Lake Toba (or Danau Toba in Indonesian) is a lake and former supervolcano, 100 kilometres long and 30 kilometres wide, and 505 metres at its deepest point. Toba is located in the middle of the northern part of Sumatra with a surface elevation of about 900 metres. It is the largest volcanic lake in the world. In addition it is the site of a supervolcanic eruption that occurred 69,000-77,000 years ago, a massive climate-changing event. This eruption is believed to have been the largest anywhere on Earth in the last 25 million years. According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences, killing most humans then alive and creating a population bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today.
Geology of Toba
The Toba caldera complex 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 kilometres) and intercepts the three older calderas. An estimate of 2,500 to 3,000 km2 of dense-rock equivalent pyroclastic material, known as 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.
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.
The Toba Eruption
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. The last eruption 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 have deduced that the total amount of erupted material was about 2,800 km3 — 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. The pyroclastic flows of the eruption destroyed an area of 20,000 square kilometres, with ash deposits as thick as 600 metres by the main vent.
To give an idea of its magnitude, consider that although the eruption took place in Indonesia, it deposited an ash layer approximately 15 centimetres thick over the entire Indian subcontinent; at one site in central India, the Toba ash layer today is up to 6 metres thick and parts of Malaysia were covered with 9 m of ashfall. In addition it has been calculated that 10,000 million metric tons of sulfuric acid was ejected into the atmosphere by the event, causing acid rain fallout.
The Toba caldera is the only supervolcano in existence that can be described as Yellowstone’s “bigger” sister. With 2,800 km3 of ejecta, it was an even greater eruption than the supereruption (2,500 km3) of 2.1 million years ago that created the Island Park Caldera in Idaho, USA. The eruption was also about three times the size of the latest Yellowstone eruption of Lava Creek 630,000 years ago. For further comparison, the largest volcanic eruption in historic times, in 1815 at Mount Tambora (Indonesia), ejected the equivalent of around 100 km3 of dense rock and made 1816 the “Year Without a Summer” in the whole northern hemisphere, whilst the 1980 eruption of Mount St. Helens in Washington State ejected around 1.2 km3 of material.
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. 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 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.
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 the Great Sumatran fault which runs along the centre of Sumatra in 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 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.
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.
Flora and fauna
Flora organisms include various types of phytoplankton, emerged macrophytes, floating macrophytes, and submerged macrophytes.
Fauna include several variations of zooplankton and benthos animals. Lake Toba offers a nurturing environment for fish.
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