Lake Toba

Lake Toba
Lake Toba - Landsat photo
Landsat photo
Location North Sumatra, Indonesia
Lake type Volcanic/ tectonic
Primary outflows Asahan River
Basin countries Indonesia
Max. length 100 km
Max. width 30 km
Surface area 1,130 km²
Max. depth 505 m[1]
Water volume 240 km³
Surface elevation 905 m
Islands Samosir
References [1]

Lake Toba (Indonesian: Danau Toba) is a lake and supervolcano, 100 km long and 30 km wide, and 505 m. (1,666 ft.) at its deepest point, in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about 900 m (3,000 feet), stretching from to . It is the largest volcanic lake in the world.[1] In addition, it is the site of the 75,000-year-old supervolcanic eruption, the largest in the last 25 million years; where it staged a massive climate-changing event that scientists believe might have wiped out much of humanity. The massive eruption is believed to have a VEI intensity of 8.

Contents

Geology

The Toba caldera complex in Northern Sumatra, Indonesia consists of four overlapping volcanic craters that adjoin the Sumatran "volcanic front". The youngest and fourth caldera is the world’s largest Quarternary caldera (100 by 30 kilometres) and intercepts the three older calderas. An estimate of 2500-3000 cubic kilometres of dense-rock equivalent pyroclastic material, nicknamed the Youngest Toba tuff, was blasted from the youngest caldera during one of the largest single eruptions in geologic history. Following the "Youngest Toba tuff eruption", a typical resurgent dome formed within the new caldera, joining two half-domes separated by a longitudinal graben.[2]

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.[3] It had an estimated Volcanic Explosivity Index of 8 (described as "mega-colossal"), making it possibly the largest explosive volcanic eruption within the last twenty-five million years.[4] Bill Rose and Craig Chesner of Michigan Technological University have deduced that the total amount of erupted material was about 2800 cubic km (670 cubic miles)[4] — around 2000 cubic km of ignimbrite that flowed over the ground, and around 800 cubic km that fell as ash, with the wind blowing most of it to the west. The pyroclastic flows of the eruption destroyed an area of 20,000 square km (7700 square miles), with ash deposits as thick as 600 metres (2000 feet) by the main vent.[4] By contrast, the 1980 eruption of Mount St. Helens ejected around 1.2 cubic km of material, whilst the largest volcanic eruption in historic times, at Mount Tambora in 1815, emitted the equivalent of around 100 cubic km of dense rock and created the "Year Without a Summer" as far away as North America.[5] The eruption was also about three times the size of the latest Yellowstone eruption of Lava Creek 630,000 years ago.[6]

The Toba eruption was the latest 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.[7]

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 inch) 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[8] and parts of Malaysia were covered with 9 m of ashfall.[9] In addition it has been calculated that 1010 metric tons of sulphuric acid was ejected into the atmosphere by the event, causing acid rain fallout.[10]

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 center of the lake is formed by a resurgent dome.

Though the year may never be precisely determined, 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.[11] 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.[12]

More recent activity

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.[13]

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[7] 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.[14] Other earthquakes have occurred in the area in 1892, 1916, and 1920-1922.[7]

Lake Toba lies near a fault line which runs along the centre of Sumatra called the Sumatra Fracture Zone.[7] 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.

On September 12, 2007, a magnitude 8.4 Earthquake shook the ground by Sumatra and was felt in the Indonesian capital, Jakarta. The epicenter for this earthquake was not as close as the previous two earthquakes, but it was in the same vicinity.

A view of Lake Toba from the island of Prapat
Batak House at Lake Toba

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.[15]

Flora and fauna

Lake Toba offers a nurturing environment for fish such as the tilapia mossambica, aplocheilus pachax, lebistes reticulatus, osphronemus goramy, trichogaster trichopterus, channa striata, chana gachua, clarias batrachus, clarias nieuhofi, clarias. sp., nemachilus fasciatus, cyprinus carpio, puntius javanicus, puntius binotatus, osteochilus nasselti, lissochilus sp., labeobarbus sora, and rasbora sp.[16]

Many other types of plants and animals live within the boundaries of Lake Toba. Flora organisms include various types of phytoplankton, emerged macrophytes, floating macrophytes, and submerged macrophytes. Fauna include several variations of zooplankton and benthos.[16]

See also

References

Notes

  1. ↑ 1.0 1.1 1.2 Worldlakes.org
  2. ↑ Chesner, C.A., Westgate, J.A., Rose, W.I., Drake, R., Deino, A. (March 1991). "Eruptive history of Earth's largest Quarternary caldera (Toba, Indonesia)". Michigan Technological University. Retrieved on 2008-08-23.
  3. ↑ Zielinski, G. A.; P.A. Mayewski, L.D. Meeker, S. Whitlow, M. Twickler and K. Taylor (1996). "Potential Atmospheric impact of the Toba mega-eruption ~71,000 years ago". Geophysical Research Letters (United States: American Geophysical Union) 23 (8): 837–840. doi:10.1029/96GL00706. 
  4. ↑ 4.0 4.1 4.2 "Supersized eruptions are all the rage!". USGS.
  5. ↑ "Facts about the year without a summer". Discovery Channel UK.
  6. ↑ "Supervolcano - The 1,080-square-mile Toba caldera is the only supervolcano in existence that can be described as Yellowstone's "big" sister.". Discovery Channel.
  7. ↑ 7.0 7.1 7.2 7.3 "Toba, Sumatra, Indonesia". Oregon State University.]
  8. ↑ 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
  9. ↑ Scrivenor, J.B. 1931. The Geology of Malaya (London: MacMillan), noted by Weber.
  10. ↑ 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.
  11. ↑ Bühring C., and Sarnthein M. 2000. "Toba ash layers in the South China Sea: evidence of contrasting wind directions during eruption ca. 74 ka." Geology 28:275-278.
  12. ↑ "Yellowstone Is a Supervolcano?" (HTML). SEMP. Retrieved on 2008-02-21.
  13. ↑ "Toba volcano (Indonesia, Sumatra)". VolcanoDiscovery.com.
  14. ↑ "Significant Earthquakes of the World". United States Geological Survey (USGS).
  15. ↑ "Batak People". IndonesianMusic.com.
  16. ↑ 16.0 16.1 "Danau Toba (Lake Toba)". International Lake Environment Committee.

Sources

  • Rampino, Michael R. and Stephen Self (1993). "Climate-volcanism feedback and the Toba eruption of 74,000 Years Ago". Quaternary Research 40 (3): 269–280. doi:10.1038/359050a0. 
  • Vazquez, Jorge A. and Mary R. Reid (2004). "Probing the accumulation history of the voluminous Toba Magma". Science 305 (5686): 991–994. doi:10.1126/science.1096994. 
  • Petraglia, M. et al. (2007). "Middle Paleolithic Assemblages from the Indian Subcontinent Before and After the Toba Super-Eruption". Science 317 (5834): 114–116. doi:10.1126/science.1141564. 

External links