The exact year of the eruption is unknown, but the pattern of ash deposits suggests that it occurred during the northern summer because only the
summer monsoon could have deposited Toba ashfall in the South China Sea.[4] The eruption lasted perhaps 9 to 14 days.[5] The most recent two high-precision
argon–argon datings dated the eruption to 73,880 ± 320[6] and 73,700 ± 300 years ago.[7] Five distinct
magma bodies were activated within a few centuries before the eruption.[8][9] The eruption commenced with small and limited air-fall and was directly followed by the main phase of ignimbrite flows.[10] The ignimbrite phase is characterized by low eruption fountain,[11] but co-ignimbrite column developed on top of pyroclastic flows reached a height of 32 km (20 mi).[12]Petrological constraints on sulfur emission yielded a wide range from 1×1013 to 1×1015 g, depending on the existence of separate sulfur gas in the Toba magma chamber.[13][14] The lower end of estimate is due to the low solubility of sulfur in the magma.[13]Ice core records estimate the sulfur emission on the order of 1×1014 g.[15]
Effects of the eruption
Bill Rose and Craig Chesner of
Michigan Technological University have estimated that the total amount of material released in the eruption was at least 2,800 km3 (670 cu mi)[16]—about 2,000 km3 (480 cu mi) of
ignimbrite that flowed over the ground, and approximately 800 km3 (190 cu mi) that fell as ash mostly to the west. However, as more outcrops become available, the most recent estimate of eruptive volume is 3,800 km3 (910 cu mi)
dense-rock equivalent (DRE), of which 1,800 km3 (430 cu mi) was deposited as ash fall and 2,000 km3 (480 cu mi) as
ignimbrite, making this eruption the largest during the
Quaternary period.[17] Previous volume estimates have ranged from 2,000 km3 (480 cu mi)[5] to 6,000 km3 (1,400 cu mi).[18] Inside the caldera, the maximum thickness of
pyroclastic flows is over 600 m (2,000 ft).[19] The outflow sheet originally covered an area of 20,000–30,000 km2 (7,700–11,600 sq mi) with thickness nearly 100 m (330 ft), likely reaching into the
Indian Ocean and the
Straits of Malacca.[10] The air-fall of this eruption blanketed the
Indian subcontinent in a layer of 5 cm (2.0 in) ash,[20] the
Arabian Sea in 1 mm (0.039 in),[21] the
South China Sea in 3.5 cm (1.4 in),[4] and Central Indian Ocean Basin in 10 cm (3.9 in).[22] Its horizon of ashfall covered an area of more than 38,000,000 km2 (15,000,000 sq mi) in 1 cm (0.39 in) or more thickness.[17] In
Sub-Saharan Africa, microscopic glass shards from this eruption are also discovered on the south coast of
South Africa,[23] in the
lowlands of northwest
Ethiopia,[24] in
Lake Malawi,[25] and in
Lake Chala.[26] In
South China, Toba tephras is found in Huguangyan
Maar Lake.[27]
The subsequent collapse formed a caldera that filled with water, creating Lake Toba. The island in the center of the lake is formed by a
resurgent dome.
