Everything about Lake Toba totally explained
Lake Toba (
Indonesian:
Danau Toba) is a
lake, 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. 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.
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 2,000 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 1.2 cubic km of material, whilst the largest volcanic eruption in historic times, at
Mount Tambora in
1815, emitted the equivalent of around 100 cubic kilometres of dense rock and created the "
Year Without a Summer" as far away as North America.
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.
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 and parts of
Malaysia were covered with 9 m of ashfall. In addition it has been calculated that 10
10 metric tons of
sulphuric acid was ejected into the atmosphere by the event, causing
acid rain fallout.
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 can 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. 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's 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.
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.
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 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. Other earthquakes have occurred in the area in 1892, 1916, and 1920-1922.
Lake Toba lies near a fault line which runs along the centre of Sumatra called the
Sumatra Fracture Zone. 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 12th, 2007, a magnitude 8.4 Earthquake shook the ground by Sumatra and was felt in the Indonesian capital, Jakarta. The epicenter for this earthquake wasn't as close as the previous two earthquakes, but it was in the same vicinity.
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.
Further Information
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