Lake Toba (Indonesian: Danau Toba, Toba Batak: ᯖᯀᯬ ᯖᯬᯅ; romanized: Tao Toba) is a large natural lake in North Sumatra and Indonesia. The lake is the massive caldera of the Toba supervolcano. The lake is located in the middle of the northern part of the island of Sumatra, with a surface elevation of about 900 metres (2,953 ft), and stretches from 2.88°N 98.52°E / 2.88; 98.52 to 2.35°N 99.1°E / 2.35; 99.1. The lake is about 100 kilometres (62 miles) long, 30 kilometres (19 mi) wide, and up to 505 metres (1,657 ft) deep. It is the largest lake in Indonesia and the largest volcanic lake in the world. Toba Caldera is one of twenty geoparks in Indonesia, and was recognised in July 2020 as one of the UNESCO Global Geoparks. Lake Toba is the site of a supervolcanic eruption estimated at VEI 8 that occurred 69,000 to 77,000 years ago, temporarily changing the earth's climate. Recent advances in dating methods suggest a more precise eruption date of 74,000 years ago. It is the largest-known explosive eruption on Earth in the last 25 million years. A theory known as the Toba catastrophe theory postulates that this event held global consequences for Earth's human population. As the eruption likely would have killed most humans along with many other creatures living at the time, the theory suggests, it could account for an apparent population bottleneck in central east Africa and India around that time. A catastrophic loss of genetic diversity would affect the genetic make-up of humans worldwide, affecting human evolution to the present. More recent studies have cast doubt on this theory, as no evidence has emerged of lasting reductions in populations of humans and other fauna at the time. Other possibilities exist, including migration patterns, to account for apparent reductions in human genetic diversity. It has also been suggested that the eruption of the Toba Caldera led to a volcanic winter with a worldwide decrease in temperature between 3 and 5 °C (5.4 and 9.0 °F), and up to 15 °C (27 °F) at higher latitudes. Additional studies in Lake Malawi in East Africa show significant amounts of ash deposits from the Toba Caldera eruptions even at that great distance, yet little indication of a significant climatic effect in East Africa.
Geology
The Toba Caldera in North Sumatra comprises four overlapping volcanic craters that adjoin the Sumatran "volcanic front". At 100 by 30 kilometres (62 by 19 mi) it is the world's largest Quaternary caldera, and the fourth and youngest caldera. It intersects the three older calderas. An estimated 2,800 km3 (670 cu mi) of dense-rock equivalent pyroclastic material, known as the youngest Toba tuff, was released during one of the largest explosive volcanic eruptions in recent geological history. Following this eruption, a resurgent dome formed within the new caldera, joining two half-domes separated by a longitudinal graben. At least four cones, four stratovolcanoes, and three craters are visible in the lake. The Tandukbenua cone on the northwestern edge of the caldera has only sparse vegetation, suggesting a young age of several hundred years. Also, the Pusubukit (Hill Center) volcano (1,971 m (6,467 ft) above sea level) on the south edge of the caldera is solfatarically active. The magma reservoir of the Toba supervolcano appears to consist of a layer of stacked magma sills, extending at least 7–30 kilometres (4.3–18.6 mi) beneath Samosir Island, in the center of the lake, and measuring 10–20 kilometres (6.2–12.4 mi) in diameter, with a shallow magma chamber sitting on top of the magma sills. This structure has been described as "magma pancakes", and similar structures are thought to potentially exist in the Taupō Supervolcano and Long Valley Caldera. The "magma pancakes" were uncovered through a study involving ambient seismic noise, and they are thought to be the driving force behind the Toba supervolcano's past supereruptions.
Major eruption
The Toba eruption (the Toba event) occurred at what is now Lake Toba about 73,700±300 years ago. It was the last in a series of at least four caldera-forming eruptions at this location, with the earlier known caldera having formed around 1.2 million years ago. This last eruption had an estimated VEI of 8, making it the largest-known explosive volcanic eruption in the Quaternary Period. Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was at least 2,800 km3 (670 cu mi)—about 2,000 km3 (480 cu mi) of ignimbrite that flowed over the ground, and approximately 800 km3 (190 cu mi) that fell as ash mostly to the west. However, as more outcrops become available, Toba possibly erupted 3,200 km3 (770 cu mi) of ignimbrite and co-ignimbrite. The pyroclastic flows of the eruption destroyed an area of least 20,000 km2 (7,700 sq mi), with ash deposits as thick as 600 m (2,000 ft) by the main vent. The eruption was large enough to have deposited an ash layer approximately 15 cm (6 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 ash fall. The subsequent collapse formed a caldera that filled with water, creating Lake Toba. The island in the center of the lake is formed by a resurgent dome.
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