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Volcanoes in Indonesia
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The geography of Indonesia is dominated
by volcanoes that are formed due to subduction zones between the
Eurasian plate and the Indo-Australian plate. Some of the volcanoes are
notable for their eruptions, for instance, Krakatau for its global
effects in 1883, Lake Toba for its supervolcanic eruption estimated to
have occurred 74,000 Before Present which was responsible for six years
of volcanic winter, and Mount Tambora for the most violent eruption in
recorded history in 1815.
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Krakatau
Krakatau volcano in the Sunda Strait
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Krakatoa or Krakatau or Krakatao is a
volcanic island in the Sunda Strait between Java and Sumatra in
Indonesia. The name is used for the island group, the main island (also
called Rakata), and the volcano as a whole. It has erupted
repeatedly, massively and with disastrous consequences throughout
recorded history. The best known eruption culminated in a series of
massive explosions on August 26-27, 1883.
The 1883 eruption ejected more than 25 cubic kilometres of rock, ash,
and pumice, and generated the loudest sound historically reported: the
cataclysmic explosion was distinctly heard as far away as Perth in
Australia (approx. 1930 miles or 3100 km), and the island of Rodrigues
near Mauritius (approx. 3000 miles or 4800 km). Near Krakatoa, according
to official records, 165 villages and towns were destroyed and 132
seriously damaged, at least 36,417 (official toll) people died, and many
thousands were injured by the eruption, mostly from the tsunamis which
followed the explosion.
The eruption destroyed two thirds of the island of Krakatoa. Eruptions
at the volcano since 1927 have built a new island in the same location,
called Anak Krakatau (child of Krakatoa).
Origin and spelling of the name
The earliest mention of the island in the Western world was on a map by Lucas Janszoon Waghenaer, who labelled the island Pulo Carcata. ("Pulo" is a form of pulau,
the Indonesian word for "island".) There are two generally accepted
spellings, Krakatoa and Krakatau. While Krakatoa is more common in the
English-speaking world, Krakatau (or Krakatao in an older Portuguese
based spelling) tends to be favored by Indonesians and geologists. The
origin of the spelling Krakatoa is unclear, but may have been the result
of a typographical error made in a British source reporting on the
massive eruption of 1883.
Theories as to the origin of the Indonesian name Krakatau include:
- Onomatopoeia, imitating the noise made by white parrots which used to inhabit the island.
- From Sanskrit karka or karkata or karkataka, meaning "lobster" or "crab".
- From Malay kelakatu, meaning "white-winged ant".
There is a popular story that Krakatau was the result of a linguistic
error. According to legend, "Krakatau" was adopted when a visiting
ship's captain asked a local inhabitant the island's name, and the
latter replied "Kaga tau" � a Jakartan/Betawinese slang phrase meaning
"I don't know". This story is largely discounted; it closely resembles
famous linguistic myths about the origin of the word kangaroo and the
name of the Yucat�n Peninsula. The name is spelled Karata on a map drawn
before 1708.
Before 1883
Before the 1883 eruption, Krakatoa consisted of three main islands: Lang
('Long', now called Rakata Kecil or Panjang) and Verlaten ('Forsaken'
or 'Deserted', now Sertung), which were edge remnants of a previous very
large caldera-forming eruption; and Krakatoa itself, an island 9 km
long by 5 km wide. Also there was a tree-covered islet near Lang named
Poolsche Hoed ('Polish Hat', apparently because it looked like one from
the sea), and several small rocks or banks between Krakatoa and
Verlaten. There were three volcanic cones on Krakatoa: running South to
North they were: Rakata (823 m), Danan (445 m), and Perboewatan (also
spelled Perbuatan) (122 m). (Danan may have been a twin volcano).
Krakatoa is directly above the subduction zone of the Eurasian Plate and
Indo-Australian Plate, where the plate boundaries undertake a sharp
change of direction, possibly resulting in an unusually weak crust in
the region.
