1883 eruption

1883 eruption

While seismic activity around the volcano was intense in the years preceding the cataclysmic 1883 eruption, a series of lesser eruptions began on 20 May 1883. The volcano released huge plumes of steam and ash lasting until late August.

On 27 August a series of four huge explosions almost entirely destroyed the island. The explosions were so violent that they were heard 3,110 km (1,930 mi) away in Perth, Western Australia, and the island of Rodrigues near Mauritius, 4,800 km (3,000 mi) away.The pressure wave from the final explosion was recorded on barographs around 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;[23]:63 the wave rounded the globe three and a half times. Ash was propelled to a height of 80 km (260,000 ft). The sound of the eruption was so loud it was reported that if anyone was within 16 kilometres (10 mi), they would have gone deaf.

The combined effects of pyroclastic flows, volcanic ashes, and tsunamis had disastrous results in the region and worldwide. The death toll recorded by the Dutch authorities was 36,417, although some sources put the estimate at more than 120,000. 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. Average global temperatures fell by as much as 1.2 °C (2.2 °F) in the year following the eruption. Weather patterns continued to be chaotic for years and temperatures did not return to normal until 1888

The 1883 eruption of Krakatoa (Krakatau) in the Dutch East Indies (now Indonesia) began in the afternoon of Sunday, 26 August 1883 (with origins as early as May of that year), and peaked in the late morning of Monday, 27 August 1883 when over 70% of the island and its surrounding archipelago were destroyed as it collapsed into a caldera. Additional seismic activity was reported to have continued until February 1884, though reports of seismic activity after October 1883 were later dismissed by Rogier Verbeek's investigation into the eruption. The 1883 eruption was one of the deadliest and most destructive volcanic events in recorded history. At least 36,417 deaths are attributed to the eruption and the tsunamis it created. Significant additional effects were also felt around the world in the days and weeks after the volcano's eruption.

Early phase

In the years before the 1883 eruption, seismic activity around the Krakatoa 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 at Krakatoa started again around 16 June with loud explosions and a thick black cloud covering the islands for five days. On 24 June, a prevailing east wind cleared the cloud, and two ash columns could be seen issuing from Krakatoa. The seat of the eruption is believed to have been a new vent or vents that formed 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

On 11 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. 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.

Climactic phase

By August 25, the Krakatoa eruptions intensified. At about 13:00 (local time) on 26 August, the volcano went into its paroxysmal phase. By 14:00, a black cloud of ash could be seen 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. Between 18:00 and 19:00 hours, a small tsunami hit the shores of Java and Sumatra, some 40 km (25 mi) away.

On August 27, four enormous explosions occurred. At 5:30 am, the first explosion was at Perboewatan, triggering a tsunami heading straight to Telok Betong, now known as Bandar Lampung. At 6:44 am, Krakatoa exploded again at Danan, 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, where they were thought to be cannon fire from a nearby ship. The third explosion has been reported as the loudest sound heard in historic times.79 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 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.

Pressure wave

The pressure wave generated by the colossal explosion radiated out from Krakatoa at 1,086 km/h (675 mph). The eruption is estimated to have reached 310 dB, loud enough to be heard clearly 5,000 kilometres (3,100 mi) away. It was so powerful that it ruptured the eardrums of sailors 64 km (40 miles) away on ships in the Sunda Strait,[6]:235 and caused a spike of more than 8.5 kilopascals (2.5 inHg) in pressure gauges 160 km (100 miles) away, attached to gasometers in the Batavia gasworks, sending them off the scale.

The pressure wave 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.[3]:63 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. By then, less than 30% of the original island remained.

Effects

The combination of pyroclastic flows, volcanic ash, and tsunamis associated with the Krakatoa eruptions had disastrous regional consequences. Some land in Banten, approximately 90 km south, was never repopulated; it reverted to jungle and is now the Ujung Kulon National Park. The official death toll recorded by the Dutch authorities was 36,417.

"The Burning Ashes of Ketimbang"

Verbeek and others believe that the final major Krakatoa eruption was a lateral blast, or pyroclastic surge (similar to the catastrophic 1980 eruption of Mount St. Helens). 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 in Sumatra; there were no survivors from the 3,000 people located on the island of Sebesi. 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.

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 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 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.[note 2] 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.

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) 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 totally disappeared.

The huge amount of material deposited by the volcano drastically altered the surrounding ocean floor. It is estimated that as much as 18–21 km3 (4.3–5.0 cu mi) of ignimbrite were deposited over 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 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.

Global climate

In the year following the 1883 Krakatoa eruption, average Northern Hemisphere summer temperatures fell by as much as 1.2 °C (2.2 °F).[11] 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 that there was a causal relationship.

The Krakatoa 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) reflected more incoming light from the sun than usual, and cooled the entire planet until the suspended sulfur fell to the ground as acid precipitation.

Global optical effects

The 1883 Krakatoa 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 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 strongly 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, recorded noctilucent clouds.

Possible causes

The fate of northern Krakatoa 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 hypotheses – based on the findings of contemporary investigators – assume that part of the island subsided before the first explosions on the morning of August 27. This forced the volcano's vents to be 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 that was 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, opening a pathway for seawater to enter the magma chamber and setting the stage for a magma-seawater interaction.

the final explosions may have been caused by magma mixing: 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.

See also

• Krakatoa documentary and historical materials
• List of natural disasters by death toll
• List of volcanic eruptions by death toll
• List of volcanoes in Indonesia
• Phreatic eruption
• Volcanic Explosivity Index
• Fair Wind to Java
• The Devil at 4 O'Clock
• Krakatoa, East of Java

Adapted from wiipedia