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This page is designed for students and others interested in
the ways we explore the impact of the Toba super-eruption on hominins (early
humans) and environments 74,000 years ago. It includes answers to a
series of questions commonly asked about the Toba super-eruption. A
brochure in Telugu (the major language spoken in the Kurnool District of
Andhra Pradesh, India) is available here.
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What
is a super-volcano?
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The terms ‘super-volcano’ and ‘super-eruption’ were first
introduced by scientists and the media to describe the Toba eruption of
74,000 years ago. Previous terms such as ‘mega-eruption’ have also been
used to describe the same phenomenon: volcanic eruptions far bigger
than any occurring in recorded history (the past few thousand years).
Other former super-volcanoes include Yellowstone (Wyoming, USA) and
Oruanui (Taupo, New Zealand).
Super-eruptions are defined by the volume of material they
release. Although there is no strict definition, in general an eruption
that produces more than 300-400 cubic kilometers of tephra (fragments
ejected into the air) may be considered ‘super’. This is quite a large
volume of rock – it would be enough to cover all of Greater London
beneath 210m of ash. The Toba eruption is estimated to have been
perhaps eight times larger than this. By comparison, Mount St. Helens
in 1980 erupted less than one cubic kilometer, and Krakatau in 1883
about 12 cubic kilometers.
For those who enjoy calculations, a formula has been developed
to calculate the magnitude (M) of an eruption: M=log10(m) -7.0, where
(m) is the erupted mass in kg, worked out from measurements taken on
exposed deposits. Using this scale, super-volcanoes are those of
magnitude 8 or larger, erupting more than one thousand million million
(1015) kg of material. Toba was ten times bigger than this,
and was therefore a magnitude 9 eruption.
There are two types of super-eruption. The first is an
enormous outpouring of lava across a wide area – this releases a large
volume of material and so qualifies as a super-eruption, even though it
does not involve the huge ash clouds some people associate with
‘typical’ volcanoes. The second type is explosive, sending tephra
across a wide area and causing tremendous local destruction. Both types
release a large volume of gases as well as solid material, and these
may have the greatest effect as they influence the atmosphere and
potentially global climate.
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What
makes Toba so special?
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Although dozens of super-volcanoes (greater than magnitude 8)
have been recorded by geologists, only two are known to have erupted
since the emergence of anatomically modern humans some 200,000 years
ago in Africa. These are the Toba eruption in Sumatra (74,000 years
ago) and the Oruanui eruption in New Zealand (26,500 years ago). While
the Oruanui volcano was large, the fallout from the eruption was
concentrated on the surrounding area, which was at the time uninhabited
by humans. By contrast, the Toba eruption was five times as big as even
the Oruanui event, and sent ash across a wide area that covers India
and parts of southeast Asia, places that were inhabited by stone-tool
making hominins. Even more importantly, the Toba eruption occurred at
about the time that modern humans may have been starting to move out of
Africa and the Middle East into other parts of the world. Added to
this, observations suggest that it occurred during the initial stages
of an ice age, and some scientists believe Toba may have contributed to
this climatic swing.
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What
does a volcano have to do with human evolution?
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Toba had erupted a number of times previously (one of these,
about 840,000 years ago, was itself a super-eruption). What was
significant about the event 74,000 years ago was the coincidence that
an important period in human evolution was occurring at the same time.
The Earth was already inhabited by a number of species closely related
to us, such as Homo neanderthalensis (the
Neanderthals) in Europe, and Homo erectus and Homo floresiensis (sometimes
called the ‘hobbits’) in southeast Asia. All of these survived Toba,
but some archaeologists have claimed that almost all the anatomically
modern humans (our direct ancestors) were killed by the environmental
effects the volcano caused, with the remaining people surviving in
refuges in Africa. This scenario is based on data from genetics, and
because it suggests that people were narrowed down from many to very
few numbers, it is known as a genetic ‘bottleneck’.
The questions for human evolution therefore include:
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Did Toba have a dramatic (global) environmental
impact, significant enough to result in a genetic bottleneck? And how
did this change the course of our evolution?
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Were modern humans living in or dispersing across
India at the time that the Toba ash fell, and if so were they killed
off by the volcano and its climatic effects?
