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. 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. 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 on the island of Sebesi. 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
Tsunamis and distant effects
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.[dubious – discuss] 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.[citation needed] This caused several cubic kilometres of material to enter the sea, displacing an equal volume of seawater. The town of Merak was destroyed by a tsunami that was 46 metres high. Some of the pyroclastic flows reached the Sumatran coast as much as 40 km (25 mi) away, having 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.[14]
Smaller waves were recorded on tidal gauges as far away as the English Channel.[15] 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 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. 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, 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.
Global climate
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 sulphur 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 sulphuric 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
Global optical effects
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
Possible causes
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:
major 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.
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:
an 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. 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.
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.
Resource: wikipedia.org
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