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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.
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. |
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).
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Landsat image of Toba caldera.
Image courtesy of the Landsat Pathfinder Project.
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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.
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.
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Map of the Toba caldera
from Knight and others (1986).
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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.
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. |
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
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