Lake Toba is a lake and super volcano. The lake is 100 kilometers long and 30 kilometers wide, and 505 meters at its deepest point. Located in the middle of the northern part of the Indonesian island of Sumatra, with a surface elevation of about 900 meters. 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.
The 35 x 100 km Toba caldera, the Earth's largest Quaternary caldera, was formed during four major Pleistocene ignimbrite-producing eruptions beginning at 1.2 million years ago. The latest of these produced the Young Toba Tuff (YTT) about 74000 years ago. The YTT represents the world's largest known Quaternary eruption, ejecting about 2500-3000 km3 (dense rock equivalent) of ignimbrite and airfall ash from vents at the NW and SE ends of present-day Lake Toba. Resurgent doming forming the massive Samosir Island and Uluan Peninsula structural blocks postdated eruption of the YTT.
YTT eruption was two orders of magnitude greater in erupted mass than the largest volcanic eruption in historic times, in 1815 at Mount Tambora in Indonesia, which caused the 1816 "Year Without a Summer" in the northern hemisphere. Although the Toba eruption took place in Indonesia, it deposited an ash layer approximately 15 centimetres thick over the entirety of South Asia. A blanket of volcanic ash was also deposited over the Indian Ocean, and the Arabian and South China Sea. Deep-sea cores retrieved from the South China Sea extended the known distribution of the eruption and suggest that the ~2,800 km3 calculation of the eruption magnitude is a minimum value or even an underestimate.
Additional post-YTT eruptions include emplacement of a series of lava domes, growth of the solfatarically active Pusukbukit volcano on the south margin of the caldera, and formation of Tandukbenua volcano at the NW-most rim of the caldera. Lack of vegetation suggests that this volcano may be only a few hundred years old.
The white box in the inset outlines the Toba caldera complex area. The arrow shows the direction of convergence of the Australian plate relative to the Eurasia plate.
Toba caldera complex, North Sumatra, Indonesia, with the gray region representing land within the present-day topographic depression. Red triangles show young dacite-andesite volcanoes. Areas of recent upflit are indicated by purple shading.
Dashes green lines show approximate source-caldera outlines for each of three Toba tuffs, based on distribution of ash flow deposits: 1) Sibandung caldera, created 73000 years ago (0.075 Ma dashes green line) by the Toba YTT event (Yang Toba Ash); 2) Haranggoal csldera, formed 500000 years ago (0.5 Ma dashes green line) by the Toba MTT event (Middle Toba Ash); ); 3) Sibandung csldera, formed 800000 years ago (0.84 Ma dashes green line) by the Toba OTT event (Old Toba Ash).
Bouguer gravity anomaly map are generally low near Toba, reaching – 75mGal over Samosir island. The contor interval is 10 mGal. The small dots show observation points. The white curves show outlines of the present-day topographic depression at Toba, of the lake, of Samosir Island, and of the Sumatra fault.
SW – NE gravity profiles and corresponding tuff and magma models.
a) Model in which gravity anomaly over Samosir Island is matched tuff only. The tuff is 2.5 km thick with density contrast of 500 kg/m3.
b) An example of a two-dimensional crustal magma body that also axplains the low gravity over the Samosir. The magma body is 10 x 10 km in cross-sectional area, centerd at 15-km depth, with density contrast of 650 kg/m3 is tapered to SW and NE of depression in both models to match regional gravity.
5. P velocity beneath the Toba caldera complex (Northern Sumatra) from local earthquake tomography.
P-velocity anomalies in horizontal sections as a result of real data inversion. The anomalies are given in percent with respect to the 1-D model obtained as a result of optimization. Black contour lines within the 'red' area indicate the level of 10 and 20 percent anomalies. Black dots indicate the final locations of sources in a corresponding depth interval. Blue triangles show positions of active volcanoes. Dotted lines indicate the location of the Mentawai and Sumatra fault zones. Positions of the profiles for two cross sections are given in the plots for 5 and 75 km depth.
Interpretation of the results along profile 1. Background is the distribution of P-velocity anomalies. Yellow stars are the final location of events used in the study. Blue, violet and red drops mark schematically the path of ascending fluids and partially molten material. Black triangle and SF indicates the Sumatra fault.
Sources of information:
1. McCaffrey R., Wark D A , Roeckere S. W., (2001) Distribution of magma beneath the Toba caldera complex, north Sumatra, Indonesia, constraned by three-dimensional P wave velocities, seismicity, and gravity data.,Geochemistry Geophysics Geosystems ,2001, Volume: 2, Issue: 4, Pages: doi: 2000GC000096.
2. Koulakov I, Yudistira T, Luehr B.-G., (2009) P, S velocity and VP/VS ratio beneath the Toba caldera complex (Northern Sumatra) from local earthquake tomography., Geophys. J. Int. ,2009, 177, p. 1121-1139, doi: 10.1111/j.1365-246X.2009.04114.x.