Supervulkane

verfasst von RK, 24.02.2004, 00:40

-->Supervolcanoes

Almost didn't read the transcript of this four year old interview
because I thought I'd read enough about Supervolcanoes. I was wrong.
Thanks Rita

The following interview reads like a very well written detailed
clearly revealing story of discovery, which in this case being true
makes it just that much more interesting. It's much broader in scope
than just a discussion of Yellowstone. Toward the end there's even a
tie-in with world wide DNA research. Apparently the earth's
population was high before and then suddenly was paired down to just
a few thousand people.

It does not contain a potential date for a future eruption,
although several things that would be expected prior to one have
started and the timings right.

I'm not suggesting any plan of action for anyone to take either. Of
course if I saw death clearly coming my way I'd smile inside myself
and think 'bring on the next adventure.' This doesn't mean I'd go
looking for it.

Mark H

Supervolcanoes
BBC2 9:30pm Thursday 3rd February 2000
NARRATOR (SINÉAD CUSACK): Yellowstone is America's
first and most famous National Park. Every year over
three 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
fossilised 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 their,
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.

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gesamter Thread:

• Supervulkane - RK, 24.02.2004, 00:40