Lake Toba (
Indonesian:
Danau Toba) is a
lake and
supervolcano.
The lake is 100 kilometres long and 30 kilometres wide, and 505 metres
(1,666 ft) 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 metres (2,953 ft), 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.
[1]
Lake Toba is the site of a supervolcanic eruption that occurred an estimated 69,000 to 77,000 years ago,
[2][3][4] a massive, climate-changing event. The eruption is believed to have had a
VEI intensity of 8. It is the largest known explosive eruption anywhere on
Earth in the last 25 million years. According to the
Toba catastrophe theory, it had global consequences, killing most humans then alive and creating a
population bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today.
[5]
However, this theory is not widely accepted due to lack of evidence for
any other animal decline or extinction, even in environmentally
sensitive species.
[6] However, it has been accepted that the eruption of Toba led to a
volcanic winter with a worldwide decline in temperatures between 3 to 5 °C (5 to 9 °F), and up to
15 °C (27.0 °F) in higher latitudes.
[edit] Geology
The Toba
caldera
complex in Northern Sumatra, Indonesia consists of four overlapping
volcanic craters that adjoin the Sumatran "volcanic front". The youngest
and fourth caldera is the world's largest Quaternary caldera (100 by 30
km (62 by 19 mi)) and intersects the three older calderas. An estimate
of 2,800 km
3 (670 cu mi) of
dense-rock equivalent
pyroclastic material, known the Youngest Toba tuff, was blasted from
the youngest caldera during one of the largest single explosive volcanic
eruptions in geologic history. Following the "Youngest Toba tuff
eruption", a typical resurgent dome formed within the new caldera,
joining two half-domes separated by a longitudinal
graben.
[3]
There are at least four cones, four
stratovolcanoes
and three craters visible in the lake. The Tandukbenua cone on the NW
edge of the caldera is relatively lacking in vegetation, suggesting a
young age of only several hundred years. Also, the Pusubukit volcano on
the south edge of the caldera is
solfatarically active.
[7]
Panoramic view of the town of Ambarita on Samosir, Lake Toba
Aerial view of the southern shore with Sibandang Island visible in the background.
[edit] The eruption
Location of Lake Toba shown in red on map.
The
Toba eruption (the
Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago.
[8] The Toba eruption was the latest of a series of at least three
caldera-forming eruptions which have occurred at the volcano, with earlier calderas having formed around 700,000 and 840,000 years ago.
[9] The last eruption had an estimated
Volcanic Explosivity Index of 8 (described as "mega-colossal"), making it possibly the largest explosive
volcanic eruption within the last 25 million years.
Bill Rose and Craig Chesner of
Michigan Technological University have deduced that the total amount of erupted material was about 2,800 km
3 (670 cu mi)
[10]—around 2,000 km
3 (480 cu mi) of
ignimbrite that flowed over the ground, and around 800 km
3 (190 cu mi) that fell as ash, with the wind blowing most of it to the west. The
pyroclastic flows
of the eruption destroyed an area of 20,000 square kilometres
(7,722 sq mi), with ash deposits as thick as 600 metres (1,969 ft) by
the main vent.
[10]
The eruption was large enough to have deposited an ash layer approximately 15 cm (5.9 in) thick over the entire
South Asia; at one site in central India, the Toba ash layer today is up to 6 m (20 ft) thick
[11] and parts of
Malaysia were covered with 9 m (30 ft) of ashfall.
[12] In addition it has been variously calculated that 10,000 million
metric tons of
sulphuric acid[13][citation needed] or 6,000 million tons of
sulphur dioxide[14] were ejected into the atmosphere by the event, causing
acid rain fallout.
