Welcome to Lake Toba
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 2.88° N 98.52° E to [show location on an interactive map] 2.35° N 99.1° E. It is the largest volcanic lake in the world.
Geology
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, 3,000 km away. Oceanographers discovered Toba ash, with its characteristic chemical "fingerprint", on the floor of the eastern Indian Ocean and the Bay of Bengal.
The eruption
The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago. 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 1010 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 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.
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 was not 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.
History
Within the last three to five million years, after human and other ape lineages diverged from the hominid stem-line, the human line produced a variety of species, including Homo ergaster, Homo erectus, Homo neanderthalensis and possibly Homo floresiensis.
According to the Toba catastrophe theory a massive volcanic eruption severely reduced the human population. This may have occurred around 70,000–75,000 years ago when the Toba caldera in Indonesia underwent an eruption of category 8 (or "mega-colossal") on the Volcanic Explosivity Index. This released energy equivalent to about one gigaton of TNT, which is three thousand times greater than the 1980 eruption of Mount St. Helens. According to Ambrose, this reduced the average global temperature by 5 degrees Celsius for several years and may have triggered an ice age.
Ambrose postulates that this massive environmental change created population bottlenecks in the various species that existed at the time; this in turn accelerated differentiation of the isolated human populations, eventually leading to the extinction of all the other human species except for the two branches that became Neanderthals (Homo neanderthalensis) and modern humans (Homo sapiens).
Evidence
Some geological evidence and computed models support the plausibility of the Toba catastrophe theory. The Greenland ice core data displays an abrupt change around this time, but in the corresponding Antarctic data the change is not easily discernible. Ashes from this eruption of Lake Toba, located near the equator, should have spread all over the world.
Genetic evidence suggests that all humans alive today, despite their apparent variety, are descended from a very small population, perhaps between 1,000 and 10,000 breeding pairs.
Using the average rates of genetic mutation, some geneticists have estimated that this population lived at a time coinciding with the Toba event. These estimates do not contradict the consensus estimates that Y-chromosomal Adam lived some 60,000 years ago, and that Mitochondrial Eve is estimated to have lived 140,000 years ago, because Toba is not conjectured to be an extreme bottleneck event, where the population was reduced to one breeding pair.
Gene analysis of some genes shows divergence anywhere from 2 million to 60,000 years ago, but this does not contradict the Toba theory, again because Toba is not conjectured to be a single-pair bottleneck event. The complete picture of gene lineages (including present-day levels of human genetic variation) allows the theory of a Toba-induced human population bottleneck.
Lice Genes Confirm Key Events in Human Evolution
Alan Rogers, a co-author of this study and professor of anthropology at the University of Utah, says: “The record of our past is written in our parasites.” Rogers and others have proposed the bottleneck may have occurred because of a mass die-off of early humans due to a globally catastrophic volcanic eruption. The analysis of lice genes confirmed that the population of Homo sapiens mushroomed after a small band of early humans left Africa sometime between 150,000 and 50,000 years ago.
Migration
According to this theory, humans once again fanned out from Africa after Toba when the climate and other factors permitted. They migrated first to Arabia and India and onwards to Indochina and Australia (Ambrose, 1998, p. 631), and later to the Middle East and what would become the Fertile Crescent following the end of the Würm glaciation period (70,000–10,000 years bp).
posted by Yanto Sanusi @ 2:39 PM
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