416 AD event
The Javanese Book of Kings (Pustaka Raja) records that in the year 338
Saka (416 AD). "A thundering sound was heard from the mountain Batuwara
... a similar noise from Kapi ... The whole world was greatly shaken and
violent thundering, accompanied by heavy rain and storms took place,
but not only did not this heavy rain extinguish the eruption of the fire
of the mountain Kapi, but augmented the fire; the noise was fearful, at
last the mountain Kapi with a tremendous roar burst into pieces and
sank into the deepest of the earth. The water of the sea rose and
inundated the land, the country to the east of the mountain Batuwara, to
the mountain Raja Basa, was inundated by the sea; the inhabitants of
the northern part of the Sunda country to the mountain Raja Basa were
drowned and swept away with all property ... The water subsided but the
land on which Kapi stood became sea, and Java and Sumatra were divided
into two parts."
There is no geological evidence of a Krakatoa eruption of this size
around that time; it may describe loss of land which previously joined
Java to Sumatra across what is now the narrow east end of the Sunda
Strait; or it may be a mistaken date, referring to an eruption in 535
AD, also referred to in the Javanese Book of Kings, and for which there
is geological and some corroborating historical evidence.
535 AD event
David Keys and others have postulated that the violent eruption of
Krakatoa in 535 may have been responsible for the global climate changes
of 535-536. Keys explores what he believes to be the radical and far
ranging global effects of just such a putative 6th century eruption in
his book Catastrophe: An Investigation into the Origins of Modern
Civilization. Additionally, in recent times, it has been argued that it
was this eruption which created the islands of Verlaten and Lang
(remnants of the original) and the beginnings of Rakata - all indicators
of early Krakatoa's caldera's size. However, there seems to be little,
if any, datable charcoal from that eruption, even if there is plenty of
circumstantial evidence.
1600s
At least two Dutch travelers reported that Danan and Perboewatan were seen erupting in May 1680 and February 1681.
Visit by HMS Discovery
In February 1780, the crews of HMS Resolution and HMS Discovery, on the
way home after Captain James Cook's death in Hawaii, stopped for a few
days on Krakatoa. They found two springs on the island, one fresh water
and the other hot. They described the natives who then lived on the
island as "friendly" and made several sketches. (In his journal, John
Ledyard calls the island 'Cocoterra'.)
Dutch activity
In 1809, the Dutch established a penal colony on the islands. It was in
operation for about a decade. In 1880, Rogier Verbeek made an official
survey of the islands and published a comprehensive report in 1884/5.
This proved helpful in judging the geological and biological impact of
the 1883 eruption.
The 1883 eruption
Early eruptions
In the years before the 1883 eruption, seismic activity around the
volcano was intense, with some earthquakes felt as far distant as
Australia. Beginning 20 May 1883, three months before the final
explosion, steam venting began to occur regularly from Perboewatan, the
northern of the island's three cones. Eruptions of ash reached an
altitude of 6 km (20,000 ft) and explosions could be heard in Batavia
(Jakarta) 160 km (100 miles) away. Activity died down by the end of May.
Also, to help the eruption along, water seeped into the magma chamber
and created large amounts of steam. It had been thought Krakatoa was 3
different volcanoes, but it was actually just one with a huge magma
chamber.
The volcano began erupting again around 20 July. The seat of the
eruption is believed to have been a new vent or vents which formed
between Perboewatan and Danan, more or less where the current volcanic
cone of Anak Krakatau is. The violence of the eruption caused tides in
the vicinity to be unusually high, and ships at anchor had to be moored
with chains as a result. On 11 August larger eruptions began, with ashy
plumes being emitted from at least eleven vents. On 24 August, eruptions
further intensified. At about 1pm (local time) on 26 August, the
volcano went into its paroxysmal phase, and by 2pm observers could see a
black cloud of ash 27 km (17 miles) high. At this point, the eruption
was virtually continuous and explosions could be heard every ten minutes
or so. Ships within 20 km (11 nautical miles) of the volcano reported
heavy ash fall, with pieces of hot pumice up to 10 cm in diameter
landing on their decks. A small tsunami hit the shores of Java and
Sumatra some 40 km (28 miles) away between 6pm and 7pm.
