The Toba Volcano Super Eruption
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Table of Contents
Introduction ............................................................................................................................... 2
Regional vs. Global Catastrophe ............................................................................................. 3
Similar Event ........................................................................................................................... 3
Statement of Aim ..................................................................................................................... 4
Geological Context ................................................................................................................... 4
Previous Volcanic Activity ........................................................................................................ 5
Volcanic Winter ......................................................................................................................... 7
Geological Evidence ................................................................................................................ 7
Effect on Early Humans ........................................................................................................... 8
Regional vs. Global Catastrophe Revisited .............................................................................. 9
Conclusion .............................................................................................................................. 10
References .............................................................................................................................. 11
Illustrations ............................................................................................................................. 12
Figure 1 ................................................................................................................................... 2
Figure 2 ................................................................................................................................... 4
Figure 3 ................................................................................................................................... 6
Figure 4 ................................................................................................................................... 7
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Introduction
Roughly 74,000 years ago, the Earth shook with one of the most powerful
volcanic eruptions in history. It was so violent that it reshaped landscapes, may have
affected the planet’s climate on a global scale, and arguably threatened the continued
existence of humanity (Oppenheimer 2002).
The area around Lake Toba, Sumatra, has
been volcanically active for many years,
with at least three previous major eruptions,
the most recent of which being the largest
in scale. The aftermath of these
sequentially massive eruptions can be seen
in fig. 1 where the volcano has collapsed in
on itself forming the modern-day Toba
Figure 1 Map of Toba Caldera. (Mucek et al., 2021)
Caldera Complex (Chesner 2012). The eponymous volcanic eruption spewed an
estimated 2,800 km3 of magma, ranking it at 8.8 on the Volcanic Explosivity Index (VEI)
(Chesner 2012). As pyroclastic material erupted out of the Toba Volcano, winds carried
ash and smaller debris across almost the entirety of South Asia, reaching as far west as
India and the Arabian Sea. Due to the sheer magnitude of the eruption, scientists have
long since attributed the Toba Eruption to causing a volcanic winter, with climate models
suggesting a global decrease of 3.5 to 9°C (Osipov et al. 2021). This drastic decrease in
global temperature would have wreaked havoc across the ecosystems, greatly
impeding the early hunter gatherer humans’ ability to proliferate and thus forming a
population bottleneck (Lane et al. 2013). This impact upon human existence is now
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under contention, as modern technologies and methodologies allowed a more accurate
re-examination of the Toba eruption. Many scientists are now considering that the Toba
Eruption’s climatic impact was much less intense and shorter than previously assumed
(Ge & Gao 2020). Recent research even suggests that there was not a volcanic winter
after the Toba Eruption (Lane et al. 2013). Regardless of the debate, the Toba Eruption
has been cemented as one of the largest volcanic eruptions in history.
Regional vs. Global Catastrophe
The debate of the scale of the Toba Eruption directly affects whether it can be
considered a regional or global catastrophe. If the eruption did cause a volcanic winter
that cooled the earth on a global scale, it is undoubtedly a global catastrophe. Per the
more recent studies into the event, if it is the case that the Toba Eruption did not actually
cause a volcanic winter, then the effects would be more localized and thus defined as a
regional catastrophe. Being a prehistoric event without any written record of its
devastation, a comprehensive understanding of the effects of the Toba Eruption is still
being studied, as such cannot be definitively classified as a regional or global
catastrophe.
Similar Event
A similar volcanic eruption that happened more recently would be the Mount
Tambora eruption of 1815. Located on Sumbawa Island, Indonesia, the eruption ejected
roughly 41 km3 of pyroclastic material. This thick cloud of ash led to a volcanic winter,
causing tens of thousands of deaths either directly from the eruption, or its impact on
agriculture (kandlbauer & Sparks 2014). Although only ranked a 7 on the VEI in
comparison to Toba’s 8, the Tambora Eruption was still a massive eruption that greatly
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affected the world. With the principles of uniformity, understanding the well documented
Tambora Eruption provides a unique perspective into the mechanisms of how volcanic
winters form, and its aftereffects.
Statement of Aim
The aim of this paper is to provide geological context into the Toba Eruption 74,000
years ago and consider the extent of which the eruption caused a volcanic winter. This
will be done through an examination of geological conditions leading up to and during
the eruption and determine how modern discoveries may change that hypothesis.
