With the intense media interest in the volcanic activity in Indonesia,
this is a reminder that the contact details of the responsible warning authority
for volcanic crises can be found at the web site of the World Organization of
Volcano Observatories, a commission of IAVCEI. The web address is http://www.wovo.org/.
Additionally, Last year, the Directorate of Volcanology and Geological Hazard Mitigation (informally, the Volcanological Survey of Indonesia) set up a news site that gives the latest official advice in either Indonesian or English, or usually both - HERE
Toba caldera produced the largest eruption in the last 2 million years. The caldera is 18 x 60 miles (30 by 100 km) and has a total relief of 5,100 feet (1700 m). The caldera probably formed in stages. Large eruptions occurred 840,000, about 700,000, and 75,000 years ago. The eruption 75,000 years ago produced the Young Toba Tuff. The Young Toba Tuff was erupted from ring fractures that surround most or all of the present-day lake.
Map of the Toba caldera from Knight and others (1986). Samosir Island and the Uluan Peninsula are parts of one or two resurgent domes. Lake sediments on Samosir indicate at least 1,350 feet (450 m) of uplift. Pusukbukit, a small stratovolcano along the west margin of the caldera, formed after the eruption 75,000 years ago. There are active solfataras on the north side of the volcano.
Comparison of volumes produced by some of the greatest volcanic eruptions. The Young Toba Tuff has an estimated volume of 2,800 cubic kilometers (km) and was erupted about 74,000 years ago. The Huckleberry Ridge Tuff, erupted at Yellowstone 2.2 million years ago, has a volume of 2,500 cubic km. The Lava Creek Tuff, erupted at Yellowstone 600,000 years ago, has a volume of 1,000 cubic km. The May 1980 eruption of Mount St. Helens produced 1 cubic km of ash. Not shown is the Fish Canyon Tuff of the San Juan Mountains of Colorado. The Fish Canyon Tuff was erupted 27.8 million years ago and has an estimated volume of 3,000 cubic km.
The volume of the youngest eruption is estimated at 2,800 cubic km, making the eruption the largest in the Quaternary. Pyroclastic flows covered an area of at least 20,000 square km. Up to 1200 feet (400 m) of Young Toba Tuff is exposed in the walls of the caldera. On Samosir Island the tuff is more than 1800 feet (600 m) thick. Ash fall from the eruption covers an area of at least 4 million square km (about half the size on the continental United States). Ash from the eruption has been recovered from deep-sea cores taken in the Bay of Bengal and in India, roughly 300 miles (500 km) inland (1,900 miles, 3100 km from Toba). Rose and Chesner suggested the ash may have reached central Asia and the Middle East. Ninkovich and others (1978) estimated of the height of the eruption column to be 30 to 50 miles (50 to 80 km) for the Young Toba Tuff. Rose and Chesner, after a study of the shapes of the ash shards, concluded this estimate was too high by a factor of 5 or more.
The pumice erupted 75,000 years ago is calc-alkalic quartz-latite to rhyolite in composition (68%-76% silica).
There have been no eruptions at Toba in historical time. The area is seismically active with major earthquakes in 1892, 1916, 1920-1922, and 1987.
Toba is located near the Sumatra Fracture Zone (SFZ). Stratovolcanoes in Sumatra are part of the Sunda arc. Volcanism is the result of the subduction of the Indian Ocean plate under the Eurasian plate. The subduction zone is marked by the Java Trench. The geologic symbol for a subduction zone is a line with "teeth" (black triangles). The teeth are on the over-riding plate (the Eurasian plate in this case). The rate of subduction is 6.7 cm per year. From Knight and others (1986).
Knight, M.D., Walker, G.P.L., Ellwood, B.B., and Diehl, J.F., 1986, Stratigraphy, paleomagnetism, and magnetic fabric of the Toba Tuffs: Constraints on their sources and eruptive styles: Journal of Geophysical Research, v. 91, p. 10,355-10,382.
Ninkovich, D., Sparks, R.S.J., and Ledbetter, M.T., 1978, The exceptional magnitude and intensity of the Toba eruption, Sumatra: An example of using deep-sea tephra layers as a geological tool: Bulletin of Volcanologique, v. 41, p. 286-298.
Rose, W.I., and Chesner, C.A., 1987, Dispersal of ash in the great Toga eruption, 75 ka: Geology, v. 15, p. 913-917. Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.
Williams, M.A.J., and Royce, K., 1982, Quaternary geology of the Middle Son Valley, north central India: Implications for prehistoric archaeology: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 38, p. 139-162.
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