The 1883 eruption of Krakatoa began in May 1883 and culminated with the destruction of Krakatoa on 27 August 1883. Minor seismic activity continued to be reported until February 1884, though reports after October 1883 were later dismissed by Rogier Verbeek’s investigation.
In the years before the 1883 eruption, seismic activity around the volcano was intense, with some earthquakes felt as far as Australia. Beginning 20 May 1883, three months before the final explosion, steam venting began to occur regularly from Perbuatan, the northernmost of the island’s three cones. Eruptions of ash reached an altitude of 6 km (20,000 ft) and explosions could be heard in New Batavia (Jakarta) 160 km (99 mi) away. Activity died down by the end of May, with no records of activity until mid-June.
Eruptions started again around 16 June, when loud explosions were heard and a thick black cloud covered the islands for five days. On 24 June an east wind blew this cloud away and two ash columns were seen issuing from Krakatoa. The new seat of the eruption is believed to have been a new vent or vents which formed between Perbuatan and Danan, near the location of the volcanic cone of Anak Krakatau. 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. Earthquake shocks began to be felt at Anyer (Java), and large pumice masses started to be reported by ships in the Indian Ocean to the west.
On 11 August, H.J.G. Ferzenaar investigated the islands. He noted three major ash columns (the newer from Danan), which obscured the western part of the island (the wind blows primarily from the east at this time of year), and steam plumes from at least eleven other vents, mostly between Danan and Rakata. Where he landed, he found an ash layer about 0.5 m (1 ft 8 in) thick; all vegetation had been destroyed, with only tree stumps left. He advised against any further landings. The next day, a ship passing to the north reported a new vent “only a few meters above sea level” (this may be the most northerly spot indicated on Ferzenaar’s map). Activity continued through mid August.
By 25 August, eruptions further intensified. At about 13:00 (local time) on 26 August, the volcano went into its paroxysmal phase, and by 14:00 observers could see a black cloud of ash 27 km (17 mi) high. At this point, the eruption was virtually continuous and explosions could be heard every ten minutes or so. Ships within 20 km (12 mi) of the volcano reported heavy ash fall, with pieces of hot pumice up to 10 cm (3.9 in) in diameter landing on their decks. A small tsunami hit the shores of Java and Sumatra some 40 km (25 mi) away between the time of 18:00 and 19:00 hours.
On 27 August four enormous explosions took place at 05:30, 06:44, 10:02, and 10:41 local time. The explosions were so violent that they were heard 3,500 km (2,200 mi) away in Perth, Western Australia and the Indian Ocean island of Rodrigues near Mauritius, 4,800 km (3,000 mi) away, where they were thought to be cannonfire from a nearby ship. 22 Each was accompanied by very large tsunamis, which are 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 pressure wave generated by the colossal final explosion radiated from Krakatoa at 1,086 km/h (675 mph). It was so powerful that it ruptured the eardrums of sailors on ships in the Sunda Strait and caused a spike of more than two and half inches of mercury (ca 85 hPa) in pressure gauges attached to gasometers in the Jakarta gasworks, sending them off the scale. The pressure wave radiated across the globe and was recorded on barographs all over the world, which continued to register it up to 5 days after the explosion. Barograph recordings show that the shockwave from the final explosion reverberated around the globe 7 times in total. Ash was propelled to a height of 80 km (50 mi).
The eruptions diminished rapidly after that point, and by the morning of August 28 Krakatoa was silent. Small eruptions, mostly of mud, continued through October, though further reports continued through February 1884. These reports were discounted by Verbeek.
“The Burning Ashes of Ketimbang”
Around noon on August 27, a rain of hot ash fell around Ketimbang (now Katibung in Lampung Province) 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 surge (similar to the catastrophic 1980 eruption of Mount St. Helens), which crossed the water. The region of the ashfall ended to the northwest of Ketimbang, where the bulk of Sebesi Island offered protection from any horizontal surges.
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 (8.1 mi) from Krakatoa. Pyroclastic flows killed around 1,000 people at Ketimbang on the coast of Sumatra some 40 km (25 mi) north from Krakatoa. The official death toll recorded by the Dutch authorities was 36,417, although some sources put the estimate at 120,000 or more. Many settlements were destroyed, including Teluk Betung and Ketimbang in Sumatra, and Sirik and Serang in Java. The areas of Banten on Java and the Lampung 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 and distant effects
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 four great explosions was accompanied by a massive pyroclastic flow resulting from the gravitational collapse of the eruption column. This caused several cubic kilometers of material to enter the sea, displacing an equally huge volume of seawater. The town of Merak was destroyed by a tsunami 46 m (151 ft) high. Some of the pyroclastic flows reached the Sumatran coast as much as 40 km (25 mi) away, having apparently moved across the water on a “cushion” of superheated steam. There are also indications of submarine pyroclastic flows reaching 15 km (9.3 mi) from the volcano.
A recent documentary film showed tests made by a research team at the University of Kiel, Germany, of pyroclastic flows moving over water. The tests revealed that hot ash travelled over the water on a cloud of superheated steam, continuing to be a pyroclastic flow after crossing water; the heavy matter precipitated out of the flow shortly after initial contact with the water, creating a tsunami due to the precipitate mass.
Smaller waves were recorded on tidal gauges as far away as the English Channel. These occurred too soon to be remnants of the initial tsunamis, and may have been caused by concussive air waves from the eruption. These air waves circled the globe several times and were still detectable using barographs five days later.
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-metre (820 ft) deep caldera. Of the northern two-thirds of the island, only a rocky islet named Bootsmansrots (‘Bosun’s Rock’, a fragment of Danan) was left; Poolsche Hoed had disappeared.
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 (4.3–5.0 cu mi) of ignimbrite was deposited over an area of 1,100,000 km2 (420,000 sq mi), largely filling the 30–40 m (98–130 ft) deep basin around the mountain. The land masses of Verlaten and Lang were increased, as was the western part of the remnant of Rakata. Much of this gained material quickly eroded away, but volcanic ash continues to be a significant part of the geological composition of these islands.
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.
In the year following the eruption, average global temperatures fell by as much as 1.2 °C (2.2 °F). 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 sulfurous acid (H2SO3) concentration in high-level cirrus clouds. 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 darkened the sky worldwide for years afterwards, and produced spectacular sunsets throughout the world for many months. 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, an astronomer 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. This explanation has been disputed by art scholars who note that Munch was an expressive rather than descriptive painter.
Weather watchers of the time tracked and mapped the effects on the sky. They labeled the phenomenon the “equatorial smoke stream.”This was the first identification of what is known today as the Jet stream.
This eruption also produced a Bishop’s Ring around the sun by day, and a volcanic purple light at twilight.
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.
The main 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.
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 actually been a major landslide (possibly the one which formed the second arc to Rakata’s cliff).
Examinations after 1930 of bathymetric charts made in 1919 show evidence of a bulge indicative of magma near the surface at the site that became Anak Krakatau.
Adapted from Wikipedia, the free encyclopedia