Cataclysmic stage
On August 27, the volcano entered the final cataclysmic stage of its
eruption. Four enormous explosions took place at 5:30 a.m., 6:42 a.m.,
8:20 a.m., and 10:02 a.m., the last of which was worst and loudest. Each
was accompanied by very large tsunamis believed to have been over 30
meters (100 ft) high in places. A large area of the Sunda Strait and a
number of places on the Sumatran coast were affected by pyroclastic
flows from the volcano. The explosions were so violent that they were
heard 2,200 statute miles (3,500 km) away in Australia and the island of
Rodrigues near Mauritius, 4,800 km away; the sound of Krakatoa's
destruction is believed to be the loudest sound in recorded history,
reaching levels of 180 dBSPL 100 miles (160 km) away. Ash was propelled
to a height of 50 miles (80 km). The eruptions diminished rapidly after
that point, and by the morning of August 28 Krakatoa was quiet.
"The Burning Ashes of Ketimbang"
Around noon on August 27, a rain of hot ash fell around Ketimbang in
Sumatra. Around a thousand people were killed, the only large number of
victims killed by Krakatoa itself, and not the waves or after-effects.
Verbeek and later writers believe this unique event was a lateral blast
or pyroclastic flow (perhaps traveling over the floating pumice rafts),
similar to what happened in 1980 at Mt. St. Helens. The region of the
ashfall ended to the northwest of Ketimbang, where the bulk of Sebesi
Island offered protection from any horizontal surges.
After eruptions
Small eruptions continued through October, and continued to be reported
through February 1884 (although any after mid October were discounted by
Verbeek). In the aftermath of the eruption, it was found that the
island of Krakatoa had almost entirely disappeared, except for the
southern half of Rakata cone cut off along a vertical cliff, leaving
behind a 250-meter-deep caldera.
Effects
The combined effects of pyroclastic flows, volcanic ashes and tsunamis
had disastrous results in the region. There were no survivors from 3,000
people located at the island of Sebesi, about 13 km from Krakatoa.
Pyroclastic flows killed around 1,000 people at Ketimbang on the coast
of Sumatra some 40 km north from Krakatoa. The official death toll
recorded by the Dutch authorities was 36,417 and many settlements were
destroyed, including Teluk Betung and Ketimbang in Sumatra, and Sirik
and Semarang in Java. The areas of Banten on Java and the Lampong on
Sumatra were devastated. There are numerous documented reports of groups
of human skeletons floating across the Indian Ocean on rafts of
volcanic pumice and washing up on the east coast of Africa, up to a year
after the eruption. Some land on Java was never repopulated; it
reverted to jungle and is now the Ujung Kulon National Park.
Tsunamis
Ships as far away as South Africa rocked as tsunamis hit them, and the
bodies of victims were found floating in the ocean for weeks after the
event. The tsunamis which accompanied the eruption are believed to have
been caused by gigantic pyroclastic flows entering the sea; each of the
five great explosions was accompanied by a massive pyroclastic flow
resulting from the gravitational collapse of the eruption column. This
caused several km� of material to enter the sea, displacing an equally
huge volume of seawater. Some of the pyroclastic flows reached the
Sumatran coast as much as 25 miles (40 km) away, having apparently moved
across the water on a "cushion" of superheated steam. There are also
indications of submarine pyroclastic flows reaching 10 miles (15 km)
from the volcano.
On a recent film and documentary, a research team at Kiel University of
Germany conducted tests of pyroclastic flows moving over water. The
tests revealed that hot ash traveled over the water on a cloud of
superheated steam with the heavy matter precipitating out of the flow,
shortly after initial contact with the water, to create a tsunami due to
the precipitate mass.