Climatic effects
Climate at time of eruption
Greenland stadial 20 (GS20) is a millennium-long cold event in the north
Atlantic ocean that started around the time of Toba eruption.[28] The timing of the initiation of GS20 is dated to 74.0–74.2 kyr, and the entire event lasted about 1,500 years.[28][29] It is the stadial part of
Dansgaard–Oeschger event 20 (DO20), commonly explained by an abrupt reduction in the strength of the
Atlantic meridional overturning circulation (AMOC). Weaker AMOC caused warming in
Southern Ocean and
Antarctica, and this asynchrony is known as
bipolar seesaw.[30][31] The start of GS20 cooling event corresponds to the start of Antarctic Isotope Maxima 19 (AIM19) warming event.[32] GS20 was associated with iceberg discharges into the North Atlantic, thus it was also named
Heinrich stadial 7a.[33] Heinrich events tend to be longer, colder and with weaker AMOC in the Atlantic ocean than other DO stadials.[30]
From 74 to 58
kyr, Earth transitioned from interglacial MIS 5 to glacial MIS 4, experiencing cooling and glacial expansion.[34][35] This transition is a part of Pleistocene interglacial-glacial cycle driven by variations in the earth's orbit.[36] Ocean temperature cooled by 0.9 °C (1.6 °F).[37] Sea level fell 60 m (200 ft).[38] Northern Hemisphere ice sheets embarked on significant expansion and surpassed the extent of
Last Glacial Maximum in
eastern Europe,
Northeast Asia and the
North American Cordillera.[39] Southern Hemisphere glaciation grew to its maximum extent during MIS 4.[40]Australasian region, Africa and Europe were characterized by increasingly cold and
arid environment.[41][42][43]
Possible climate records of eruption
While Toba eruption occurred in the backdrop of rapid climate transitions of GS20 and MIS 4 triggered by changes in ocean currents and
insolation,[44][28] whether the eruption played any role in accelerating these events is much more debated. South China Sea marine records of climate, sampled at every centennial interval, shows 1 °C (1.8 °F) cooling above Toba ash layer for a thousand year but the authors concede that it may just be GS20.[45] Arabian Sea marine records confirm that Toba ash occurred after the onset of GS20 but also that GS20 is not colder than GS21 in the records, from which authors conclude that the eruption did not intensify GS20 cooling.[46] Dense sampling of environmental records, at every 6
–9 year interval, in Lake Malawi, show no cooling-induced change in
lake ecology and in
grassy woodlands after the deposition of Toba ash,[25][47] but cooling-forced aridity killed high elevation
afromontane forests.[48] The Lake Malawi studies concluded that the environmental effects of the eruption were mild and limited to less than a decade in East Africa,[47] but these studies are questioned due to sediment mixing which would have diminished the cooling signal.[49] Environmental records from a
Middle Stone Age site in Ethiopia, however, shows that a severe drought occurred concurrently with Toba ash layer which altered early human
foraging behaviours.[24]
No Toba ash has been identified in ice core records, but four sulfate events within the ice strata have been proposed to possibly represent the deposition of aerosols from Toba eruption.[50][32][51] One sulfate event at 73.75–74.16 kyr, which has all the characteristics of the Toba eruption, is among the largest sulfate loadings that have ever been identified.[51] In the ice core records, GS20 cooling was already underway by the time of sulfate deposition, nonetheless a 110-year period of accelerated cooling followed the sulfate event, and the authors interpret this acceleration as AMOC weakened by the Toba eruption.[15]
Climate modeling
The modeled climate effects of the Toba eruption hinges on the mass of sulfurous gases and aerosol microphysical processes. Modeling on an emission of 8.5×1014 g of sulfur, which is 100 times the
1991 Pinatubo sulphur, volcanic winter has a maximum global mean cooling of 3.5 °C (6.3 °F) and returns gradually within the range of natural variability 5 years after the eruption. An initiation of 1,000-year cold period or ice age is not supported by the model.[52][53] Two other emission scenarios, 1×1014 g and 1×1015 g, are investigated using state-of-art simulations provided by the
Community Earth System Model. Maximum global mean cooling is 2.3 °C (4.1 °F) for the lower emission and 4.1 °C (7.4 °F) for the higher emission. Strong decrease in precipitation occurs in high emission. Negative temperature anomalies return to less than 1 °C (1.8 °F) within 3 and 6 years for each emission scenario after the eruption.[54] But so far no model can simulate aerosol microphysical processes with sufficient accuracy, empirical constraints from historical eruptions suggest that aerosol size may substantially reduce magnitude of cooling to less than 1.5 °C (2.7 °F) no matter how much sulfur emitted.[55]