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Were the hominins living in India non-modern, but they
were killed off allowing modern humans an easy passage during a later
dispersal?
A related question, but from a different perspective, is what
would happen if a super-volcano were to erupt today? Can we learn
lessons from the last time this happened that would help us survive?
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What
is archaeology?
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Archaeology is the scientific search, recovery and
interpretation of material evidence for past behaviour. Most
archaeologists examine objects and sites created by modern humans (in
the last few hundred thousand years), but archaeology also deals with
human ancestors back to the time of the first stone tools, 2.6 million
years ago in Africa. As a science, archaeological research involves
creating hypotheses about how, why and when people lived in various
parts of the world. These hypotheses are then tested against the
evidence left behind by those people. This process usually involves a
series of steps:
1. Research in
libraries and discussions with other archaeologists and local people
helps us decide on an area where we may find evidence to test our
hypotheses. This will depend on the question we want to answer, and is
a very important stage as the following fieldwork stages are very
time-consuming and often expensive.
2. We go to our
chosen field areas (in our case in southern and northern India), and
search for indications that past humans or their ancestors lived in a
particular place or site. This stage is known as survey.
3. Once promising
sites are identified, we very carefully dig holes to find buried
material from earlier times (deeper objects are usually older than
those closer to the surface). Detailed records are made of the
sediments in which the artefacts are buried, as well as the location
and condition of the artefacts themselves. This stage is known as excavation.
4. The most
time-consuming stage is the period of analysis and interpretation that
follows the fieldwork, when all the data collected during survey and
excavation are put together. As part of this process, a series of tests
may be run on the recovered sediments to find out what the past
environment was like, and samples such as charcoal are processed to
provide dates for the different levels within a site.
5. The final stage
involves writing reports on our work and publishing these so that other
archaeologists and scientists can see the work we’ve done and compare
it to previously discovered information. These reports usually generate
further hypotheses that will drive the next round of research, and the process
begins again.
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How
does archaeology help in understanding the impact of Toba?
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Much of the speculation about Toba’s impact on human evolution
and dispersals is based on indirect evidence from genetics and
environmental reconstruction. Archaeology allows us to recover and
examine the direct evidence left behind by the early humans who were in
India at the time that the ash fell. This information, especially the
forms of stone tools and their presence or absence immediately above
and below the ash layer in our sites, provides much more specific data
about the impact of Toba than we can obtain from other sources. One of
the additional benefits of an archaeological approach is that we can
collect samples for dating and environment reconstruction at the same
time as we recover human artefacts, tying these different strands of
evidence together.
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What
other methods can help?
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Other methods that work alongside archaeology include computer
modeling of the way that Toba’s huge gas and ash cloud would have
interacted with earth’s atmosphere, volcanological analysis of the ash
or tephra itself to reveal details of the volcanic eruption (including
the chemistry of the rock that was ejected and estimates of the length
of the time over which the eruption occurred), and comparison of the
evidence for a dramatic reduction in the numbers of humans at the time
of Toba with records from other animal species to see if they were
similarly affected. You can read more about some of these methods,
along with others that out team is using, in our research page.
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Why
do scientists disagree about the impact of Toba?
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The huge size of the Toba super-eruption 74,000 years ago has
been known for many decades, but it is only in the past ten years or so
that scientists have seriously begun considering the influence of this
event on human evolution. We are therefore at the stage of collecting
as much high-quality data on the natural environment and human
responses to the eruption as possible, and as different scientists work
with data from different sources there will be times when not all the
data points to the same conclusions. More and more sources of
information are being explored each year, including the climate records
from gases trapped within northern hemisphere ice sheets, and
sophisticated genetic analyses of living human populations to
reconstruct past evolutionary trends. It is one of the most important
features of science that theories and hypotheses can change as new
evidence comes in, and this can lead to old ideas being replaced by new
ones. We do not yet have all the answers, and in some cases (such as
determining the species of humans living in India at the time of the
eruption) we don’t even have the evidence required to make definitive statements.
We can be sure, however, that each step is bringing us closer to
solving the Toba mystery.
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Where can I get more information?
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The LINKS page of this website is a
guide to other reliable places on the internet where you can learn
about human evolution, archaeology and volcanoes. We will be adding to
this page as our project progresses.
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