A cataclysm written in ash
Around 74,000 years ago, the volcano now known as Lake Toba erupted with a force rated 8 on the Volcanic Explosivity Index (VEI), the highest possible value. This event, the most powerful explosive eruption in the last 2.5 million years, ejected an estimated 2,800 cubic kilometers of rock and ash into the atmosphere. Of this, approximately 800 cubic kilometers fell as ash, while 2,000 cubic kilometers of hot pyroclastic flows devastated at least 20,000 square kilometers of the surrounding landscape.
The ash, known to geologists as the Youngest Toba Tuff, spread across the globe. It blanketed all of South Asia with a layer up to 15 cm thick and settled as far as the Arabian Sea and the South China Sea. In parts of central India, the ash layer is still up to 6 meters thick today. This widespread tuff layer is an important time marker for geologists and archaeologists, allowing them to date sedimentary layers across continents. The collapse of the emptied magma chamber formed the vast caldera that is now filled by Lake Toba, which at 100 kilometers long and 30 kilometers wide is the world's largest volcanic lake.
Volcanic winter and a human bottleneck
The eruption released enormous quantities of sulfurous gases, leading to a "volcanic winter." Climate models suggest this may have caused a global average temperature drop of 3–5°C for several years. This sudden climatic shift is the basis for the Toba catastrophe theory, which proposes the event caused a genetic bottleneck in human evolution. Proponents of the theory suggest the global human population may have crashed to as few as 3,000 to 10,000 individuals, drastically reducing genetic diversity.
However, the severity of this bottleneck is a subject of ongoing scientific debate. Archaeological evidence from sites in India and Africa shows that some human populations were present both before and after the eruption, suggesting they survived the event. More recent climate studies based on sediment cores from Lake Malawi in Africa show little evidence of the dramatic cooling predicted by the volcanic winter models, complicating the direct link between the eruption and a near-extinction event for humanity.
Today, the caldera is a dynamic geological site. In the center of the lake is Samosir Island, a massive resurgent dome. This island, roughly the size of Singapore, was formed by magma pushing up the caldera floor after the main eruption. The process of resurgence began sometime after the eruption, with the dome underwater as recently as 33,000 years ago, and involved uplift of up to 700 meters. A 2016 study revealed that Toba's magma chamber still contains an estimated 50,000 cubic kilometers of eruptible magma.