Geographic effects
As a result of the huge amount of material deposited by the volcano, the
surrounding ocean floor was drastically altered. It is estimated that
as much as 18-21 km� of ignimbrite was deposited over an area of 1.1
million km�, largely filling the 30-40 m deep basin around Krakatoa. The
land masses of Verlaten and Lang were increased, and volcanic ash
continues to be a significant part of the geological composition of
these islands. Polish Hat disappeared. A new rock islet called
Bootsmansrots ('Bosun's Rock', a fragment of Danan) was left.
Two nearby sandbanks (called Steers and Calmeyer after the two naval
officers who investigated them) were built up into islands by ashfall,
but the sea later washed them away. Seawater on hot volcanic deposits on
Steers and Calmeyer caused steam which some people mistook for
continued eruption.
The fate of Krakatoa itself has been the subject of some dispute among
geologists. It was originally proposed that the island had been blown
apart by the force of the eruption. However, most of the material
deposited by the volcano is clearly magmatic in origin and the caldera
formed by the eruption is not extensively filled with deposits from the
1883 eruption. This indicates that the island subsided into an empty
magma chamber at the end of the eruption sequence, rather than having
been destroyed during the eruptions.
Global climate
In the year following the eruption, average global temperatures fell by
as much as 1.2 degrees Celsius. Weather patterns continued to be chaotic
for years, and temperatures did not return to normal until 1888. The
eruption injected an unusually large amount of sulfur dioxide (SO2) gas
high into the stratosphere which was subsequently transported by
high-level winds all over the planet. This led to a global increase in
sulfuric acid (H2SO4) concentration in high-level cirrus cloud. The
resulting increase in cloud reflectivity (or albedo) would reflect more
incoming light from the sun than usual, and cool the entire planet until
the suspended sulfur fell to the ground as acid precipitation.
Global optical effects
The eruption produced spectacular sunsets throughout the world for many
months afterwards. British artist William Ashcroft made thousands of
colour sketches of the red sunsets half-way around the world from
Krakatoa in the years after the eruption. In 2004, researchers proposed
the idea that the blood-red sky shown in Edvard Munch's famous 1893
painting The Scream is also an accurate depiction of the sky over Norway
after the eruption. Munch said: "suddenly the sky turned blood red ... I
stood there shaking with fear and felt an endless scream passing
through nature." Also, a so called blue moon had been seen for two years
as a result of the eruption.
Legacy of the 1883 eruption
The 1883 eruption of Krakatoa is among the most violent volcanic events
in modern times (a VEI of 6, equivalent to 200 megatons of TNT � about
13,000 times the yield of the Little Boy bomb which devastated
Hiroshima, Japan). Concussive air waves from the explosions traveled
seven times around the world, and were detectable for five days. The sky
was darkened for days afterwards. Sea waves caused by the eruption were
recorded as far away as the English Channel. The explosion is
considered to be among the loudest noises ever heard by humans.
Cause of the explosion
The violence of the final explosions has also attracted debate. Four theories are:
- Contemporary investigators believed that the volcano's vents had
sunk below sea level on the morning of 27 August, letting seawater flood
into it and causing a massive series of phreatic (interaction of ground
water and magma) explosions.
- The seawater could have chilled the magma, causing it to crust
over and producing a "pressure cooker" effect relieved only when
explosive pressures were reached.
Both these ideas assumed that the island subsided before the explosions;
however, the evidence does not support that conclusion and the pumice
and ignimbrite deposits are not of a kind consistent with a
magma-seawater interaction.
- A massive underwater land slump or partial subsidence suddenly left the highly pressurized magma chamber wide open.
- The final explosions may have been caused by magma mixing caused
by a sudden infusion of hot basaltic magma into the cooler and lighter
magma in the chamber below the volcano. This would have resulted in a
rapid and unsustainable increase in pressure, leading to a cataclysmic
explosion. Evidence for this theory is the existence of pumice
consisting of light and dark material, the dark material being of much
hotter origin. However, such material reportedly is less than 5% of the
content of the Krakatoa ignimbrite and some investigators have rejected
this as a prime cause of the 27 August explosions.
