.
NARRATOR: By knowing
the rate of mutation of mitochondrial DNA and by a complex
analysis of
the distribution of these mutations, the geneticists can estimate
the
size of populations in the past. Several years ago they began seeing
a
strange pattern in their results.
LYNN JORDE: We
expected that we would see a pattern consistent with a
relatively
constant population size. Instead, we saw something that
departed
dramatically from that expectation. We saw a pattern much more
consistent with
a dramatic reduction in population size at some point
in our past.
NARRATOR: This confirmed what other geneticists have
noticed. Given the length
of time humans have existed, there should be
a wide range of genetic variation,
yet DNA from people throughout the
world is surprisingly similar. What could
have caused this? The answer
is a dramatic reduction of the population some
time in the past: a
bottleneck.
LYNN JORDE: We imagine the population diagrammed like
this. In the distant past
back here we have a large population, then a
bottleneck looking like this and
then a subsequent enlargement of
population size again, so we would have
families of people in the
distant past with a significant amount of genetic
diversity, but when
the bottleneck occurs, when there's a reduction in
population size
perhaps only a few of those families would survive
the
bottleneck.
We have a dramatic reduction in genetic
diversity during this time when the
population is very small and then
after the bottleneck the people who would we,
who we would see today
would be descendants only of those who survived, so
they're going to be
genetically much more similar to one another reducing the
amount of
genetic variation.
NARRATOR: Human DNA is so similar the scientists
concluded the population
reduction had been catastrophic. PROF. HENRY
HARPENDING (University of Utah):
It seemed so incredible, you know the
idea that all of us, now there's 6
billion people on Earth, and what
the data were telling us was that we, you
know our species was reduced
to, you know, a few thousand. Suddenly it hit us,
we had something to
say about human history.
LYNN JORDE: Our population may have been
in such a precarious position that
only a few thousand of us may have
been alive on the whole face of the Earth at
one point in time, that we
almost went extinct, that some event was so
catastrophic as to nearly
cause our species to cease to exist completely.
NARRATOR: It is an
astonishing revelation, but the key was to find out when and
why it
happened. Because mitochondrial DNA mutates at an average rate
these
scientists believe, controversially, that they can narrow down
the date of the
bottleneck.
LYNN JORDE: Mutations in the
mitochondria take place with clocklike regularly,
so the number of
mutations give us a clock essentially that we can use to
approximately
date the major event. In the case of a population bottleneck we
think
that this would have occurred roughly 70-80,000 years ago, give or
take
some number of thousands of years. So then the real question is:
what could
have caused such a reduction, an extreme reduction, in the
human population
down to as few as 5 or 10,000
individuals?
NARRATOR: As for what caused this dramatic reduction
in population the
geneticists had no idea. Henry Harpending began
touring universities to talk
about the bottleneck. He was invited by
anthropologist Stanley Ambrose to give
a lecture to his
students.
HENRY HARPENDING: Well Stanley is full of ideas, he's the
kind of scientist
that plucks things from all over and puts them
together.
PROF. STANLEY AMBROSE (University of Illinois): I sat in
on the lecture and he
start4ed talking about this human population
bottleneck and I thought what
could have caused it and at that point I
broke out into a sweat. I went up to
Henry and said I've just read a
paper, and it's on the top of my desk now, that
may have an explanation
for why this population bottleneck occurred.
HENRY HARPENDING: I
didn't read it till a week later and when I read it you
know it was
like somebody kicking you in the face. There it was.
STANLEY
AMBROSE: The paper was about the super eruption of a volcano
called
Toba in Sumatra.
NARRATOR: This team of scientists
believe the bottleneck occurred between 70
and 80,000 years ago,
although this date is hotly debated. Toba erupted in the
middle of this
period, 74,000 years ago. If there really is a connection this
research
has terrifying implications for a future Yellowstone eruption. It
could
well be of a similar size and ferocity to Toba. Like Toba, it would
have
a devastating impact, not just on the surrounding region, North
America, but on
the whole world.
MICHAEL RAMPINO: If
Yellowstone goes off again, and it will, it'll be
disastrous for the
United States and eventually for the whole world.
NARRATOR:
Vulcanologists believe it would all start with the magma
chamber
becoming unstable.
BILL McGUIRE: You'd start
seeing bigger earthquakes, you may see parts of
Yellowstone uplifting
as magma intrudes and gets nearer and nearer the surface.
ROBERT
SMITH: And maybe an earthquake sends a rupture through the
brittle
layer, you've broken the lid of the pressure
cooker.
BILL McGUIRE: This would generate sheets of magma which
will be probably rising
up to 30, 40, 50 kilometres sending gigantic
amounts of debris into the
atmosphere.
ROBERT
CHRISTIANSEN: Where we are right now would be gone. We would
be
instantly incinerated.
MICHAEL RAMPINO: Pyroclastic
flows will cover that whole region, maybe kill
tens of thousands of
people in the surrounding area.
BILL McGUIRE: You're getting a, an
eruption which we can barely imagine. We've
never seen this sort of
thing. You wouldn't be able to get within 1,000
kilometres of it when
it was going like this.
ROBERT CHRISTIANSEN: The ash carried in the
atmosphere and deposited over large
areas of the United States,
particularly over the great plains, would have
devastating
effects.
BILL McGUIRE: The area that would be affected is, is the
bread basket of North
America in effect and it produces an enormous
amount of grain on a global scale
really. That's, that's, that's the
problem and you would see nothing. The
harvest would vanish virtually
overnight.
ROBERT CHRISTIANSEN: All basic economic activity would
certainly be impacted by
this and let alone changes in the climate that
could possibly be induced.
MICHAEL RAMPINO: The climatic effects
globally from that eruption will be
produced by the plume of material
that goes up into the atmosphere. That'll
spread worldwide and will
have a cooling effect that will probably knock out
the growing season
on a global basis. We can't really overstate the effect of
these huge
eruptions. Civilisation will start to creak at the seams in a
sense.
ROBERT SMITH: The fact that we haven't seen one in historic
time or documented
means the human race really is not attuned to these
things because they're such
a rare event.
MICHAEL RAMPINO:
It's really not a question of if it'll go off, it's a question
of when
because sooner or later one of these large super eruptions will
happen.