The Toba caldera is the only supervolcano in existence that can be described as
Yellowstone's "bigger" sister. With 2,800 km
3 (670 cu mi) of ejecta, it was an even greater eruption than the supereruption (2,500 km
3) of 2.1 million years ago that created the
Island Park Caldera
in Idaho, USA. The eruption was also about three times the size of the
latest Yellowstone eruption of Lava Creek 630,000 years ago. For further
comparison, the largest volcanic eruption in historic times, in
1815 at
Mount Tambora (Indonesia), ejected the equivalent of around 100 km
3 (24 cu mi) of dense rock and made 1816 the "
Year Without a Summer" in the whole northern hemisphere, whilst the
1980 eruption of Mount St. Helens in Washington State ejected around 1.2 km
3 (0.29 cu mi) of material. The largest known eruption since the Toba event, the
Oruanui eruption in New Zealand around 24,500 BC, ejected the equivalent of 530 km
3 of magma.
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.
Landsat photo of Sumatra surrounding Lake Toba
Though the year may 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.
[15]
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 species may have passed through a
genetic bottleneck around this time, 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.
[16]
[edit] 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.
[17]
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 (1,476 ft)
[9]
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 (6.8 mi).
[18] Other earthquakes have occurred in the area in 1892, 1916, and 1920–1922.
[9]
Lake Toba lies near the
Great Sumatran fault which runs along the centre of Sumatra in the
Sumatra Fracture Zone.
[9] 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 9.1
2004 Indian Ocean Earthquake and the 8.7
2005 Sumatra earthquake, the epicenters of which were around 300 km (190 mi) from Toba.
On 12 September 2007, a
magnitude 8.5 earthquake shook the ground in 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.
On 26 October 2010, a magnitude 7.7 earthquake occurred 36 kilometers
(22 mi) southwest of the nearby island of Pagai-selatan. A 3-metre
(10 ft) tsunami immediately followed the quake.
On 10 January 2012, a magnitude 7.3 earthquake occurred within the area.
[edit] Eruption prospect
The 2004 earthquake physically rolled the Sumatran island
[19] and altered
[20] the shape of the Earth
[19] as was detected by the
GRACE satellite. Recent highly localized earthquake activity may initiate magmatic activity of this colossal global climate modifier.
View of the lake with an example of Batak architecture in the foreground.
Traditional Batak house at Ambarita, Lake Toba
[edit] People
Most of the people who live around Lake Toba are ethnically
Bataks.
Traditional Batak houses are noted for their distinctive roofs (which
curve upwards at each end, as a boat's hull does) and their colorful
decor.
[21]
[edit] Flora and fauna
The
flora of the lake includes various types of
phytoplankton, emerged
macrophytes, floating macrophytes, and submerged macrophytes, while the surrounding countryside is rainforest including areas of
Sumatran tropical pine forests on the higher mountainsides.
[22]
The
fauna includes several species of
zooplankton and
benthic animals. Since the lake is
oligotrophic (nutrient-poor), the native fish fauna is relatively scarce, and the only
endemics are
Rasbora tobana (strictly speaking near-endemic, since also found in some tributary rivers that run into the lake)
[23] and
Neolissochilus thienemanni, locally known as the Batak fish.
[24] The latter species is threatened by deforestation (causing
siltation), pollution, changes in water level and the numerous fish species that have been
introduced to the lake.
[24] Other native fishes include species such as
Aplocheilus panchax,
Nemacheilus pfeifferae,
Homaloptera gymnogaster,
Channa gachua,
Channa striata,
Clarias batrachus,
Barbonymus gonionotus,
Barbonymus schwanenfeldii,
Danio albolineatus,
Osteochilus vittatus,
Puntius binotatus,
Rasbora jacobsoni,
Tor tambra,
Betta imbellis,
Betta taeniata and
Monopterus albus.
[25] Among the many introduced species are
Anabas testudineus,
Oreochromis mossambicus,
Oreochromis niloticus,
Ctenopharyngodon idella,
Cyprinus carpio,
Osphronemus goramy,
Trichogaster pectoralis,
Trichopodus trichopterus,
Poecilia reticulata and
Xiphophorus hellerii.
[25]
[edit] Gallery
-
Toba Lake viewed from Parapat
-
Toba Lake viewed from Ambarita
-
Toba Lake Villages Samosir
-
-
-
07.24
mario zebedia
