:: Lake Toba - North Sumatra Travel Guides ::
Lake Toba (Indonesian: Danau
Toba) is a lake, 100 km long and 30 km wide, and 505 m.
(1,666 ft.) at its deepest point, in the middle of the
northern part of the Indonesian island of Sumatra with a
surface elevation of about 900 m (3,000 feet),
stretching from [show location on an interactive map]
2.88° N 98.52° E to [show location on an interactive
map] 2.35° N 99.1° E. It is the largest volcanic lake in
the world.
Geology
In 1949 the Dutch
geologist Rein van Bemmelen reported that Lake Toba was
surrounded by a layer of ignimbrite rocks, and that it
was a large volcanic caldera. Later researchers found
rhyolite ash similar to that in the ignimbrite around
Toba (now called Young Toba Tuff to distinguish it from
layers deposited in previous explosions) in Malaysia and
India, 3,000 km away. Oceanographers discovered Toba
ash, with its characteristic chemical "fingerprint", on
the floor of the eastern Indian Ocean and the Bay of
Bengal.
The Toba eruption (the Toba event) occurred at what is
now Lake Toba about 67,500 to 75,500 years ago.
It
had an estimated Volcanic Explosivity Index of 8
(described as "mega-colossal"), making it possibly the
largest explosive volcanic eruption within the last
twenty-five million years. Bill Rose and Craig Chesner
of Michigan Technological University deduced that the
total amount of erupted material was about 2800 cubic km
(670 cubic miles) — around 2,000 km³ of ignimbrite that
flowed over the ground and around 800 km³ that fell as
ash, with the wind blowing most of it to the west. By
contrast, the 1980 eruption of Mount St. Helens ejected
around 1.2 cubic km of material, whilst the largest
volcanic eruption in historic times, at Mount Tambora in
1815, emitted the equivalent of around 100 cubic
kilometres of dense rock and created the "Year Without a
Summer" as far away as North America.
The eruption
The Toba eruption
w
as
the latest of a series of at least three caldera-forming
eruptions which have occurred at the volcano. Earlier
calderas were formed around 700,000 and 840,000 years
ago.
To give an idea of its magnitude, consider that although
the eruption took place in Indonesia, it deposited an
ash layer approximately 15 cm (6 in) thick over the
entire Indian subcontinent; at one site in central
India, the Toba ash layer today is up to 6 m (20 feet)
thick and parts of Malaysia were covered with 9 m of
ashfall. In addition it has been calculated that 1010
metric tons of sulphuric acid was ejected into the
atmosphere by the event, causing acid rain fallout.
The subsequent collapse formed a caldera that,
after
filling with water, created Lake Toba. The island in the
center of the lake is formed by a resurgent dome.
Though the year can never be precisely determined, the
season can: only the summer monsoon could have deposited
Toba ashfall in the South China Sea, implying that the
eruption took place sometime during the northern summer.
The eruption lasted perhaps two weeks, but the ensuing
"volcanic winter" resulted in a decrease in average
global temperatures by 3 to 3.5 degrees Celsius for
several years. Greenland ice cores record a pulse of
starkly reduced levels of organic carbon sequestration.
Very few plants or animals in southeast Asia would have
survived, and it is possible that the eruption caused a
planet-wide die-off. There is some evidence, based on
mitochondrial DNA, that the human race may have passed
through a genetic bottleneck within this timeframe,
reducing genetic diversity below what would be expected
from the age of the species. According to the Toba
catastrophe theory proposed by Stanley H. Ambrose of the
University of Illinois at Urbana-Champaign in 1998,
human populations may have been reduced to only a few
tens of thousands of individuals by the Toba eruption.
More recent activity
Smaller eruptions
have occurred at
Toba since. The small cone of Pusukbukit has formed on
the southwestern margin of the caldera and lava domes.
The most recent eruption may have been at Tandukbenua on
the northwestern caldera edge, since the present lack of
vegetation could be due to an eruption within the last
few hundred years.
Some parts of the caldera have experienced uplift due to
partial refilling of the magma chamber, for example
pushing Samosir Island and the Uluan Peninsula above the
surface of the lake. The lake sediments on Samosir
Island show that it has been uplifted by at least 450
metres since the cataclysmic eruption. Such uplifts are
common in very large calderas, apparently due to the
upward pressure of unerupted magma. Toba is probably the
largest resurgent caldera on Earth. Large earthquakes
have occurred in the vicinity of the volcano more
recently, notably in 1987 along the southern shore of
the lake at a depth of 11 km. Other earthquakes have
occurred in the area in 1892, 1916, and 1920-1922.
Lake Toba lies
near
a fault line which runs along the centre of Sumatra
called the Sumatra Fracture Zone. The volcanoes of
Sumatra and Java are part of the Sunda Arc, a result of
the northeasterly movement of the Indo-Australian Plate
which is sliding under the eastward-moving Eurasian
Plate. The subduction zone in this area is very active:
the seabed near the west coast of Sumatra has had
several major earthquakes since 1995, including the 9.3
2004 Indian Ocean Earthquake and the 8.7 2005 Sumatra
earthquake, the epicenters of which were around 300 km
from Toba.
On September 12th, 2007, a magnitude 8.4 Earthquake
shook the ground by Sumatra and was felt in the
Indonesian capital, Jakarta. The epicenter for this
earthquake was not as close as the previous two
earthquakes, but it was in the same vicinity.
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