Between 19 May and 21 October 1883, the volcanic island of Krakatoa (located in the Sunda Strait, then part of the Dutch East Indies—modern-day Indonesia) began erupting, lasting more than five months. On 27 August, the island had its most significant eruption, which destroyed over 70% of the island and its surrounding archipelago, the island collapsing into a caldera. The eruption of 27 August had an estimated volcanic explosivity index of 6, and is one of the deadliest and most destructive volcanic events in recorded history; the third explosion of that day, that occurred at 10:02 a.m., remains the loudest-known sound in history.
The explosion was heard away in Perth, Western Australia, and Rodrigues near Mauritius, 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.
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 the Australian Northern Territory, 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 , and explosions could be heard in Batavia (Jakarta) away, rattling windows and doors. Dust fell 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 five to ten 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 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 Krakatau. 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 Krakatau 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 thick and the destruction of all vegetation, leaving only tree stumps. He advised against any further landings.
Climactic phase By 25 August, the Krakatau eruptions intensified. At about 1:00 p.m. on 26 August, the eruption entered its climactic phase. By 2:00 p.m., a black ash cloud could be seen high. At this point, the eruption was almost continuous, and explosions could be heard every ten minutes. By 5:00 p.m., the explosions were audible all over Java. Waves approximately a metre high, which may have been produced by steam explosions, began battering the coasts of Java and Sumatra at 5:30 p.m., and continued throughout the night. Ships within of the volcano reported heavy ash fall, with pieces of hot pumice up to in diameter landing on their decks. Between 7:00 and 8:00 p.m., a small tsunami hit the shores of Java and Sumatra, away.
Explosions On 27 August, a series of eleven enormous explosions occurred, which marked the climax of the eruption. At 5:30 a.m., the first explosion was at Perboewatan, triggering a tsunami heading to Telok Betong, now known as Bandar Lampung. At 6:44 a.m., Krakatau exploded again at Danan, with the resulting tsunami propagating eastward and westward. The third and largest explosion, at 10:02 a.m. (some sources say 9:58 a.m.), was so powerful that it was heard away in Perth, Western Australia, and the Indian Ocean island of Rodrigues near Mauritius, away, where the blast was thought to have been cannon fire from a nearby ship. The third explosion was the loudest sound that has ever been measured. The loudness of the blast heard from the volcano has been calculated to have been 180 dBSPL (180dB re 20μPa, or 20kPa).
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.
Pressure wave Some islands in the western Indian Ocean, approximately away, still heard it at nearly the same level as a gun blast. Due to the limitations of the speed of sound, it is likely that people on these far away islands did not hear Krakatau for nearly four hours after its eruption. The pressure wave generated by the colossal third explosion radiated out from Krakatau at . The eruption is estimated to have reached 180 dBSPL (180dB re 20μPa, or 20kPa) at 100 miles from the source - possibly 310 dB at the explosion site itself - loud enough to be heard away. It was so powerful that the sound it produced ruptured the eardrums of sailors on RMS Norham Castle of the Castle Line, which was hove to off Sumatra roughly away, and caused a spike of more than in the pressure gauge attached to a gasometer in the Batavia (correspondent to modern day Jakarta) gasworks 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.
Tsunamis
Massive tsunamis struck the coastlines of the Sunda Strait, entirely submerging some islands. Some away, the low-lying Thousand Islands were submerged under 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 high.A documentary film showed tests made by a research team at Kiel University, Germany, of pyroclastic flows moving over the water. See The tests revealed that hot ash travelled over the water on a cloud of superheated steam, continuing to be a pyroclastic flow after crossing water; the heavy matter precipitated out of the flow shortly after initial contact with the water, creating a tsunami due to the precipitate mass. The waves reached heights of up to along the south coast of Sumatra and up to along the west coast of Java.Bryant, Edward. Tsunami: The Underrated Hazard, Springer: New York, 2014, , pp. 162–163. 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 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 , 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
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. Two thousand 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. The flows had moved across the water on a cushion of superheated steam. There are also indications of submarine pyroclastic flows reaching from the volcano.
An eyewitness enveloped by the outermost edges of the pyroclastic flow described her experience:
An estimated of tephra was deposited. Ash was propelled to an estimated height of . It is estimated that as much as of ignimbrite were deposited over .