1.0 INTRODUCTION
Over 74,000 years ago in northern Sumatra, Indonesia, the eruption of Mount Toba created one the largest recorded volcanic eruptions of the last 2 million years.1 This volcanic eruption created a 100 x 30km long volcanic crater – known as a caldera.2 Since the time of
Figure 1. Location of Toba Lake5 the event, this caldera has filled with water, forming what is known today as the Toba Lake (Fig.1)3. The catastrophic consequences of this ∼74 ka “super-eruption” are thought to have driven the human population to near extinction.7
1.1 Overview of Super-volcanos and Super-eruptions
A super-eruption is the eruption of a super-volcano that reaches a Volcano Explosivity Index (VEI) magnitude of 8, and the measured deposits of the explosion exceed more than 1000 cubic kilometres.4 Figure 2 demonstrates a comparison of eruption volumes for some of the largest super-eruptions in history. The Toba eruption produced between 2500-3000 km3 of magma in comparison to some of the smaller devastating eruptions, such as the Yellowstone eruptions that produced at most
Figure 2. Comparison of Eruption Volumes4 2450 km3 of magma, thus demonstrating the magnitude of Toba.4
1.2 Significance of the Toba Eruption
The Toba eruption has great historical significance, specifically the impact on both the change in world climate and the human population. Although, there is speculation regarding the influence the impact actually had on these global changes. It was reported that the ash and gas expelled from the eruption extended 30 miles into the stratosphere resulting in multiple repercussions on the biosphere.5 Many researchers propose that this gas and ash led to a “volcanic winter” causing a 3 to 5ºC decrease in global temperatures across several years.5,6 The environmental changes following this putative volcanic winter is believed to have led to the virtual extinction of many species, including the modern human, and also gave rise to a human population bottleneck.5,6 A population bottleneck is a drastic reduction in the size of a population following an event; this event also decreases the population gene pool, leaving the residual population with lower genetic diversity8. Some researchers argue that based on ice core geochemical and geological evidence, the 74ka Toba eruption must have been the cause of the regional and global climate change.5 While others maintain that due to sea surface temperature, termite survival and archeological findings, there is minimal evidence supporting the impact and alleged date of the Toba eruption.5 Nonetheless, the Toba eruption has great historical significance and the magnitude of its impact had lasting consequences that are evident worldwide.
1.3 Statement of Aim
The aim of this report is to examine the impact that the Toba eruption had on human history, and the surrounding controversy of the event; specifically the vegetation, fossil and climate change, as well as the specific time in which the event took place.
2.0 GEOLOGICAL SETTING OF TOBA
Toba is located within the three Barisan Mountains that make up the backbone of Sumatra.7 The Toba caldera is part of the volcanic arc that is linked with the subduction of the Indo-Australian Plate beneath the South-Asian Plate.8 Subduction of these plates occurs at a rate of 5.2 to 5.8 cm per year.8 The Toba caldera also lies at the junction of the
Figure 3. Tectonic setting of the Toba Caldera 9 Sumatran Fault Zone (SFZ) and the Investigator Fracture Zone (shown in Figure 3), making this region very seismically active.9
Over the past million years the Toba volcano has erupted four times.9 These caldera forming eruptions are known as the Haranggoal Dacite Tuff (HDT), Oldest Toba Tuff (OTT), Middle Toba Tuff (MTT), and the Youngest Toba Tuff (YTT) dating at 1.2, 0.840, 0.501 and 0.074Ma respectively.10 Ignimbrite deposits from the three earlier eruptions are estimated to have produced volumes of HDT=35km3, OTT= 500km3, and MTT=60km3.10 Whereas deposits from the YTT accumulated to a volume of ∼2800km3, making it Earth’s largest Quaternary volcanic eruption.10 The aerosols released from this “super-eruption” are thought to have led to a global volcanic winter and had immense consequences on the human population.
