Youngest Toba eruption
The Toba eruption was a supervolcanic eruption that occurred around 74,000 years ago, during the Late Pleistocene, at the site of present-day Lake Toba, in Sumatra, Indonesia. It was the last in a series of at least four caldera-forming eruptions there, the earlier known caldera having formed about 1.2 million years ago. This, the last eruption, had an estimated volcanic explosivity index of 8, making it the largest known explosive volcanic eruption in the Quaternary, and one of the largest known explosive eruptions in the Earth's history.
Eruption
Chronology of the Toba eruption
The exact date 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. The eruption lasted perhaps 9 to 14 days. The most recent two high-precision argon–argon datings dated the eruption to 73,880 ± 320 and 73,700 ± 300 years ago. Five distinct magma bodies were activated within a few centuries before the eruption. The eruption commenced with small and limited air-fall and was directly followed by the main phase of ignimbrite flows. The ignimbrite phase is characterized by low eruption fountain, but co-ignimbrite column developed on top of pyroclastic flows reached a height of. Petrological constraints on sulfur emission yielded a wide range from to, depending on the existence of separate sulfur gas in the Toba magma chamber. The lower end of the estimate is due to the low solubility of sulfur in the magma. Ice core records estimate the sulfur emission on the order of.Effects of the eruption
and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was at least —about of ignimbrite that flowed over the ground, and approximately that fell as ash mostly to the west. However, as more outcrops become available, the most recent estimate of eruptive volume is dense-rock equivalent, of which was deposited as ash fall and as ignimbrite, making this eruption the largest during the Quaternary period. Previous volume estimates have ranged from to. Inside the caldera, the maximum thickness of pyroclastic flows is over. The outflow sheet originally covered an area of with thickness nearly, likely reaching into the Indian Ocean and the Straits of Malacca. The air-fall of this eruption blanketed the Indian subcontinent in a layer of ash, the Arabian Sea in, the South China Sea in, and Central Indian Ocean Basin in. Its horizon of ashfall covered an area of more than in or more thickness. In Sub-Saharan Africa, microscopic glass shards from this eruption are also discovered on the south coast of South Africa, in the lowlands of northwest Ethiopia, in Lake Malawi, and in Lake Chala. In South China, Toba tephras is found in Huguangyan Maar Lake.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 the time of the eruption
Greenland stadial 20 is a millennium-long cold event in the north Atlantic ocean that started around the time of the Toba eruption. The timing of the initiation of GS20 is dated to 74.0–74.2 kyr, and the entire event lasted about 1,500 years. It is the stadial part of Dansgaard–Oeschger event 20, commonly explained by an abrupt reduction in the strength of the Atlantic meridional overturning circulation. Weaker AMOC caused warming in the Southern Ocean and Antarctica, and this asynchrony is known as bipolar seesaw. The start of the GS20 cooling event corresponds to the start of the Antarctic Isotope Maxima 19 warming event. GS20 was associated with iceberg discharges into the North Atlantic, thus it was also named Heinrich stadial 7a. Heinrich events tend to be longer, colder and with weaker AMOC in the Atlantic ocean than other DO stadials.From 74 to 58 kyr, Earth transitioned from interglacial marine isotope stage 5 to glacial MIS 4, experiencing cooling and glacial expansion. This transition is a part of the Pleistocene interglacial-glacial cycle driven by variations in the Earth's orbit. Ocean temperatures cooled by. Sea level fell. Northern Hemisphere ice sheets embarked on significant expansion and surpassed the extent of the Last Glacial Maximum in eastern Europe, Northeast Asia and the North American Cordillera. Southern Hemisphere glaciation grew to its maximum extent during MIS 4. Australasia, Africa and Europe were characterized by increasingly cold and arid environments.
Possible climate records of the eruption
While the Toba eruption occurred in the backdrop of the rapid climate transitions of GS20 and MIS 4, triggered by changes in ocean currents and insolation, whether the eruption played any role in accelerating these events is much more heavily debated. South China Sea marine records of climate, sampled at every centennial interval, shows cooling above the Toba ash layer for a thousand years but the authors concede that it may just be GS20. 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. 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, but cooling-forced aridity killed high-elevation afromontane forests. The Lake Malawi studies concluded that the environmental effects of the eruption were mild and limited to less than a decade in East Africa, but these studies are questioned due to sediment mixing which would have diminished the cooling signal. Environmental records from a Middle Stone Age site in Ethiopia, however, show that a severe drought occurred concurrently with the Toba ash layer, which altered early human foraging behaviours.Toba ash records have not been identified in ice core samples. However, four sulfate events in the ice strata are proposed to represent the deposition of aerosols from the Toba eruption. 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. In the ice core records, GS20 cooling was already underway by the time of sulfate deposition; however, a 110-year period of accelerated cooling followed this sulfate event. The authors interpret this acceleration as AMOC weakened by the Toba eruption.
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 of sulfur, which is 100 times the 1991 Pinatubo sulphur, volcanic winter has a maximum global mean cooling of and returns gradually within the range of natural variability 5 years after the eruption. An initiation of a 1,000-year cold period or ice age is not supported by the model. Two other emission scenarios, and, were investigated using state-of-art simulations provided by the Community Earth System Model. Maximum global mean cooling was for the lower emission and for the higher emission scenarios. A strong decrease in precipitation occurs in the high emission scenario. Negative temperature anomalies return to less than within 3 and 6 years for each emission scenario after the eruption. 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 the magnitude of cooling to less than, no matter how much sulfur is emitted.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. However, some physical evidence disputes the association with the millennium-long cold event and genetic bottleneck, and some consider the theory disproven.History
In 1972, an analysis of human hemoglobins found very few variants, and to account for this low frequency of variation, the human population must have been as low as a few thousand until very recently. More genetic studies confirmed an effective population on the order of 10,000 for much of human history. 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 and 65 kyr ago. Recent research shows the extent of climate change was much smaller than believed by proponents of the theory.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 super-eruption which at the time was dated to between 73 and 75 kyr near the beginning of glacial period MIS 4. The subsequent explosive human expansion was believed to be the result of the end of the ice age. Geologist Michael R. Rampino of New York University and volcanologist Stephen Self of the University of Hawaiʻi at Mānoa supported her theory. In 1998, anthropologist Stanley H. Ambrose of the University of Illinois Urbana-Champaign used coalescence evidence of some genes to hypothesize 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.