Geological Context
The Toba Volcano is located
within the Sunda Arc, a well
documented volcanic arc formed
from the subduction zone between
the Indo-Australian Plate and the
Eurasian Plate. As seen in fig. 2, the
Figure 2 The Sunda Arc. (Widiyantoro et al., 2024)
Sunda Arc follows along the
subduction zone between the two plates, on top of the continental plate. The Toba
Caldera, as it is known today, can be found in North Sumatra. As the denser oceanic
plate subducts underneath the continental plate, water and other volatiles are
introduced to the overlying mantle edge. This reduces the melting point of the
surrounding mantle, building up magma in large reservoirs. As these reservoirs grow,
they will break through the solid rick layer of the crust and, similarly to magma plumes,
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form large chains of volcanos (Grove et al., 2012). The Sunda Arc also has a large
concentration of silicon in its surrounding area, feeding into its magma chambers. This
greatly increases the viscosity of the magma, allowing it to trap volatiles and gases at a
much higher rate, leading to more pressure and thus larger eruptions (Deegan et al.,
2021). Being on such a tectonically active zone, the Toba Volcano has also had an
extensive history of massive eruptions before its eponymous super eruption.
Previous Volcanic Activity
Before the Toba Super Eruption, there were three prior major eruptions, named the
Haranggaol Dacite Tuff (HDT), Oldest Toba Tuff (OTT), Middle Toba Tuff (MTT), and
finally the focus of this study, the Youngest Toba Tuff (YTT). These eruptions are named
after tuff, rock formed from the volcanic debris that collects in the surrounding area after
a large eruption (Chesner et al., 1991). Due to all the Toba eruptions having been
massive, each expelling large amounts of volcanic debris, it created very clear
stratigraphic layers. This allowed researchers to not only date samples, but as well
measure the coverage of each eruption and model estimates of each eruption’s scale.
Evidence for HDT was the most eroded, which is to be expected as it was dated to have
been deposited roughly 1.2 million years ago. This eruption was the first known from the
Toba Volcano and was what started the formation of the Toba Caldera Complex. As the
eruption occurred, the magma chambers directly below the crust were emptied and
collapsed in, forming the unique caldera structure (Widiyantoro et al., 2024). The
subsequent volcanic eruptions were similar, but notably OTT was the only other super
eruption, spewing an estimated 2,300 km3 of pyroclastic material. Through analysis of
the uranium-lead (U-Pb) content found in zircon crystals following the eruptions,
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researchers have found that the magma chambers
below the Toba Super Volcano were growing
laterally after each subsequent eruption
(Szymanowski et al., 2021). Enlarged magma
chambers means more magma can be injected
and built up, trapping more gasses and other
volatiles, thus ultimately leading to a larger
Figure 3 Zircon Volume (Liu et al., 2021)
eruption. Notably, it was also found that the
underlying mechanisms of the Toba Super Eruption was not due to a sudden influx of
magma like most silicon rich super volcanic eruptions. Instead, it was found that a
continuous influx of magma over the course of hundreds of thousands of years between
eruptions led to a much greater build up of pressure and larger eruptions. As the two
super eruptions, OTT and YTT, were studied, it was found that there was a higher
concentration of zircon at these eruptions. This can be seen in fig. 3, where the integral,
and thus total volume, of uranium-lead found within crystalized zircon is much higher at
OTT and YTT (Liu et al., 2021). This is supported by models where a variable influx of
magma is measured against the zircon content, showing that sharp spikes in magma
influx after relatively lower sustained influx will yield less overall zircon than a
continuously high influx of magma (Allen et al., 2015). However, this does mean that
future super eruptions will be very difficult to predict, as some of the known key
indicators such as increased surface deformation or degassing, may not occur before
super eruptions (Liu et al., 2021). This constant influx of magma, combined with the
lateral growth of the magma chambers themselves, allowed for the Toba Super Eruption
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to grow exponentially in scale. As well, the high silicon concentrations further increased
the explosivity of the eventual eruption as the increasingly viscous magma could trap
more and more volatiles. As a result, the Toba Super Eruption was caused by a number
of compounding factors that worked in concert to truly become one of the largest natural
catastrophes this world has seen. There is no one trigger that set off the eruption, but
instead a gradual build up that reached critical levels before erupting.
Volcanic Winter
The largest point of contention in relation to the Toba Super Eruption is the extent in
which the eruption affected early human populations, and if the volcanic winter is
directly triggered to an ice age that led to a significant downturn in human populations
(Lane et al. 2013). Being a multifaceted problem, melding anthropological and
geological aspects, this requires a comprehensive view of all the evidence.
Geological Evidence
The largest point towards the Toba Super Eruption causing a large-scale volcanic
winter was the sheer scale of the eruption. Being a VEI 8, the Toba Super Eruption
would have expelled massive amounts of sulfur
dioxide into the air, blocking the sun and
cooling the world. By creating a model based
on the 1991 Mount Pinatubo eruption and
scaling the eruption to what the Toba Super
Eruption was believed to be, it was found that
the eruption would have caused global cooling
Figure 4 Global Temperature Post Toba Super Eruption
(Black et al., 2021)
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within the 8°C and 17°C range (Robock et al., 2009). Unequivocally, that is a massive
change in global temperatures and would have rapidly affected ecosystems, not just in
the surrounding regions around Toba, but also the world. Further evidence of the global
nature of this cooling was found in ice core samples from Greenland and Antarctica.