3.0 THE TOBA SUPER-ERUPTION
As aforementioned, the Toba eruption was “one of the largest single volcanic eruptions in geologic history”.7 Scientists have recently uncovered that the eruption was due to changes in the magnetic system just prior to the eruption.9 By analyzing the quartz crystals found in magma, it was found that “the magma melted and assimilated a large volume of a local rock that is characterized by a relatively low ratio of 18O and 16O”.9 The high water content of this type of rock that was released into the magma produced steam, which led to a rapid increase in pressure within the magma chamber causing the magma to rupture through the overlaying crust.9
This eruption had a VEI magnitude of 8, categorizing it as a super-eruption.7 VEI is a function of an eruption’s plume height along with the volume of erupted tephra.7 It is estimated that the total erupted volume was 2800 km3.7 This erupted material is termed YYT and includes the dense rock equivalent (DRE) of pyroclastic material combined with the ignimbrite flows of the caldera.7 To put into perspective the magnitude of this eruption, the Tambora eruption of 1815 produced only 30-33 km3 DRE of pyroclastic material and reached a VEI of 7. The ejection height of the Toba eruption has been found to have reached a maximum height of 20-30 km.9 The ejected material was found to contain fine ash and various volatile substances such as: sulphur dioxide,
Figure 4. Distribution of volcanic ash from Toba Eruption11 fluorine, chlorine.7 The distribution of ash extends throughout the Bay of Bengal and deposits are spread across peninsular India (demonstrated in Figure 4).11
The caldera that formed as a result this eruption can be explained by the continuous addition of silicic magma bodies to the upper crust.7 The increased thickness of the crust led to a collapse of the magma bodies creating a caldera that measures 100 x 30 km.7 When looking at the shape of the caldera, scientists have found that the eruption must have occurred through multiple vents.13 This large caldera surrounds the calderas produced from the three previous eruptions (MTT, OTT, and HDT).7 Lake Toba now fills 2/3 of the caldera and extends about 500 m deep.7
Presently, the shoreline of the Samosir Island that lies in the centre of Lake Toba is tectonically tilted which indicates that the caldera is in a state of resurgence.7 This region of Toba is the most seismically active segment of the caldera and it is approximated that 600 m of resurgence has occurred over the past 33,000 years.7 Although there is no strong evidence to argue that a “super-volcano scale magma chamber” lies beneath Toba, it is highly likely that notable eruptions at this site will continue to occur until the IFZ subducts under Toba.7,14 However, because the volatile substances and molten magma of the magma chamber have not reached critical volumes, these eruptions are not to be expected for hundred of thousands of years.12
4.0 The Consequences of the Catastrophe and the Debate
The ∼74 ka eruption was the largest eruption of the Quaternary period that had regional and global consequences. The catastrophic nature of this event has undergone scrutiny and debate as some question whether or not the magnitude of the eruption led to a population bottleneck and caused the long-term global “cooling”. There is also ongoing debate about the timing that this event took place.
4.1 Climatic Consequences of the Eruption
The climatic changes following the Toba super-eruption are associated with a worldwide cooling that is known as a “volcanic winter”. This Toba-induced volcanic winter is estimated to have decreased global temperatures by 3 to 5ºC for six years.15 The accumulation of sulphate aerosols and volcanic ash released into the stratosphere during the eruption, generated clouds that reflected solar radiation back into space therefore preventing the penetration of heat to the Earth (see Figure 5).16 Ultimately, the volcanic winter caused destruction of surrounding vegetation and animal life.16
Figure 5. Climatic consequences of a large volcanic eruption16
It is argued that the correlation between the Toba eruption and the Quaternary climate change could have just been an unrelated coincidence.17 Researchers claim that because the volcanic aerosols have a short residence period in the atmosphere, the cooling temperatures that occurred after the eruption were not a direct impact of the eruption itself.16 Nevertheless, a study conducted by Rampino and Self, argued that the Toba eruption occurred at a transition period categorized by rapid ice growth and decreasing sea level.9 They further maintained that the subsequent cooling effects of the volcanic cloud would have been sufficient enough to accelerate the global transition “towards full glacial conditions”, or volcanic winter.9
