Toba Lake "unforgettable"

Toba lake

(Indonesian: Danau Toba) is a lake and supervolcano. The lake is 100 kilometres long, 30 kilometres wide, and up to 505 meters (1,666 ft) deep. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about 900 metres (2,953 ft), the lake stretches from 2.88°N 98.52°E to 2.35°N 99.1°E. It is the largest lake in Indonesia and the largest volcanic lake in the world.^[1]

Panoramic view of Parapat from Samosir Island, Lake Toba.

Lake Toba is the site of a massive supervolcanic eruption estimated at VEI 8 that occurred 69,000 to 77,000 years ago,^[2][3][4] representing a climate-changing event. It is the largest known explosive eruption on Earth in the last 25 million years. According to the Toba catastrophe theory, it had global consequences for human populations: it killed most humans living at that time and is believed to have created a population bottleneck in central east Africa and India, which affects the genetic make up of the human world-wide population to the present.^[5] This hypothesis is not widely accepted because evidence is lacking for a decline or extinction of other animals, including species that are sensitive to changes in the environment.^[6]

It has been accepted that the eruption of Toba led to a volcanic winter with a worldwide decrease in temperature between 3 to 5 °C (5.4 to 9.0 °F), and up to 15 °C (27 °F) in higher latitudes. Additional studies in Lake Malawi in East Africa show significant amounts of ash being deposited from the Toba eruptions, even at that great distance, but little indication of a significant climatic effect in East Africa.

Geology

Panoramic view of the town of Ambarita on Samosir, Lake Toba

The Toba caldera complex in Northern Sumatra, comprises four overlapping volcanic craters that adjoin the Sumatran "volcanic front." The youngest and fourth caldera is the world's largest Quaternary caldera (100 by 30 km (62 by 19 mi)) and intersects the three older calderas. An estimated 2,800 km³ (670 cu mi) of dense-rock equivalent pyroclastic material, known as the youngest Toba tuff, was released during one of the largest explosive volcanic eruptions in recent geological history. Following this eruption, a resurgent dome formed within the new caldera, joining two half-domes separated by a longitudinal graben.^[3]

At least four cones, four stratovolcanoes, and three craters are visible in the lake. The Tandukbenua cone on the northwestern edge of the caldera has only sparse vegetation, suggesting a young age of several hundred years. Also, the Pusubukit (Hill Center) volcano (1971 meters above sea level) on the south edge of the caldera is solfatarically active and is a Geology Sanctuary.

The eruption

Main article: Toba catastrophe theory

Location of Lake Toba shown in red on map.

The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago.^[10] It was the last in a series of at least three caldera-forming eruptions at this location, with earlier calderas having formed around 700,000 and 840,000 years ago.^[11] This last eruption had an estimated VEI 8, making it possibly the largest explosive volcanic eruption within the last 25 million years.
Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was about 2,800 km³ (670 cu mi)^[12]—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. The pyroclastic flows of the eruption destroyed an area of 20,000 km² (7,722 sq mi), with ash deposits as thick as 600 m (1,969 ft) by the main vent.^[12]
The eruption was large enough to have deposited an ash layer approximately 15 cm (5.9 in) thick over all of South Asia; at one site in central India, the Toba ash layer today is up to 6 m (20 ft) thick^[13] and parts of Malaysia were covered with 9 m (30 ft) of ash fall.^[14] In addition it has been variously calculated that 10,000 million tonnes (1.1×10¹⁰ short tons) of sulfurous acid^[15] or 6,000 million tonnes (6.6×10⁹ short tons) of sulfur dioxide^[16] were ejected into the atmosphere by the event.
The subsequent collapse formed a caldera that, after filling with water, created Lake Toba. The island in the center of the lake is formed by a resurgent dome.

Landsat photo of Sumatra surrounding Lake Toba

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.^[17] The eruption lasted perhaps two weeks, and the ensuing "volcanic winter" resulted in a decrease in average global temperatures by 3.0 to 3.5 °C (5 to 6 °F) for several years. Greenland ice cores record a pulse of starkly reduced levels of organic carbon sequestration. Very few plants or animals in southeast Asia would have survived, and it is possible that the eruption caused a planet-wide die-off.
Evidence from studies of mitochondrial DNA suggests that humans may have passed through a genetic bottleneck around this time that reduced genetic diversity below what would be expected given the age of the species. According to the Toba catastrophe theory, proposed by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign in 1998, the effects of the Toba eruption may have decreased the size of human populations to only a few tens of thousands of individuals.^[18] However, this hypothesis is not widely accepted because similar effects on other animal species have not been observed.^[6]

Lake Toba
Lake Toba
Location	North Sumatra, Indonesia
Coordinates	2.6845°N 98.8756°ECoordinates: 2.6845°N 98.8756°E
Type	Volcanic/ tectonic
Primary outflows	Asahan River
Basin countries	Indonesia
Max. length	100 km (62 mi)
Max. width	30 km (19 mi)
Surface area	1,130 km2 (440 sq mi)
Average depth	500 meters
Max. depth	505 m (1,657 ft)[1]
Water volume	240 km3 (58 cu mi)
Surface elevation	905 m (2,969 ft)
Islands	Samosir
Settlements	Ambarita, Pangururan
References