Toba catastrophe theory
The Toba catastrophe theory holds that the eruption caused a severe global
volcanic winter of six to ten years and contributed to a 1,000-year-long cooling episode, resulting in a
genetic bottleneck in
humans.[56][57] However, some physical evidence disputes the association with the millennium-long cold event and genetic bottleneck, and some consider the theory disproven.[58][48][59][60][61]
History
In 1972, an analysis of human
hemoglobins found very few variants, and to account for the low frequency of variation human population must have been as low as a few thousand until very recently.[62] More genetic studies confirmed an effective population on the order of 10,000 for much of human history.[63][64] Subsequent research on the differences in human
mitochondrial DNA sequences dated a rapid growth from a small
effective population size of 1,000 to 10,000, sometime between 35,000 and 65,000 years ago.[65][66][67]
The large magnitude of the Toba eruption has been known since 1939, and various techniques dated the timing of the event to 73,000 to 75,000 years ago.[5] A study published in 1993 suggested that the eruption accelerated climate and environmental transition from the last interglacial period
MIS 5 to the
last glacial period MIS 4.[68]
In 1993, science journalist Ann Gibbons posited that population growth was suppressed by the cold climate of the last Pleistocene Ice Age, possibly exacerbated by the Toba eruption. The subsequent explosive human expansion was believed to be the result of the end of the ice age.[69] Geologist
Michael R. Rampino of
New York University and volcanologist Stephen Self of the
University of Hawaiʻi at Mānoa supported her theory.[70] In 1998, anthropologist Stanley H. Ambrose of the
University of Illinois Urbana-Champaign hypothesized that the Toba eruption caused a human population crash to only a few thousand surviving individuals, and the subsequent recovery was suppressed by the global glacial condition of MIS 4 until the climate eventually transitioned to the warmer condition of MIS 3 about 60,000 years ago, during which rapid human population expansion occurred.[56]
Possible effects on Homo
At least two other Homo lineages,
H. neanderthals, and
Denisovans, survived the Toba eruption and subsequent MIS 4 ice age, as their latest presence are dated to ca. 40 kyr,[71] and ca. 55 kyr.[72] Other lineages including H. floresiensis,[73]H. luzonensis,[74] and
Penghu 1[75] may had also survived through the eruption. More recently, reconstructions of human demographic history using
whole-genome sequencing[76][77][78] and discoveries of archaeological cultures with Toba ash layer[79][23][24] add further light to how humans had fared during the eruption and the following GS20 and MIS 4 ice age.
Human demographic history
The Toba eruption has been associated with a
genetic bottleneck in human evolution about 70,000 years ago;[80][81] it is hypothesized that the eruption resulted in a severe reduction in the size of the total human population due to the effects of the eruption on the global climate.[82] According to the genetic bottleneck theory, between 50,000 and 100,000 years ago, human populations decreased to 3,000–10,000 surviving individuals.[83][84] It is supported by some genetic evidence suggesting that modern humans are descended from a very small population of between 1,000 and 10,000 breeding pairs that existed about 70,000 years ago.[85][86]
Proponents of the genetic bottleneck theory (including Robock) suggest that the Toba eruption resulted in a global ecological disaster, including destruction of vegetation along with severe drought in the
tropical rainforest belt and in monsoonal regions. A 10-year volcanic winter triggered by the eruption could have largely destroyed the food sources of humans and caused a severe reduction in population sizes.[87] These environmental changes may have generated population bottlenecks in many species, including
hominids;[88] this in turn may have accelerated differentiation from within the smaller human population. Therefore, the genetic differences among modern humans may represent changes within the last 70,000 years, rather than gradual differentiation over hundreds of thousands of years.[89]
