Krakatoa
2007 Schools Wikipedia Selection. Related subjects: Geography of Asia
| Krakatoa | |
|---|---|
 An early 19th century illustration of Krakatoa |
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| Elevation | 813 m (2,667 feet) |
| Location | Sunda Strait, Indonesia |
| Coordinates | |
| 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
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
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
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.