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Krakatoa

2007 Schools Wikipedia Selection. Related subjects: Geography of Asia

                       Krakatoa
   An early 19th century illustration of Krakatoa
   Elevation     813 m (2,667 feet)
   Location      Sunda Strait, Indonesia
   Coordinates   6°6′27″S, 105°25′3″E
   Type          Volcanic caldera
   Last eruption 2001

   Krakatoa (Indonesian name: Krakatau, Portuguese name: 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 ever 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). Atmospheric
   shock waves reverberated around the world seven times and were
   detectable for five days . 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 in the tsunamis which
   followed the explosion.

   The eruption destroyed two-thirds of the island of Krakatoa. New
   eruptions at the volcano since 1927 have built a new island, 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. 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 that 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 Yucatán Peninsula.

   The name is spelled Karata on a map drawn before 1708.

Before 1883

Geography

   The Sunda Strait
   Enlarge
   The Sunda Strait

   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 that 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 the HMS Discovery

   In February 1780, the crew of HMS Resolution and HMS Discovery on the
   way home after Captain James Cook's death in Hawaiʻi, 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. (No
   information on exactly where.) It was in operation for about a decade.
   R.D.M. Verbeek made an official survey of the islands in 1880 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

Pre-eruption

   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 and smoke.

Early eruptions

   The volcano began erupting again around 19 June. 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 worst and loudest of these was the last
   explosion. Each was accompanied by very large tsunamis believed to have
   been over 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.

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 m-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 that 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 km³ 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 documentary, a German research team conducted tests of
   pyroclastic flows moving over water. The tests revealed that hot ash
   traveled over the water on a cloud of superheated steam, preceding a
   tsunami.

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 km³ of ignimbrite was deposited over an area of
   1.1 million km², 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.

Worldwide effects

   Sunset in Hong Kong after the 1991 eruption of Mount Pinatubo
   Enlarge
   Sunset in Hong Kong after the 1991 eruption of Mount Pinatubo

   The eruption produced erratic weather and spectacular sunsets
   throughout the world for many months afterwards, as a result of
   sunlight reflected from suspended dust particles ejected by the volcano
   high into Earth's atmosphere. This worldwide volcanic dust veil acted
   as a solar radiation filter, reducing the amount of sunlight reaching
   the surface of the earth. In the year following the eruption, global
   temperatures were lowered by as much as 1.2 degrees Celsius on average.
   Weather patterns continued to be chaotic for years, and temperatures
   did not return to normal until 1888. 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
   13000 times the yield of the Little Boy bomb which devastated
   Hiroshima, Japan). Concussive air waves from the explosions travelled
   seven times around the world, and the sky was darkened for days
   afterwards. Waves from the tsunamis were recorded as far away as the
   English Channel. The explosion is considered to be the loudest noise
   ever heard by man.

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

   Anak Krakatau, June 2005. Fresh lava flows are clearly visible.
   Enlarge
   Anak Krakatau, June 2005. Fresh lava flows are clearly visible.
   Map of Krakatoa as it is today, showing boundaries of the ancient
   caldera in which the island sits
   Enlarge
   Map of Krakatoa as it is today, showing boundaries of the ancient
   caldera in which the island sits

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 that 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 that 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.

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.
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