1883 eruption of Krakatoa

Early phase In the years before the 1883 eruption, seismic activity around the Krakatau volcano was intense, with earthquakes felt as far away as the Australian Northern Territory, one of which, in 1880, damaged a lighthouse. Although heard at very great distances, the sounds of the largest explosions were much less noticeable, or not heard at all in close proximity to the volcano. The reasons for this are complex and still debated by volcanologists today but a possible explanation is that the sound waves may have been projected upwards to the stratosphere and reflected away from the area, in part by ash suspended in the air. Although often directly linked in popular accounts, the explosions and the tsunamis were probably caused by distinct mechanisms. The explosions may have emanated from huge pulses of gas-laden magma, whilst the waves followed later when the ejecta slumped back down into the surrounding sea. The worst of the many tsunamis that resulted were estimated to have been over 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 Krakatau eruption was a lateral blast, or pyroclastic surge. Material shot out of the volcano at . The energy released from the explosion has been estimated to be equal to about , 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 a.m., a landslide tore off half of Rakata's cone, along with the remainder of the island to the north of Rakata. It is still debated whether this event caused the loudest of the explosions but, either way, the collapse of the island into the magma chamber heralded the end of the paroxysmal phase. 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 high. The waves reached heights of up to along the south coast of Sumatra and up to along the west coast of Java. The tsunamis washed the land clean of vegetation and destroyed human settlements. These occurred too soon to be remnants of the initial tsunamis, which owing to a wavelength of less than , 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. Pyroclastic flows and tephra pumice from the late August 1883 eruption of Krakatoa. This rock floated across the Indian Ocean for almost a year before it washed ashore at Takwa Beach, Kenya, East Africa. About 10% of the eruption fatalities were from pyroclastic flows. The pyroclastic flows and surges produced by the third explosion moved faster than and travelled over the sea up to from the source, affecting an area constrained to a minimum of . 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 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, around north-northeast and 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. An eyewitness enveloped by the outermost edges of the pyroclastic flow described her experience: An estimated of tephra was deposited. ==Aftermath==

The combination of pyroclastic flows, volcanic ash, tsunamis, and earthquakes associated with the Krakatoa eruptions had disastrous regional and global consequences, with the main one being caused by the massive redistribution of mass due to the earthquakes that shifted Earth's figure axis by roughly 17 cm. On the regional scale the effects were limited to some land in Banten, approximately 80 km south, never being repopulated and reverting to jungle, which is now the Ujung Kulon National Park. Ash fell away. Huge fields of floating pumice were reported for months after the event. There were no survivors from the 3,000 people on the island of Sebesi. 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 . The record rainfall that hit Southern California during the water year from July 1883 to June 1884 – Los Angeles received and San Diego – 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 sulfur fell to the ground as acid rain. 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. 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 Krakatoa. ==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. ==In popular culture==

• The explosion has been theorized to be a source of inspiration for Edvard Munch's 1893 painting The Scream. The reddish sky in the background is the artist's memory of the effects of the powerful volcanic eruption of Krakatoa, which deeply tinted sunset skies red in parts of the Western hemisphere for months during 1883 and 1884, about a decade before Munch painted The Scream. • The explosion plays a pivotal role in Don Rosa's comic The Cowboy Captain of the Cutty Sark, where Scrooge McDuck witnesses the explosion and turns it to his advantage. • In The Dresden Files, the explosion is revealed to have been caused by Ebenezar McCoy in his role as 'the Blackstaff', the White Council's assassin, along with a number of other events, including the Tunguska event. ==See also==