Lake Toba (Indonesian: Danau Toba) is a lake and supervolcano. The lake is 100 kilometres long, 30 kilometres wide, and up to 505 metres (1,666 ft) deep. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about 900 metres (2,953 ft), the lake stretches from 2.88°N 98.52°E to 2.35°N 99.1°E. It is the largest lake in Indonesia and the largest volcanic lake in the world.
Lake Toba is the site of a massive supervolcanic eruption estimated at VEI 8 that occurred 69,000 to 77,000 years ago, representing a climate-changing event. It is the largest known explosive eruption on Earth in the last 25 million years. According to the Toba catastrophe theory, it had global consequences for human populations: it killed most humans living at that time and is believed to have created a population bottleneck in central east Africa and India, which affects the genetic make up of the human world-wide population to the present. This hypothesis is not widely accepted because evidence is lacking for a decline or extinction of other animals, including species that are sensitive to changes in the environment. It has been accepted that the eruption of Toba led to a volcanic winter with a worldwide decrease in temperature between 3 to 5 °C (5 to 9 °F), and up to 15 °C (27 °F) in higher latitudes.
The Toba caldera complex in Northern Sumatra, Indonesia, comprises four overlapping volcanic craters that adjoin the Sumatran "volcanic front." The youngest and fourth caldera is the world's largest Quaternary caldera (100 by 30 km (62 by 19 mi)) and 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 geologic 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 north-western edge of the caldera has only sparse vegetation, suggesting a young age of several hundred years. Also, the Pusubukit volcano on the south edge of the caldera is solfatarically active.
The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago. It was the last in a series of at least three caldera-forming eruptions at this location, with earlier calderas having formed around 700,000 and 840,000 years ago. This last eruption had an estimated VEI 8, making it possibly the largest explosive volcanic eruption within the last 25 million years.
Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was about 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. 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 ash fall. 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.
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, as a boat's hull does) and their colorful decor.
The flora of the lake includes various types of phytoplankton, emerged macrophytes, floating macrophytes, and submerged macrophytes, while the surrounding countryside is rainforest including areas of Sumatran tropical pine forests on the higher mountainsides.
The fauna includes several species of zooplankton and benthic animals. Since the lake is oligotrophic (nutrient-poor), the native fish fauna is relatively scarce, and the only endemics are Rasbora tobana (strictly speaking near-endemic, since also found in some tributary rivers that run into the lake) and Neolissochilus thienemanni, locally known as the Batak fish. The latter species is threatened by deforestation (causing siltation), pollution, changes in water level and the numerous fish species that have been introduced to the lake. Other native fishes include species such as Aplocheilus panchax, Nemacheilus pfeifferae, Homaloptera gymnogaster, Channa gachua, Channa striata, Clarias batrachus, Barbonymus gonionotus, Barbonymus schwanenfeldii, Danio albolineatus, Osteochilus vittatus, Puntius binotatus, Rasbora jacobsoni, Tor tambra, Betta imbellis, Betta taeniata and Monopterus albus. Among the many introduced species are Anabas testudineus, Oreochromis mossambicus, Oreochromis niloticus, Ctenopharyngodon idella, Cyprinus carpio, Osphronemus goramy, Trichogaster pectoralis, Trichopodus trichopterus, Poecilia reticulata and Xiphophorus hellerii.