The Toba eruption, also known as the Toba supereruption and the Youngest Toba eruption, was a massive volcanic event that happened about 74,000 years ago during the Late Pleistocene. It occurred at the location of present-day Lake Toba in Sumatra, Indonesia. This eruption was the last in a series of at least four eruptions that formed calderas in that area. An earlier caldera in the same region formed approximately 1.2 million years ago. The Toba eruption had a volcanic explosivity index of 8, which is the highest level on the scale used to measure volcanic eruptions. This makes it the largest known explosive volcanic eruption during the Quaternary period and one of the largest explosive eruptions in Earth's history.
Eruption
The exact date of the eruption is unknown. However, the pattern of ash deposits suggests it happened during the northern summer because only the summer monsoon could have carried Toba ash to the South China Sea. The eruption lasted about 9 to 14 days. Recent scientific studies using a method called argon–argon dating placed the eruption at 73,880 ± 320 and 73,700 ± 300 years ago. Five separate magma bodies were active within a few centuries before the eruption. The eruption began with small amounts of ash falling from the sky, followed by a major phase of ignimbrite flows. The ignimbrite phase had low eruption fountains, but a column of gas and ash rose above the pyroclastic flows to a height of 32 km (20 mi). Studies of the magma suggest sulfur emissions ranged from 1 × 10 to 1 × 10 grams, depending on whether sulfur gas existed in the magma chamber. The lower estimate is due to sulfur’s low solubility in magma. Ice core records estimate sulfur emissions at about 1 × 10 grams.
Bill Rose and Craig Chesner of Michigan Technological University estimated that the eruption released at least 2,800 km³ (670 cu mi) of material. This included about 2,000 km³ (480 cu mi) of ignimbrite that flowed over the ground and approximately 800 km³ (190 cu mi) of ash that fell mostly to the west. More recent estimates suggest the total erupted material was 3,800 km³ (910 cu mi) of dense-rock equivalent (DRE), with 1,800 km³ (430 cu mi) as ash fall and 2,000 km³ (480 cu mi) as ignimbrite. This makes the Toba eruption the largest during the Quaternary period. Earlier estimates of erupted material ranged from 2,000 km³ (480 cu mi) to 6,000 km³ (1,400 cu mi). Inside the caldera, pyroclastic flows reached a thickness of over 600 m (2,000 ft). The outflow sheet originally covered an area of 20,000–30,000 km² (7,700–11,600 sq mi) with a thickness of nearly 100 m (330 ft), possibly extending into the Indian Ocean and the Straits of Malacca. Ash from the eruption covered the Indian subcontinent with a layer 5 cm (2.0 in) thick, the Arabian Sea with 1 mm (0.039 in), the South China Sea with 3.5 cm (1.4 in), and the Central Indian Ocean Basin with 10 cm (3.9 in). The ash layer covered more than 38,000,000 km² (15,000,000 sq mi) with at least 1 cm (0.39 in) of thickness (~7.5% of Earth’s surface). In Sub-Saharan Africa, microscopic glass shards from the eruption were found on the south coast of South Africa, in northwest Ethiopia, in Lake Malawi, and in Lake Chala. In South China, Toba tephras were discovered in Huguangyan Maar Lake.
After the eruption, the area collapsed to form a caldera, which later filled with water to create Lake Toba. An island in the center of the lake was formed by a resurgent dome.
Greenland stadial 20 (GS20) was a cold event in the North Atlantic that began around the time of the Toba eruption. GS20 started about 74.0–74.2 kyr ago and lasted approximately 1,500 years. It is part of Dansgaard–Oeschger event 20 (DO20) and is linked to a sudden weakening of the Atlantic meridional overturning circulation (AMOC). This weakening caused warming in the Southern Ocean and Antarctica, a pattern known as the bipolar seesaw. The start of GS20 cooling matched the start of Antarctic Isotope Maxima 19 (AIM19) warming. GS20 was also called Heinrich stadial 7a due to iceberg discharges into the North Atlantic. Heinrich events are longer, colder, and have weaker AMOC than other DO stadials. From 74 to 58 kyr, Earth transitioned from the warm marine isotope stage (MIS) 5 to the colder MIS 4, with cooling and glacial expansion. This transition is part of the Pleistocene climate cycle driven by Earth’s orbital changes. Ocean temperatures dropped by 0.9 °C (1.6 °F), sea levels fell 60 m (200 ft), and ice sheets expanded in the Northern Hemisphere, surpassing the extent of the Last Glacial Maximum in parts of Europe, Asia, and North America. Southern Hemisphere glaciation reached its maximum during MIS 4, with colder and drier conditions in Australasia, Africa, and Europe.