Additional caveats include difficulties in estimating the global and regional climatic effects of the eruption and lack of conclusive evidence for the eruption preceding the crash.[90] Furthermore, genetic analysis of
Alu sequences across the entire
human genome has shown that the effective human population size was less than 26,000 at 1.2 million years ago; possible explanations for the low population size of human ancestors may include repeated population crashes or periodic replacement events from competing Homo subspecies. (If these results are accurate, then, even before the emergence of Homo sapiens in Africa, Homo erectus population was unusually small when the species was spreading around the world.)[91]
The exact geographic distribution of anatomically modern human populations at the time of the eruption is not known, and surviving populations may have lived in
Africa and subsequently migrated to other parts of the world. Analyses of
mitochondrial DNA have estimated that the
major migration from Africa occurred 60,000–70,000 years ago,[92] consistent with dating of the Toba eruption to about 75,000 years ago.[citation needed]
Archaeological studies
Other research has cast doubt on an association between the Toba Caldera Complex and a genetic bottleneck. For example, ancient
stone tools at the Jurreru Valley in southern India were found above and below a thick layer of ash from the Toba eruption and were very similar across these layers, suggesting that the dust clouds from the eruption did not wipe out this local population.[93][94][95] However, another site in India, the Middle Son Valley, exhibits evidence of a major population decline and it has been suggested that the abundant springs of the Jurreru Valley may have offered its inhabitants unique protection.[96] Additional archaeological evidence from southern and northern India also suggests a lack of evidence for effects of the eruption on local populations, causing the authors of the study to conclude, "many forms of life survived the supereruption, contrary to other research which has suggested significant animal extinctions and genetic bottlenecks".[97] However, some researchers have questioned the techniques utilized to date artifacts to the period subsequent to the Toba supervolcano.[98] The Toba Catastrophe also coincides with the disappearance of the
Skhul and Qafzeh hominins.[99] Evidence from pollen analysis has suggested prolonged deforestation in South Asia, and some researchers have suggested that the Toba eruption may have forced humans to adopt new adaptive strategies, which may have permitted them to replace
Neanderthals and "other archaic human species".[100][101]
Genetic bottlenecks in other mammals
Some evidence indicates population crashes of other animals after the Toba eruption. The populations of the Eastern African
chimpanzee,[102]Bornean orangutan,[103] central Indian
macaque,[104]cheetah and
tiger,[105] all expanded from very small populations around 70,000–55,000 years ago.
^Ambrose, S. H. (2019), "Chapter 6 chronological calibration of Late Pleistocene Modern Human dispersals, climate change and Archaeology with Geochemical Isochrons", in Sahle, Yonatan; Reyes-Centeno, Hugo; Bentz, Christian (eds.), Modern Human Origins and Dispersal, Kerns Verlag, pp. 171–213
^Jones, Sacha. (2012). Local- and Regional-scale Impacts of the ~74 ka Toba Supervolcanic Eruption on Hominin Population and Habitats in India. Quaternary International 258: 100-118.
^National Geographic- Did early humans in India survive a supervolcano?
^ Shea, John. (2008). Transitions or Turnovers? Climatically-forced Extinctions of Homo sapiens and Neanderthals in the East Mediterranean Levant. Quaternary Science Reviews 27: 2253-2270.
Ninkovich, D.; N.J. Shackleton; A.A. Abdel-Monem; J.D. Obradovich; G. Izett (7 December 1978). "K−Ar age of the late Pleistocene eruption of Toba, north Sumatra". Nature. 276 (5688): 574–577.
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10.1016/S0277-3791(01)00154-8
Steiper, M.E. (2006). "Population history, biogeography, and taxonomy of orangutans (Genus: Pongo) based on a population genetic meta-analysis of multiple loci". Journal of Human Evolution. 50 (5): 509–522.
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Journey of Mankind by The Bradshaw Foundation – includes discussion on Toba eruption, DNA and human migrations
Geography Predicts Human Genetic Diversity ScienceDaily (Mar. 17, 2005) – By analyzing the relationship between the geographic location of current human populations in relation to East Africa and the genetic variability within these populations, researchers have found new evidence for an African origin of modern humans.