Eruption of Krakatao

Krakatoa eruption lithograph.jpg1883 Eruption of Krakatoa

The 1883 eruption of Krakatoa in the Dutch East Indies (now Indonesia) began in the afternoon of August 26, 1883, and culminated with several destructive eruptions of the remaining caldera. On August 27, two-thirds of Krakatoa collapsed in a chain of titanic explosions, destroying most of the island and its surrounding archipelago.

It was one of the deadliest and most destructive volcanic events in recorded history, with at least 36,417 deaths being attributed to the eruption itself and the tsunamis it created. Significant additional effects were also felt around the world.

Early phase

In the years before the 1883 eruption, seismic activity around the volcano was intense, with earthquakes felt as far away as Australia. Beginning 20 May 1883, 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 New Batavia (Jakarta) 160 km (99 mi) away. Activity died down by the end of May, and there was no further recorded activity for several weeks.

Eruptions started again around 16 June, featuring loud explosions and covering the islands with a thick black cloud for five days. On 24 June, a prevailing east wind cleared the cloud, and two ash columns were seen issuing from Krakatoa. The seat of the eruption is believed to have been a new vent or vents which formed between Perboewatan and Danan. Earthquake shocks began to be felt at Anyer, Banten, and ships began to report large pumice masses appearing in the Indian Ocean to the west.

On 11 August, a Dutch topographical engineer, Captain 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, and steam plumes from at least eleven other vents, mostly between Danan and Rakata. 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. The next day, a ship passing to the north reported a new vent “only a few meters above sea level.” Activity continued through mid-August.

Climactic phase

At about 13:00 local time on 26 August, the volcano went into its paroxysmal phase. 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 (4 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 5:30 am, the first explosion was at Perboewatan volcano, triggering a tsunami heading straight to Telok Betong, now known as Bandar Lampung. At 6:44 am, Krakatoa exploded again on Danan volcano, with the resulting tsunami stretching eastward and westward. The largest explosion, at 10:02 am, was so violent 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.

Each explosion was accompanied by large tsunamis, which are believed to have been over 30 meters (98 feet) 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 energy released from the explosion has been estimated to be equal to about 200 megatons of TNT, roughly four times as powerful as the Tsar Bomba, the most powerful thermonuclear weapon ever detonated. At 10:41 am, a landslide tore off half of Rakata volcano, causing the final explosion.

Final explosive eruption

The pressure wave generated by the colossal fourth and final explosion radiated out from Krakatoa at 1,086 km/h (675 mph). It was so powerful that it ruptured the eardrums of sailors 64 km (40 miles) away on ships in the Sunda Strait. The pressure wave radiated across the globe and was recorded on barographs all over the world. Several barographs recorded the wave seven times over the course of 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.

Ash was propelled to an estimated height of 80 km (50 mi). The eruptions diminished rapidly after that point, and by the morning of 28 August, Krakatoa was silent. Small eruptions, mostly of mud, continued into October 1883.

“The Burning Ashes of Ketimbang”

Around noon on 27 August 1883, a rain of hot ash fell around Ketimbang (now Katibung in Lampung Province) in Sumatra. Approximately 1,000 people were killed, the only large number of victims killed by Krakatoa itself, and not by 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 ash fall ended to the northeast of Ketimbang, where the bulk of Sebesi Island offered protection from any horizontal surges.

Effects

The combination of pyroclastic flows, volcanic ash, and tsunamis had disastrous results in the region. There were no survivors from the 3,000 people located on 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 48 km (30 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 (Bandar Lampung), and Sirik and Serang in Java. The areas of Banten on Java and 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.

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 massive pyroclastic flows resulting from the gravitational collapse of the eruption columns. 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.

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 on barographs five days later.

Map krakatau.gif

Geographic effects

In the aftermath of the eruption, it was found that the island of Krakatoa had almost entirely disappeared, except for the southern third. The Rakata cone was 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 totally 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 (98–131 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 away, but volcanic ash continues to be a significant part of the geological composition of these islands.

Two nearby sandbanks (called Steers and Calmeyer) 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.

Global climate

In the year following the eruption, average Northern Hemisphere summer 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 record rainfall that hit Southern California during the “water year” from July 1883 to June 1884 – Los Angeles received 38.18 inches (969.8 mm) and San Diego 25.97 inches (659.6 mm) – has been attributed to the Krakatoa eruption. There was no El Niño during that period as is normal when heavy rain occurs in Southern California, but many scientists doubt this proposed causal relationship.

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 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 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 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.

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.

For several years following the eruption, it was reported that the moon appeared to be blue and sometimes green. This was because some of the ash clouds were filled with particles about 1 µm wide—the right size to scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green. People also saw lavender suns and, for the first time, noctilucent clouds.

Possible causes

The fate of northern 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 established theories – based on the findings of contemporary investigators – assume that part of the island subsided before the first explosions on the morning of 27 August. This forced the volcano’s vents below sea level, causing:

  • massive flooding which created a series of phreatic explosions (interaction of ground water and magma).
  • seawater to cool the magma enough for it to crust over and produce a “pressure cooker” effect relieved only when explosive pressures were reached.

However, there is geological evidence which 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 massive underwater land slump or partial subsidence suddenly exposed the highly pressurized magma chamber to seawater.
  • 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 five per cent 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 27 August, after light returned by the 29th, reports continued for months that Krakatoa was still in eruption. Verbeek’s committee 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 further activity were seen until 1913, when an eruption was reported.

Source: https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa

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