Lake Toba. Landsat 7 satellite image in infrared and visible light of Lake Toba volcano, Sumatra, Indonesia. Water is blue, vegetation is green, bare land is pink and clouds are white. This lake is 100 kilometres long and 30 kilometres wide. The lake was formed when the land collapsed following a volcanic eruption. The island at centre, Samosir Island, is a resurgent dome resulting from the refilling of the underground magma chamber. An eruption of Toba 74,000 years ago is thought to have been the largest volcanic eruption ever, causing a volcanic winter that lowered global temperatures for several years, and wiped out a large proportion of the human population.