Toba is located at the Sunda subduction zone, where the Indian tectonic plate sinks beneath the Sumatran plate. When a plate sinks or “subducts” into the mantle, it begins to dehydrate, with much of its water moving back up into the space above it.
This water lowers the melting temperatures of the already hot rocks it travels through, which causes them to melt more profusely and produce huge, layered magma chambers. Over time, this becomes increasingly thick, gassy, and extremely pressurized magma, which almost invariably erupts in a highly explosive manner.
This type of volcanic activity is found above subduction zones all over the world, but Toba has a history of being exceptionally violent. Based on their seismic surveys, the team thinks that the subducting slab is giving Toba a particularly high amount of water, which is itself heated by a gigantic magma chamber. The combination of the two is thought to lead to particularly destructive supereruptions.
“In this reservoir… if liquids begin to transform to gasses, it immediately increases the pressure and leads to the origin of new fractures, which become pathways for new portions of liquid water, which then is transformed to gasses,” Koulakov adds. “This avalanche type process results in a huge explosion!”
“Huge” is putting it mildly. Ashfall from the 74,000-year-old event was 9 meters (30 feet) thick in some parts of the world. The broiling, supersonic pyroclastic flows that erupted from the cauldron-forming (“caldera”) eruption scorched an area of 20,000 square kilometers (7,722 square miles), which is equivalent to wiping out over 25 New York Cities.
The amount of sunlight-reflecting sulfur aerosols released, estimated at around 10,000 million tonnes (11,000 million tons), caused a powerful climatic effect, with temperatures in some parts of the world decreasing by up to 15°C (27°F). It may have even put an end to many prehistoric human civilizations, although this is heavily debated.
Image in text: A very artistic artist's impression of the Toba supervolcanic system. Credit: Ivan Koulakov