Dating the ice core similarly to stratigraphic samples of rock, it was found that a large
sulphate deposit could be dated back to roughly 74,000 years ago, coinciding with the
Toba Super Eruption (Lin et al., 2023). This decrease in global temperature can be seen
in fig. 4, where there is a significant drop in temperature directly after the Toba Super
Eruption. This level of global cooling is what led many scientists to consider that the
Toba Super Eruption may have been what started an ice age and the subsequent
population bottleneck thereafter (Rampino and Self, 1992). However, from the global
temperature model, it can also be seen that the temperature gradually trends upwards
again as the volcanic winter clears out. This coincides with more modern climate models
that supposed that the global temperature would have recovered within a few decades
at most. There was without a doubt a volcanic winter, but its effects on the rate Earth
was tending towards glocalization was minimal (Robock et al., 2009).
Effect on Early Humans
Although the statement that the Toba Super Eruption may have triggered an ice age
is under contention, it is without a doubt that a volcanic winter occurred. Then to
consider the extent in which the Toba Super Eruption affected early humans, it would be
beneficial to consider how volcanic winters affect human populations. As seen with the
1815 Tambora Eruption, the bulk of the deaths was not directly from the eruption itself,
but instead the aftereffects of the volcanic winter. Global cooling wreaked havoc in
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villages, ruined crop cycles and even wrought famine and disease (Boe, 2020).
However, these populations were very different and to directly correlate the level of
impact to humans from the Tambora Eruption to the Toba Super Eruption would not be a
fair comparison. This is because the Toba Super Eruption happened before the advent
of agriculture and the early humans were all hunter gatherers (Carey, 2023). Without
densely populated cities reliant on a sole food source that would have been destroyed
by a volcanic winter, the nomadic hunter gatherers would have been more adaptable to
this change. Moreover, evidence of continued human existence could be found across
the world, most notably in the then ash covered islands of Sumatra (Ge & Gao 2020).
This directly shows that even though many early humans may have died in the eruption
itself, many could still survive in the following volcanic winter.
Regional vs. Global Catastrophe Revisited
Now with further context into the circumstances surrounding the Toba Super
Eruption, it can be noted that the volcanic winter did in fact take place, however it was
not the sole trigger to the eventual ice age. Many of the preconceived notions of the
damage that the Toba Super Eruption wrought is based on how recent volcanic winters
have devastated populations, like Mount Tambora. However, as discussed, the Tambora
Super Eruption would have had less of an effect on hunter gatherer populations, without
infrastructure set in place to be destroyed. As well, the volcanic winter was much shorter
than originally believed, most areas likely recovering within a few years. Regardless
though, regional or global catastrophe are not differentiated by the number of human
deaths or even how long the damage lasts. Instead, it is by the scope of how far the
effects reach. In this regard, the Toba Super Eruption is without a doubt a global
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catastrophe, even without considering its impact towards the ice age. This is because
the volcanic winter not only affect the global temperature, but it was also a significant
drop, with some models stating upwards of 17°C. Despite overstatements of the after
effects of the eruption and even the time it lasted, the Toba Super Eruption is
unequivocally a global catastrophe that greatly affected the world as a whole.
Conclusion
As seen throughout this study, it is difficult to create direct links of cause and effect
between major geological events, even if they seem to fit very well together. The Toba
Super Eruption was first considered to be the sole trigger for a volcanic winter that
caused an ice age and subsequent population bottleneck. Through recent reexaminations, many researchers believe these effects to be greatly overstated.
However, that does not undercut the very real damage this global catastrophe still
wrought.
It can be said that this re-examination has unveiled much about the mechanisms
surrounding super eruptions, namely that in silicon rich magma chambers, like the one
under Toba, super eruptions are more likely to come from a continuous magma influx
that ultimately crosses a critical level than a sudden magma influx. This now allows
surveyors to consider that common indicators for volcanic eruptions, such as degassing,
may occur for super eruptions.
Being such a tectonically active zone, future local and regional disasters will be
inevitable, but a super eruption is not expected within the few hundred thousand years,
as the chambers need to be filled before that would be possible. As such there are no
mitigating changes been done in the area.
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There was much to be learned from the Toba Super Eruption, and much still to learn.
Through an analysis of the geological context, what processes were occurring directly
before and during the eruption, and as well the aftereffects on the world, it can be
concluded that the Toba Super Eruption was one of the largest global catastrophes in
Earth's history.
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Illustrations
Figure 2 The Sunda Arc. (Widiyantoro et al., 2024)
Figure 1 Map of Toba Caldera. (Mucek et al., 2021)
Figure 5 Global Temperature Post Toba Super Eruption
(Black et al., 2021)
Figure 3 Zircon Volume (Liu et al., 2021)