1883 eruption of Krakatoa

Beginning on 20 May 1883, and ending on 21 October 1883, the volcanic island of Krakatoa, located in the Sunda Strait, had repeated, months long volcanic eruptions. The most destructive of these eruptions occurred on 27 August, when over 70% of the island and its surrounding archipelago were destroyed when it erupted so violently it collapsed itself into a caldera. The August eruption had an estimated Volcanic Explosivity Index (VEI) of 6, and is one of the deadliest and most destructive volcanic events in recorded history.
The explosion was heard 3,110 kilometres (1,930 mi) away in Perth, Western Australia, and Rodrigues near Mauritius, 4,800 kilometres (3,000 mi) away. The acoustic pressure wave circled the globe more than three times. At least 36,417 deaths are attributed to the eruption and the tsunamis it created. Significant additional effects were felt worldwide in the days and weeks after the volcano's eruption. Additional seismic activity was reported until February 1884, but any reports after October 1883 were dismissed by Rogier Verbeek's subsequent investigation into the eruption.
In the years before the 1883 eruption, seismic activity around the Krakatoa volcano was intense, with earthquakes felt as far away as North Australia, one of which, in 1880, damaged a lighthouse. Strombolian activity began on 20 May 1883, and steam venting began to occur regularly from Perboewatan, the northernmost of the island's three cones. Eruptions of ash reached an estimated altitude of 6 km (20,000 ft), and explosions could be heard in Batavia (Jakarta) 160 km (100 mi) away, rattling windows and doors. Dust fell 300 miles away. An excursion party sent to the island on 26 May reported that the islands were covered with a fine white dust, with explosions coming from Perboewatan every 5 to 10 minutes. Although by this time the eruptions were calming down, some of the explosions were still very energetic, hurling pumice opposite the direction of the wind. A large crater approximately 900 metres (3,000 feet) in diameter formed, with a cavity from which issued plumes of steam. The eruptions soon quietened after the initial violence.
Around 16 June, there were loud explosions and a thick black cloud covered the islands for five days. The eruptions started gaining strength again on 19 June. On 24 June, a prevailing east wind cleared the cloud, and two ash columns could be seen issuing from Krakatoa. It was also noticed that the summit cone of Perboewatan was gone, which an extensive report of a few years later attributes it to having been blown away. The seat of the eruption is believed to have been a new vent or vents that formed on that day, between Perboewatan and Danan. The violence of the ongoing eruptions caused tides in the vicinity to be unusually high, and ships at anchor had to be moored with chains. Earthquakes were felt at Anyer, Banten, and ships began to report large pumice masses to the west in the Indian Ocean. A third vent, and later many more, appeared. The eruptions continued throughout July, and their violence continued to increase until the explosions of 27 August.
In early August, a Dutch topographical engineer, Captain H. J. G. Ferzenaar, investigated the Krakatoa islands. He noted three major ash columns (the newer from Danan), which obscured the western part of the island, and steam plumes from at least eleven other vents, mostly between Danan and Rakata. When he landed, he noted an ash layer about 0.5 m (1 ft 8 in) thick and the destruction of all vegetation, leaving only tree stumps. He advised against any further landings.
By 25 August, the Krakatoa eruptions intensified. At about 1:00 pm on 26 August, the eruption entered its climactic phase. By 2:00 pm, a black ash cloud could be seen 27 km (17 mi) high. At this point, the eruption was almost continuous, and explosions could be heard every ten minutes. By 5:00 pm, the explosions were audible all over Java. Waves a few feet in height began battering the coasts of Java and Sumatra at 5:30 pm, and continued throughout the night, which may have been produced by steam explosions. Ships within 20 km (12 mi) of the volcano reported heavy ash fall, with pieces of hot pumice up to 10 cm (4 in) in diameter landing on their decks. Between 7:00 pm and 8:00 pm, a small tsunami hit the shores of Java and Sumatra, 40 km (25 mi) away.
On 27 August, four enormous explosions occurred, which marked the climax of the eruption. At 5:30 am, the first explosion was at Perboewatan, triggering a tsunami heading to Telok Betong, now known as Bandar Lampung. At 6:44 am, Krakatoa exploded again at Danan, with the resulting tsunami propagating eastward and westward. The third and largest explosion, at 10:02 am, was so powerful that it was heard 3,110 km (1,930 mi) away in Perth, Western Australia, and the Indian Ocean island of Rodrigues near Mauritius, 4,800 km (3,000 mi) away, where the blast was thought to have been cannon fire from a nearby ship. The third explosion has been reported as the loudest sound in history. The loudness of the blast heard 160 km (100 mi) from the volcano has been calculated to have been 180 dB. Each explosion was accompanied by tsunamis estimated to have been over 30 metres (98 feet) high in places. A large area of the Sunda Strait and places on the Sumatran coast were affected by pyroclastic flows from the volcano. Verbeek and others believe that the final major Krakatoa eruption was a lateral blast, or pyroclastic surge. Material shot out of the volcano at 2,575 kilometres per hour (715 metres per second). The energy released from the explosion has been estimated to be equal to about 200 megatonnes of TNT (840 petajoules), roughly four times as powerful as the Tsar Bomba, the most powerful thermonuclear weapon ever detonated. This makes it one of the most powerful explosions in recorded history. At 10:41 am, a landslide tore off half of Rakata volcano, along with the remainder of the island to the north of Rakata, causing the final explosion.
