Lake Toba

Lake Toba, Indonesia
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Landsat photo of Lake Toba
Lake type Volcanic/ tectonic
Primary outflows Asahan River
Basin countries North Sumatra, Indonesia
Max-length 100 km
Max-width 30 km
Surface area 1130 km²
Max-depth 505 m[1]
Water volume 240 km³
Surface elevation 905 m
Islands Samosir

Lake Toba (Indonesian: Danau Toba) is a large lake, 100 km long and 30 km wide, in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about 900 m (3000 feet), stretching from 2.88° N 98.52° E to 2.35° N 99.1° E.

Contents

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Geology

In 1949 the Dutch geologist Rein van Bemmelen reported that Lake Toba was surrounded by a layer of ignimbrite rocks, and that it was a large volcanic caldera. Later researchers found rhyolite ash similar to that in the ignimbrite around Toba (now called Young Toba Tuff to distinguish it from layers deposited in previous explosions) in Malaysia and India, 3000 km away. Oceanographers discovered Toba ash, with its characteristic chemical "fingerprint", on the floor of the eastern Indian Ocean and the Bay of Bengal.

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The eruption

Location of Lake Toba shown in red on map.
Location of Lake Toba shown in red on map.

The Toba eruption (the Toba event) occurred at what is now Lake Toba about 71,500 ± 4000 years ago. It had an estimated Volcanic Explosivity Index of 8 (described as "mega-colossal"), making it possibly the largest volcanic eruption within the last two million years. Bill Rose and Craig Chesner of Michigan Technological University deduced that the total amount of erupted material was about 2800 cubic km (670 cubic miles) — around 2000 km³ of ignimbrite that flowed over the ground and around 800 km³ that fell as ash, with the wind blowing most of it to the west. By contrast, the 1980 eruption of Mount St. Helens ejected around 1 cubic km of material, whilst the largest volcanic eruption in historic times, at Mount Tambora in 1815, emitted the equivalent of around 100 cubic kilometres of dense rock and created the "Year Without a Summer" as far away as North America.

The Toba eruption was the last of a series of at least three caldera-forming eruptions which have occurred at the volcano. Earlier calderas were formed around 700,000 and 840,000 years ago[2].

To give an idea of its magnitude, consider that although the eruption took place in Indonesia, it deposited an ash layer approximately 15 cm (6 in) thick over the entire Indian subcontinent; at one site in central India, the Toba ash layer today is up to 6 m (20 feet) thick[3] and parts of Malaysia were covered with 9 m of ashfall.[4] In addition it has been calculated that an amount of 1010 metric tons of sulphuric acid was ejected into the atmosphere by the event, causing acid rain fallout.[5]

Landsat photo of Sumatra surrounding Lake Toba
Landsat photo of Sumatra surrounding Lake Toba

The subsequent collapse formed a caldera that, after filling with water, created Lake Toba. The island in the southern part of the lake is formed by a resurgent volcanic dome.

Though the year can never be precisely determined, paradoxically the season can: only the summer monsoon could have deposited Toba ashfall in the South China Sea, implying that the eruption took place sometime during the northern summer[6] The eruption lasted perhaps two weeks, but the ensuing "volcanic winter" resulted in a decrease in average global temperatures by 3 to 3.5 degrees Celsius 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. There is some evidence, based on mitochondrial DNA, that the human race may have passed through a genetic bottleneck within this timeframe, reducing genetic diversity below what would be expected from 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, human populations may have been reduced to only a few tens of thousands of individuals by the Toba eruption.

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More recent activity

Children playing in Lake Toba
Children playing in Lake Toba

Smaller eruptions have occurred at Toba since. The small cone of Pusukbukit has formed on the southwestern margin of the caldera and lava domes. The most recent eruption may have been at Tandukbenua on the northwestern caldera edge, since the present lack of vegetation could be due to an eruption within the last few hundred years [7].

Some parts of the caldera have experienced uplift due to partial refilling of the magma chamber, for example pushing Samosir Island and the Uluan Peninsula above the surface of the lake. The lake sediments on Samosir Island show that it has been uplifted by at least 450 metres [8] since the cataclysmic eruption. Such uplifts are common in very large calderas, apparently due to the upward pressure of unerupted magma. Toba is probably the largest resurgent caldera on Earth. Large earthquakes have occurred in the vicinity of the volcano more recently, notably in 1987 along the southern shore of the lake at a depth of 11 km[9]. Other earthquakes have occurred in the area in 1892, 1916, and 1920-1922[10].

Lake Toba lies near a fault line which runs along the centre of Sumatra called the Sumatra Fracture Zone [11]. The volcanoes of Sumatra and Java are part of the Sunda Arc, a result of the northeasterly movement of the Indo-Australian Plate which is sliding under the eastward-moving Eurasian Plate. The subduction zone in this area is very active: the seabed near the west coast of Sumatra has had several major earthquakes since 1995, including the 9.3 2004 Indian Ocean Earthquake and the 8.7 2005 Sumatra earthquake, the epicenters of which were around 300 km from Toba.

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People

Most of the people who live around Lake Toba are ethnically Bataks. Traditional Batak houses are noted for their distinctive roofs (which curve upwards at each end, as a boat's hull does) and their colorful decor.

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See also

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References

  1. Worldlakes.org
  2. http://volcano.und.nodak.edu/vwdocs/volc_images/southeast_asia/indonesia/toba.html
  3. Acharyya S.K., and Basu P.K. 1992. "Toba ash on the Indian subcontinent and its implications for correlation of late pleistocene alluvium." Quaternary Research 40:10-19
  4. Scrivenor, J.B. 1931. The Geology of Malaya (London: MacMillan), noted by Weber.
  5. Huang C.Y., Zhao M.X., Wang C.C., and Wei G.J. 2001. "Cooling of the South China Sea by the Toba eruption and correlation with other climate proxies ca. 71,000 years ago." Geophysical Research Letters 28:3915-3918, noted by Weber.
  6. Bühring C., and Sarnthein M. 2000. "Toba ash layers in the South China Sea: evidence of constrasting wind directions during eruption ca. 74 ka." Geology 28:275-278.
  7. http://www.volcanodiscovery.com/volcano-tours/volcanoes/indonesia/sumatra/toba/
  8. http://volcano.und.nodak.edu/vwdocs/volc_images/southeast_asia/indonesia/toba.html
  9. http://earthquake.usgs.gov/eqcenter/eqarchives/significant/sig_1987.php
  10. http://volcano.und.nodak.edu/vwdocs/volc_images/southeast_asia/indonesia/toba.html
  11. http://volcano.und.nodak.edu/vwdocs/volc_images/southeast_asia/indonesia/toba.html
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External links

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