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In 1949 the Dutch geologist Rein van Bemmelen reported that Lake Toba was surrounded by a layer of ignimbrite rocks, and was a large volcanic caldera. Later researchers found rhyolite ash similar to that in the ignimbrite around Toba in Malaysia and India, 3000km away. Oceanographers discovered Toba ash on the floor of the eastern Indian Ocean and the Bay of Bengal.
The Toba eruption is dated at 73,000 ± 4000 years ago. It had a Volcanic Explosivity Index of 8, making it the most recent eruption of a "supervolcano". Bill Rose and Craig Chesner of Michigan Technological University deduced that the total amount of erupted material was about 2800km3; around 800km3 of ignimbrite that flowed over the ground and around 2000km3 that fell as ash, with the wind blowing most of it to the west. Such a huge eruption probably lasted nearly two weeks. Very few plants or animals in Indonesia 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 was reduced to only a few thousand individuals by the Toba eruption (see Toba catastrophe theory).
A large area collapsed after the ejection of that amount of subsurface material, forming a caldera, which filled with water creating Lake Toba. Later, the floor of the caldera uplifted to form Samosir, a large island in the lake. 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.
There have been no historic eruptions at Toba, but large earthquakes have occurred, the most recent in 1987 along the southern shore of the lake. The underwater fault off the west coast of Sumatra has had several major earthquakes since 1995, including the ~9.0 2004 Indian Ocean Earthquake and the 8.7 2005 Sumatran earthquake.
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, like a boat's hull does) and their colorful decor.
Probing the Accumulation History of the Voluminous Toba Magma, Jorge A. Vazquez and Mary R. Reid, Science, 305, 991-994, 13 August, 2004.
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