Geology of Lake Toba

The Toba caldera complex in Northern Sumatra, Indonesia consists of four overlapping volcanic craters that adjoin the Sumatran "volcanic front". The youngest and fourth caldera is the world's largest Quaternary caldera (100 km (62 mi) by 30 km (19 mi)) and intersects the three older calderas. An estimate of 2,800 km³ (670 cu mi) of dense-rock equivalent pyroclastic material, known the Youngest Toba tuff, was blasted from the youngest caldera during one of the largest single explosive volcanic eruptions in geologic history. Following the "Youngest Toba tuff eruption", a typical resurgent dome formed within the new caldera, joining two half-domes separated by a longitudinal graben.

There are at least four cones, four stratovolcanoes and three craters visible in the lake. The Tandukbenua cone on the NW edge of the caldera is relatively lacking in vegetation, suggesting a young age of only several hundred years. Also, the Pusubukit volcano on the south edge of the caldera is solfatarically active.