Lake Toba - The Eruption

The Eruption

The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago. 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 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)—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.

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 and parts of Malaysia were covered with 9 m (30 ft) of ashfall. In addition it has been variously calculated that 10,000 million tonnes (1.1×1010 short tons) of sulfuric acid or 6,000 million tonnes (6.6×109 short tons) of sulfur dioxide 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. 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. However, this hypothesis is not widely accepted due to lack of evidence for any other animal decline or extinction, even in environmentally sensitive species.