The Toba eruption occurred during the rapid climate changes of GS20 and MIS 4, driven by ocean currents and changes in sunlight. Whether the eruption influenced these changes is debated. Marine records from the South China Sea show 1 °C (1.8 °F) of cooling above the Toba ash layer for a thousand years, but this may be linked to GS20. Marine records from the Arabian Sea confirm the Toba ash occurred after GS20 began but show GS20 was not colder than GS21, suggesting the eruption did not intensify the cooling. Studies of Lake Malawi show no major changes in lake ecology or grassy woodlands after the ash fell, but high-elevation forests were affected by aridity. These findings are questioned due to possible sediment mixing that may have reduced the cooling signal. Environmental records from Ethiopia show a severe drought occurred alongside the Toba ash layer, affecting early human foraging behaviors.
Toba ash has not
Toba catastrophe theory
The Toba catastrophe theory suggests that a massive volcanic eruption caused a long, severe cold period lasting six to ten years and led to a cooling event that lasted about 1,000 years. This cooling may have caused a genetic bottleneck in humans, meaning the human population became very small. However, some evidence challenges this idea, and some scientists believe the theory is not supported by all the facts.
In 1972, scientists studied human hemoglobins and found few variations. This led to the idea that the human population was very small, possibly only a few thousand people, until recently. Later studies showed that humans had a population of about 10,000 for much of their history. Research on mitochondrial DNA suggested that the human population grew rapidly from around 1,000 to 10,000 people between 35,000 and 65,000 years ago. More recent studies suggest the climate changes were not as extreme as previously thought.
In 1993, science writer Ann Gibbons suggested that the cold climate of the last Ice Age, possibly worsened by the Toba eruption (dated to 73,000–75,000 years ago), limited human population growth. She believed that human populations expanded again when the Ice Age ended. Scientists Michael R. Rampino and Stephen Self supported this idea. In 1998, anthropologist Stanley H. Ambrose used genetic evidence to suggest that the Toba eruption caused a sharp drop in the human population to only a few thousand people. He believed the population recovered only after the climate warmed around 60,000 years ago.
Other human species, such as Neanderthals and Denisovans, survived the Toba eruption and the Ice Age. Their last known presence dates to about 40,000 and 55,000 years ago, respectively. Other species, like Homo floresiensis and Homo luzonensis, may also have survived. Recent studies using whole-genome data and archaeological findings show how humans lived through the eruption and the following ice age.
Recent research uses genetic models to study human population history. For non-African populations, studies show a sharp decline in numbers starting 200,000 years ago, reaching a low point around 40,000–60,000 years ago. During this time, non-African populations may have shrunk by 5 to 15 times, with only 1,000–3,000 people surviving. This matches earlier studies of mitochondrial DNA and is linked to the "Out-of-Africa" migration. Genetic evidence also shows 56 changes in genes related to cold adaptation in non-African populations, with 31 of these changes occurring between 72,000 and 97,000 years ago. This event is called the "Arabian Standstill" and may have been caused by the cold, dry conditions of the Ice Age and the Toba eruption.
African populations experienced a smaller and earlier population decline, with numbers dropping to around 10,000 people. The cause of this decline is unclear, but it may be linked to climate changes, population movements, or other factors. Earlier studies of genetic material suggest that human populations were very small around 1.2 million years ago, possibly due to repeated population crashes or competition with other human species. Whole-genome analysis also shows low population numbers in Africa around 1 million years ago, which may have been caused by a severe ice age.
Some research challenges the idea that the Toba eruption caused a genetic bottleneck. For example, stone tools found in southern India show that human populations may have survived the eruption, as tools were found both before and after the ash layer. However, other sites in India suggest population declines. Studies of tools in the Jurreru Valley show a gap in time between tools before and after the eruption, possibly due to population loss or sediment removal. Some researchers argue that the effects of the eruption on local populations were not as severe as previously thought. The Toba eruption also coincides with the disappearance of early humans in Skhul and Qafzeh. Evidence suggests that forests in South Asia may have been destroyed for a long time, and humans may have adapted to survive.
Other animals, such as Eastern African chimpanzees, Bornean orangutans, central Indian macaques, gorillas, cheetahs, and tigers, may have also experienced population crashes after the Toba eruption. Their populations grew from very small numbers around 70,000–55,000 years ago.