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Published on 18th July, 2002

The Volcano That May Destroy Mankind

By Zaynab El-Fatah

Photography by Hj Zaynab El-Fatah and Hj S.Abidin

Lake Toba. Photo credit Hj S.Abidin

Approaching Lake Toba in North Sumatra, one could be forgiven for thinking that the beauty of the lake was heavenly, without thinking of how it was formed. The traditional Christian owners there, known as Batak people, seemingly guard the Lake and generally live peacefully with the many tourists who pass through there.

Samosir Island.Photo credit Hj Zaynab El-Fatah

Sumatra is the largest island of, what is now known, as the archipelago of the Republic of Indonesia. Sumatra island has a very long fault line and seismic activity is present for the length of the island, with the volcano on Samosir Island, Lake Toba, remaining active. One can bathe in the hot springs of Pangaruruan Volcano at Lake Toba, but many others including the volcano in Tarutung, attract many swimmers as well as engineers seeking alternative ways of providing power for the beautiful island.

The enormous inland lake [ pictured above] has a history  spoken of in local myths, but the true story of Toba and how this enormous inland lake evolved, lies in knowing the story of its Toba volcano.

Fault Zone- Samosir Island.

Map indicating the Sumatra Fracture Zone (SFZ).

Map indicating 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).

Young Toba Tuff

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.

Scientific Discussion on Toba

In Britain, a scientist has predicted that another volcanic super- eruption the size of Toba could pose twice as much of a threat to civilization as a collision with an asteroid or a comet.

Michael Rampino, of New York University, warned that a massive volcanic eruption capable of causing as much devastation as the cosmic bodies, occurs every 50,000 years.

"Volcanoes in Yellowstone Park and Long Valley in California have erupted three times in the past two million years, each time coating the whole of the U.S. with ash" he said.

REF: New Scientist Magazine on Wednesday.

"But the biggest and and the most recent super-eruption happened at Toba, on the Indonesian Island of Sumatra, 73,000 yrs ago" he added.

REF: New Scientist Magazine on Wednesday.

Map of the Toba caldera from Knight and others (1986).
Map of the Toba caldera 
from Knight and others (1986).

Samosir Island and the Uluan Peninsula

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 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. 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, but the area is seismically active with major earthquakes in 1892, 1916, 1920-1922, and 1987.

Rampinos Research

According to Rampino's research, Toba blasted a crater 100 kms long and sent 3 billion tonnes of sulphur dioxide into the atmosphere and a dense volcanic cloud around the globe.

"He also suspects that Toba's super-eruption was responsible for the population crash of 70,000 years ago, when the number of people fell to no more than 10,000", the magazine added.

Michael Rampino warned that this kind of super- eruption occurs every 50,000 years.

Other research indicates that as the cooling of the earth had already commenced and Toba then was the indicating point for the commencement of the ice-age. 

Sources of Information:

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.  

Rampino, Michael, New Scientist Magazine

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.

Further Reading:

Can We Predict Volcanic Eruptions

END

Author: Hj Zaynab El-Fatah
Photographers: Hajis Zaynab El-Fatah and S.Abidin
Illustrations: Knight and others (1986)
Chief Editor: Hj Nurzaynab El-Fatah
Production: Hj S. Abidin
Published Date: 18th July, 2002
Modification Date: 16th January 2009/ 19th Muharram, 1430
Publication ID: 02volcanoDes.  The Volcano That May Destroy Mankind
Copyright: Victory News Magazine, 2009

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