The sounds of the eruption of the Krakatoa volcano were estimated to be 310 dB SPL, and there are reports that it was heard some 1,300 miles away in the Bay of Bengal. Some islands in the western Indian Ocean, approximately 3,000 miles away, still heard it at a dB level near the same level as a gun blast. Due to the speed of sound, it is likely the people on these far away islands did not hear Krakatoa for nearly four hours after its eruption. The pressure wave generated by the colossal third explosion radiated out from Krakatoa at 1,086 km/h (675 mph). The eruption is estimated to have reached 180 dB, loud enough to be heard 5,000 kilometres (3,100 mi) away. It was so powerful that it ruptured the eardrums of sailors on RMS Norham Castle of the Castle Line which was hove to off Sumatra, and caused a spike of more than 8.5 kilopascals (2.5 inHg) in the pressure gauge attached to a gasometer in the Batavia (correspondent to modern day Jakarta) gasworks 160 km (100 miles) away, sending it off the scale. At Batavia, the air waves burst windows and cracked walls.
The pressure wave was recorded on barographs worldwide. Several barographs recorded the wave seven times over five days: four times with the wave travelling away from the volcano to its antipodal point and three times travelling back to the volcano. Hence, the wave rounded the globe three and a half times.
Massive tsunamis struck the coastlines of the Sunda Strait, entirely submerging some islands. Some 80 km (50 mi) away, the low-lying Thousand Islands were submerged under 2 meters (6.6 feet) of seawater, forcing people to climb trees. The tsunamis produced by the third explosion were the largest, although the first two explosions did produce smaller tsunamis. The town of Merak was destroyed by a tsunami that was 46 metres high. The waves reached heights of up to 24 metres (79 feet) along the south coast of Sumatra and up to 42 metres (138 feet) along the west coast of Java. The tsunamis washed the land clean of vegetation and destroyed human settlements. The Loudon, which at the time was anchored near the village of Telok Betong, survived the waves. Passengers produced this eyewitness account:
A farm worker who was near Merak on Monday morning reports:
Away from the coasts of the Sunda Strait, the waves reached a height of 4 metres on Southern Sumatra. One person was killed in Sri Lanka. Ships as far away as South Africa rocked as tsunamis hit them, and the victims' bodies were found floating in the ocean for months after the event. The tsunamis which accompanied the eruption were believed to have been caused by gigantic pyroclastic flows entering the sea; each of the four great explosions was accompanied by large pyroclastic flows resulting from the gravitational collapse of the eruption columns. This caused several cubic kilometres of material to enter the sea, displacing an equal volume of seawater.
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, which owing to a wavelength of less than 7 km, failed to cause significant damage at large distance, and may have been caused by concussive air waves from the eruption. These air waves circled the globe several times and were still detectable on barographs five days later.
About 10% of the eruption fatalities were from hot pyroclastic flows and falling tephra. The pyroclastic flows and surges produced by the third explosion moved faster than 100 km/h (62 mph) and travelled over the sea up to 80 km (50 mi) from the source, affecting an area constrained to a minimum of 4,000 km (1,500 sq mi). Pyroclastic deposits thought to be from the flows have been found on Southeast Sumatra, and northwest of the volcano on the islands of Sebesi, Sebuku and Lagoendi, while on Southwest Sumatra the flows burnt victims. They travelled 40 km (25 mi) across the Sunda Strait. Once they hit Southern Sumatra, they incinerated entire villages and burned all vegetation. 2,000 of the corpses in Southern Sumatra appear to have been scorched to death, presumably by the pyroclastic flows. The Loudon and the W.H. Besse, at ~65 km north-northeast and ~80 km east-northeast of Krakatau respectively, were hit by strong winds and tephra. They were farther away than the scorched victims of the hot flows in Sumatra, so the ships and crew survived. The flows had 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.