As previously mentioned the catastrophic effects of the volcanic winter are said to have led to the extinction of the modern human and gave rise to a human population bottleneck.15 Alone, the direct impact of the tephra falls from the eruption would have demolished all trees, mammals and bird its path.15 Rampino and Ambrose bottleneck hypothesis suggests that the population bottleneck did in fact occur as a result of the volcanic winter.18 Following the Toba eruption the Earth was faced with a 6 year volcanic winter, characterized by the driest climates and coldest temperatures of the Quaternary period; thus causing famine and low productivity leading to subsequent reduction in the human population.9 They identified a late Pleistocene human population bottleneck that drastically reduced the global population to between 3000 and 10,000 individuals, and was specifically evidence within the African population.2,18 The low density African population began to increase in size approximately 70,000 years ago.18 The current human population is said to be the descendants of smaller African groups.18 Recovery of the human population occurred as a result of remaining individuals who united and adopted cooperative skills, and developed communication strategies to withstand the wrath of the volcanic winter.18 Contrastingly, Gathrone-Harding and Harcourt-Smith reject this bottleneck hypothesis and date the population bottleneck to 2 million years ago rather than as a result of the Toba eruption.18 However, evidence suggests that this estimate of the age of the bottleneck is highly unrealistic as there is geological proof of population reduction dated back to the time of the Toba eruption, and therefore the bottleneck effects must be associated with the climatic responses of Toba.18
4.2 Impact on Vegetation and Animal Life
Not only did the climatic changes following the Toba eruption have a devastating impact on the human population, it also led to the decimation of surrounding vegetation and mammal populations.2 Rampino and Self suggest that nuclear winter caused environmental devastations and led to the almost complete demolition of deciduous and temperate forests, as well as tropical plants.2 Similarly to catastrophic eruptions such as the 1883 Krakatau eruption, extinguished life forms in the immediate vicinity of the Toba eruption was a result of the tephra, lava and hot gases expelled.2 Many researchers have noted that the environmental effects of any super-eruption are not substantial enough to produce mammalian extinctions.2 They suggest that mammals have a greater ability to recover from catastrophic events, however there are still a notable number of mammalian extinctions associated with the Toba event.2 If the damaging effects of the volcanic winter to the mammals ecosystems did not produce a complete extinction of a species, it left subsequent “mosaic patches” of various species composition at the very least.2 Furthermore, the catastrophic outcomes of the Toba super-eruption led to a devastation of regional vegetation that impacted the respective mammalian populations.
5.0 AGE OF THE YOUNGEST TOBA ERUPTION
There is still ongoing debate about the estimated date of the ∼74 ka super-eruption of Toba. Determining an accurate date of the catastrophic event has posed many issues for researchers. Potassium-Argon dating is a direct method used by Storey et al. to determine the age of the volcanic tuff found at the site of the eruption.19 Samples of tuff found on in the caldera on the east central margin was found to have a K-Ar age of 73.5 ± 3 ka (or ∼74 ka).9 Indirect hypotheses such as those estimated by Zielinski et al., date the eruption to 71.1 ± 5 ka and infer this date from the ice core sulphate spike in Greenland.9 However, the best current estimate for the age of the YTT, that is proven to be most common amongst a vast majority of research is ∼74 ka and is justified using direct dating methods.9
6.0 CONCLUSION
Furthermore, it is evident that the ∼74 ka super-eruption of Toba was regional and global catastrophic event that produced the largest explosion and caldera on Earth. The location of the volcano along the seismically active volcanic arc led to the VEI 8 super-eruption.7 Although controversial, evidence has proven that the 2800 km3 of erupted tephra, volatile substances, ash, and sulphuric aerosols into the stratosphere caused a 6 year long global “volcanic winter” that led to a human population bottleneck.10 This catastrophic eruption also led to devastating effects of the regional vegetation and animal wildlife. Over the next hundreds and thousands of years, this resurgent caldera is likely to erupt again, hopefully not to the extreme of the YTT eruption.12 Finally, in spite of the controversy surrounding the Toba eruption, it was still one of the largest eruptions of the Quaternary period that had significant climatic and global consequences.