| ke Toba | |
|---|---|
 | |
| Landsat photo | |
| Location | North Sumatra, Indonesia |
| Coordinates | 2.6845°N 98.8756°ECoordinates: 2.6845°N 98.8756°E |
| Lake 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) |
| Max. depth | 505 m (1,657 ft)[1] |
| Water volume | 240 km3 (58 cu mi) |
| Surface elevation | 905 m (2,969 ft) |
| Islands | Samosir |
| References | [1] |
Lake Toba (Indonesian: Danau Toba) is a lake and supervolcano. The lake is 100 kilometres long and 30 kilometres wide, and 505 metres (1,666 ft) at its deepest point. 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 Indonesiaand the largest volcanic lake in the world.[1]
Lake Toba is the site of a supervolcanic eruption that occurred an estimated 69,000 to 77,000 years ago,[2][3][4] a massive, climate-changing event. It is estimated to have been a VEI 8 eruption. It is the largest known explosive eruption anywhere on Earth in the last 25 million years. According to the Toba catastrophe theory, it had global consequences, killing most humans then alive and creating apopulation bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today.[5] However, this hypothesis is not widely accepted due to lack of evidence for any other animal decline or extinction, even in environmentally sensitive species.[6] However, it has been accepted that the eruption of Toba led to a volcanic winter with a worldwide decline in temperatures between 3 to 5 °C (5 to 9 °F), and up to 15 °C (27.0 °F) in higher latitudes.
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[edit]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 Quaternary caldera (100 by 30 km (62 by 19 mi)) and intersects the three older calderas. An estimate of 2,800 km3 (670 cu mi) of dense-rock equivalent pyroclastic material, known the Youngest Toba tuff, was blasted from the youngest caldera during one of the largest single explosive volcanic 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.[3]
There are at least four cones, four stratovolcanoes and three craters visible in the lake. The Tandukbenua cone on the NW edge of the caldera is relatively lacking in vegetation, suggesting a young age of only several hundred years. Also, the Pusubukit volcano on the south edge of the caldera is solfatarically active.[7]
[edit]The eruption
Main article: Toba catastrophe theory
The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago.[8] The Toba eruption was the latest of a series of at least three caldera-forming eruptions which have occurred at the volcano, with earlier calderas having formed around 700,000 and 840,000 years ago.[9] The last eruption had an estimated VEI 8 (described as "mega-colossal"), making it possibly the largest explosive volcanic eruption within the last 25 million years.
Bill Rose and Craig Chesner of Michigan Technological University have deduced that the total amount of erupted material was about 2,800 km3 (670 cu mi)[10]—around 2,000 km3 (480 cu mi) of ignimbrite that flowed over the ground, and around 800 km3 (190 cu mi) 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 km2(7,722 sq mi), with ash deposits as thick as 600 m (1,969 ft) by the main vent.[10]
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[11] and parts of Malaysia were covered with 9 m (30 ft) of ashfall.[12] In addition it has been variously calculated that 10,000 million tonnes (1.1×1010 short tons) of sulfuric acid[13][citation needed] or 6,000 million tonnes (6.6×109 short tons) of sulfur dioxide[14] 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.
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.[15] The eruption lasted perhaps two weeks, but 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.
There is some evidence, based on mitochondrial DNA, that the human species may have passed through a genetic bottleneck around this time, 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.[16] However, this hypothesis is not widely accepted due to lack of evidence for any other animal decline or extinction, even in environmentally sensitive species.[6]
[edit]More recent activity
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.[17]
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 m (1,476 ft)[9] 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 (6.8 mi).[18] Other earthquakes have occurred in the area in 1892, 1916, and 1920–1922.[9]
Lake Toba lies near the Great Sumatran fault which runs along the centre of Sumatra in the Sumatra Fracture Zone.[9] 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 9.1 2004 Indian Ocean Earthquake and the 8.7 2005 Sumatra earthquake, the epicenters of which were around 300 km (190 mi) from Toba.
On 12 September 2007, a magnitude 8.5 earthquake shook the ground in 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.
On 26 October 2010, a magnitude 7.7 earthquake occurred 36 kilometers (22 mi) southwest of the nearby island of Pagai-selatan. A 3-metre (10 ft) tsunami immediately followed the quake.
On 10 January 2012, a magnitude 7.3 earthquake occurred within the area.
[edit]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.[19]
[edit]Flora and fauna
The flora of the lake includes various types of phytoplankton, emerged macrophytes, floating macrophytes, and submerged macrophytes, while the surrounding countryside is rainforest including areas of Sumatran tropical pine forests on the higher mountainsides.[20]
The fauna includes several species of zooplankton and benthic animals. Since the lake is oligotrophic (nutrient-poor), the native fish fauna is relatively scarce, and the only endemics are Rasbora tobana (strictly speaking near-endemic, since also found in some tributary rivers that run into the lake)[21] and Neolissochilus thienemanni, locally known as the Batak fish.[22] The latter species is threatened by deforestation (causingsiltation), pollution, changes in water level and the numerous fish species that have been introduced to the lake.[22] Other native fishes include species such as Aplocheilus panchax, Nemacheilus pfeifferae, Homaloptera gymnogaster, Channa gachua, Channa striata, Clarias batrachus,Barbonymus gonionotus, Barbonymus schwanenfeldii, Danio albolineatus, Osteochilus vittatus, Puntius binotatus, Rasbora jacobsoni, Tor tambra, Betta imbellis, Betta taeniataand Monopterus albus.[23] Among the many introduced species are Anabas testudineus, Oreochromis mossambicus, Oreochromis niloticus, Ctenopharyngodon idella, Cyprinus carpio, Osphronemus goramy, Trichogaster pectoralis, Trichopodus trichopterus, Poecilia reticulata and Xiphophorus hellerii.[23]