An eyewitness enveloped by the outermost edges of the pyroclastic flow described her experience:
An estimated 20 cubic kilometres of tephra was deposited. Ash was propelled to an estimated height of 80 km (50 mi). It is estimated that as much as 18–21 km (4.3–5.0 cu mi) of ignimbrite were deposited over 1,100,000 km (420,000 sq mi). Around noon on 27 August 1883, a rain of hot ash fell around Ketimbang (now Katibung in Lampung Province) in Sumatra, known as 'The Burning Ashes of Ketimbang', killing approximately 1,000 people in Sumatra.
The combination of pyroclastic flows, volcanic ash, and tsunamis associated with the Krakatoa eruptions had disastrous regional consequences. Some land in Banten, approximately 80 km south, was never repopulated; it reverted to jungle and is now the Ujung Kulon National Park. Ash fell 2,500 km away. Huge fields of floating pumice were reported for months after the event. There are numerous 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.
An observation made two weeks after the eruption describes the state of the affected areas, where the village of Tjaringin once was.
The official reported death toll was 36,417. There were no survivors from the 3,000 people on the island of Sebesi.
In the aftermath of the eruption, it was found that Krakatoa had almost entirely disappeared, except for the southern third. Much of the Rakata cone had been sheared away, leaving behind a 250-metre (820 ft) cliff. 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.
The huge amount of material the volcano deposited drastically altered the ocean floor. The huge amount of ignimbrite deposits largely filled the 30–40 m (98–131 ft) deep basin around the mountain. The land masses of Verlaten and Lang islands were increased, as was the western part of the remnant of Rakata. Much of this gained material quickly eroded, but volcanic ash remains a significant part of the geological composition of these islands. The basin was 100 m (330 ft) deep before the eruption, and 200–300 m (660–980 ft) after.
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 had caused steam to rise, which some mistook for a continued eruption.
The eruption caused a volcanic winter. In the year following the eruption, average Northern Hemisphere summer temperatures fell by 0.4 °C (0.72 °F). The record rainfall that hit Southern California during the water year from July 1883 to June 1884 – Los Angeles received 970 millimetres (38.18 in) and San Diego 660 millimetres (25.97 in) – has been attributed to the Krakatoa eruption. There was no El Niño during that period as is usual when heavy rain occurs in Southern California, but many scientists doubt that there was a causal relationship.
The eruption injected a tremendous 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 clouds. The resulting increase in cloud reflectivity (or albedo) reflected more incoming light from the sun than usual and cooled the entire planet until the sulphur fell to the ground as acid precipitation.
The 1883 Krakatoa eruption darkened the sky worldwide for years afterwards and produced spectacular sunsets worldwide for many months. British artist William Ascroft made thousands of colour sketches of the red sunsets halfway around the world from Krakatoa in the years after the eruption. The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration". This eruption also produced a Bishop's Ring around the sun by day, and a volcanic purple light at twilight. In 2004, an astronomer proposed the idea that the red sky shown in Edvard Munch's 1893 painting The Scream is an accurate depiction of the sky over Norway after the eruption.
Weather watchers of the time tracked and mapped the effects on the sky. They labelled the phenomenon the "equatorial smoke stream". This was the first identification of what is known today as the jet stream. For several years following the eruption, it was reported that the moon appeared to be blue and sometimes green. This was because some ash clouds were filled with particles about 1 μm wide – the right size to strongly scatter red light while allowing other colours to pass. White moonbeams shining through the clouds emerged blue and sometimes green. People also saw lavender suns and, for the first time, recorded noctilucent clouds.
Although the violent phase of the 1883 eruption was over by the late afternoon of 27 August, after light returned by 29 August, reports continued for months that Krakatoa was still in eruption. Verbeek's committee's earliest duties were to determine if this was true and verify reports of other volcanoes erupting on Java and Sumatra. In general, these were found to be false. Small eruptions, mostly of mud, continued into October 1883. Verbeek discounted any claims of Krakatoa still erupting after mid-October 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 further activity were seen until 1913 when an eruption was reported. An investigation could find no evidence the volcano was awakening. 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).
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.
The fate of northern Krakatoa has been the subject of some dispute among geologists. It was initially proposed that the island had been blown apart by the force of the eruption. Most of the material deposited by the volcano is 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.
Based on the findings of contemporary investigators, the established hypotheses assume that part of the island subsided before the first explosions on the morning of 27 August. This forced the volcano's vents to be below sea level, causing:
Geological evidence does not support the assumption that only subsidence before the explosion was the cause. For instance, the pumice and ignimbrite deposits are not of a kind consistent with a magma-seawater interaction. These findings have led to other hypotheses:
A numerical model for a Krakatoa hydrovolcanic explosion and the resulting tsunami was described by Mader & Gittings, in 2006. A high wall of water is formed that is initially higher than 100 metres driven by the shocked water, basalt, and air.
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