Subsequent volcanism
Verbeek investigation
Although the violent engulfment phase of the eruption was over by late
afternoon of August 27, after light returned by the 29th, reports
continued for months that Krakatoa was still in eruption. One of the
earliest duties of Verbeek's committee was to determine if this was true
and also verify reports of other volcanoes erupting on Java and
Sumatra. In general, these were found to be false, and Verbeek
discounted any claims of Krakatoa still erupting after mid October as
due to steaming of hot material, landslides due to heavy monsoon rains
that season, and "hallucinations due to electrical activity" seen from a
distance.
No signs of activity were seen in the next several years until 1913,
when an eruption was reported. Investigation could find no evidence the
volcano was awakening, and it was determined that what had been mistaken
for renewed activity had been a major landslide (possibly the one which
formed the second arc to Rakata's cliff).
Anak Krakatau
Verbeek, in his report on the eruption, predicted that any new activity
would manifest itself in the region which had been between Perboewatan
and Danan. This prediction came true in June 1927 when evidence of a
submarine eruption was seen in this area. A few days later, a new island
volcano, named Anak Krakatau ("Child of Krakatoa"), broke water.
Initially, the eruptions were of pumice and ash, and it (and 2 more
islands) was quickly eroded away by the sea; but eventually Anak
Krakatoa #4 produced lava flows faster than the waves could erode them.
Of considerable interest to volcanologists, this has been the subject of
extensive study since the new island broke water permanently in August
1930.
Current activity
The island is still active, with its most recent eruptive episode having
begun in 1994. Since then, quiet periods of a few days have alternated
with almost continuous eruptions, with occasional much larger
explosions. Since the 1950s, the island has grown at an average rate of
five inches (13 cm) per week. Reports in 2005 indicated that activity at
Anak Krakatau was increasing, with fresh lava flows adding to the
island's area.
On 6 May 2009 the Volcanological Survey of Indonesia raised the eruption
alert status of Anak Krakatau to Level Orange. James Reynolds posted
footage to YouTube from as recently as November 1, 2010 showing some
spectacular eruptions, and Nasa has released satellite imagery of the
recent activity.
Biological research
The islands have become a major case study of island biogeography and
founder populations in an ecosystem being built from the ground up in an
environment virtually sterilized.
'The Krakatau problem'
Biologically, the 'Krakatau problem' refers to the question if the
islands were completely sterilized by the 1883 eruption, or if some life
survived. When the first researchers reached the islands in May, 1884,
the only living thing they found was a spider in a crevice on the south
side of Rakata. Life quickly recolonized the islands, however. The
eastern side of the island has been extensively vegetated by trees and
shrubs, presumably brought there as seeds washed up by ocean currents or
carried in birds' droppings. It is, however, in a somewhat fragile
position and the vegetated area has been badly damaged by recent
eruptions.
Handl's occupancy
A German, Johann Handl, obtained a permit to mine pumice in Oct 1916
(Thornton). His lease was for 870 hectares, basically the eastern half
of the island, for 30 years. He occupied the south slope of Rakata from
1915 to 1917, when he left due to "violation of the terms of the lease"
(Winchester gives Late 1917-1921). Built house & planted garden with
"4 European families and about 30 coolies". Introduced Rattus rattus
(Black Rat). Handl found unburned wood below 1883 deposits when digging,
fresh water was found below 18 feet.
National park
After Handl's departure, the western half of Rakata and Verlaten were
designated a national monument in July 1919. The eastern half was added
in 1925, and the islands were included in the Ujung Kulon Reserve, which
had been established in 1921. In 1982, Ujung Kulong was made a national
park. This led to the problem where the Krakatau Islands are part of a
Javan Park, they are politically controlled by the Lampung province of
Sumatra. This paradox was resolved in 1990, when the Krakataus were made
a separate nature reserve. Park Rangers have a station on Sertung, from
which they patrol, but as of 1996, they have no permanent patrol boats.
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