THE 1883 ERUPTION
The other two islands, Sertung and Panjang, were enlarged considerably (Sertung doubled in size) by the glowing ash and pumice which smothered them to a depth of 30 meters. On Rakata, the south and west coasts were extended almost a kilometer seawards and the ash layer reached a thickness of 60 m in some areas, although probably much shallower on the steeper slopes. Weeks after the explosion, rain water turned into steam as it trickled into crevices and a even month later the surface was too hot for bare feet. It is believed that all life, plant and animal, was destroyed on the islands. Yet the three islands are now covered in forest, and over 200 species of higher plants and 36 species of land birds have been found on Rakata in the 1980s.
The Cataclysmic Events Of August 26-27 1883
After reawakening on May 20, 1883, Krakatau generated mild detonations from Perboewatan throughout May and June. By mid-June the summit crater of Perboewatan had been largely destroyed and the cite of eruption widened to include several new vents near Danan. By mid-July, banks of pumice were common features found floating in the Sunda Straits. However, some of the earliest tephra was basaltic, indicating that recharge of basalt magmas into the magma chamber beneath Krakatau may well have played a role in the intiation of these early eruptions.
Sunday, August 26. At 12:53 p.m., Krakatau delivered the opening salvo to a climactic eruption that would last throughout the evening of August 27. The initial blast generated an ear-shattering fusillade accompanied by a black churning cloud of volcanic debris that rose quickly to 25 km above the island. Over the next several hours, it would widen dramatically to the northeast, rising to a height of at least 36 km. The intensity of the eruptions increased throughout Sunday, frightening the coastal communities of western Sumatra, western Java, and adjacent islands. Later in the day, these villages would be battered by a series of devasting tsunamis generated by pyroclastic flows plunging into the sea. The worst was yet to come.
Monday, August 27. This frightening display of volcanic power would culminate in a series of at least four stupendous eruptions that began at 5:30 a.m., climaxing in a colossal blast that literally blew Krakatau apart. The noise was heard over 4600 km away, throughout the Indian Ocean, from Rodriguez Island and Sri Lanka in the west, to Australia in the east. Two-thirds of the island collapsed beneath the sea into the underlying, partially vacated magma chamber. About 23 square kilometers of the island, including all of Perboewatan and Danan, subsided into a caldera about 6 km across. At an original height of 450 m, Danan had collapsed to depth of 250 m below sealevel.
Enormous Sea Waves
The cataclysmic blasts of August 27 generated mountainous tsunamis, up to 40 m tall, that ravaged coastlines across the Sunda Straits. Many of the closest islands were completely submerged. After first being overwhelmed by massive pyroclastic flows (see below), Sebesi Island northeast of Krakatau, was innudated by mammoth sea waves. These tsunami stripped away all vegetation, washed ~3000 people out to sea, and destroyed all signs of human occupation. Although located at seemingly safe distance, 80 km east of the Sunda Straits, the low-lying Thousand Islands were buried by at least 2 m of seawater and their inhabitants had to save themselves by climbing trees.
Eyewitness accounts of the massive waves came from passengers of the Loudon, who survived the barrage only through the heroic efforts of its Captain Lindemann. The ship was anchored in Lampong Bay, near the village of Telok Betong when the first of several waves arrived on Monday morning:
"Suddenly we saw a gigantic wave of prodigious height advancing toward the seashore with considerable speed. Immediately, the crew . . .managed to set sail in face of the imminent danger; the ship had just enough time to meet with the wave from the front. The ship met the wave head on and the Loudon was lifted up with a dizzying rapidity and made a formidable leap... The ship rode at a high angle over the crest of the wave and down the other side. The wave continued on its journey toward land, and the benumbed crew watched as the sea in a single sweeping motion consumed the town. There, where an instant before had lain the town of Telok Betong, nothing remained but the open sea."
Other ships in Lampong Bay were not as lucky. The wave lifted the steamship Berouw up the Koeripan River valley, depositing the ship over a mile inland, thirty feet above sealevel, killing all 28 of its crew members.
One of the most harrowing accounts was that of a Javanese field hand working in paddy fields located 8 km inland on Java, near the town of Merak. The following is his account of events at ~10:30 a.m., Monday morning:
" . . .all of a sudden there came a great noise. We . . .saw a great black thing, a long way off, coming towards us. It was very high and very strong, and we soon saw that it was water. Trees and houses were washed away . . .The people began to . . . run for their lives. Not far off was some steep sloping ground. We all ran towards it and tried to climb up out of the way of the water. The wave was too quick for most of them, and many were drowned almost at my side. . . . There was a general rush to climb up in one particular place. This caused a great block, and many of them got wedged together and could not move. Then they struggled and fought, screaming and crying out all the time. Those below tried to make those above them move on again by biting their heels. A great struggle took place for a few moments, but . . . one after another, they were washed down and carried far away by the rushing waters. You can see the marks on the hill side where the fight for life took place. Some . . . dragged others down with them. They would not let go their hold, nor could those above them release themselves from this death-grip." -- From A. Scarth, 1999
Nobody knows how many people were washed out to sea by these enormous waves. For months after the eruption, the Sunda Straits where congested with thick pumice banks, often containing fifty or more corpses. The official number of dead was calculated by Dutch authorities at 36,417, 90 percent of which were killed by the tsunamis. Two weeks after the disaster, one traveler describes his observations where the village of Tjaringin once stood.
"Thousands of corpses of human beings and also carcasses of animals still await burial, and make their presence apparent by the indescribable stench. They lie in knots and entangled masses impossible to unravel, and often jammed along with coconut stems among all that had served these thousands as dwellings, furniture, farming implements, and adornments for houses and compounds." -- From Zeilinga de Boer and Sanders, 2002
Pyroclastic Airfall And Devasting Pyroclastic Flows
Tsunamis were clearly responsible for most of the fatalities at Krakatau. However, ~4,500 deaths (over 10% of the total) have been attributed to falling tephra and hot pyroclastic flows. The amount of tephra generated is thought to be about 20 cubic kilometers, or twenty times that of the destructive Mt. St. Helens eruption in 1980. Near Sumatra, the Sunda Straits were clogged with so much debris that it looked like solid ground. Relief ships were unable to reach coastal communities like Telok Betong for weeks. Over the ensuing months, storms and high-tides would disperse thick banks of floating pumice beyond the Straits, into the Java Sea and Indian Ocean. Ships thousands of kilometers from Krakatau would report huge fields of this floating debris for months after the eruption. One such accumulation floated 8,170 km, until it reached Durban, South Aftica in September, 1884.
About 2000 of the corpses in southern Sumatra had severe burns, indicating that they had been scorched to death, peresumably from pyroclastic flows. Although the behavior of pyroclastic flows and surges over water is poorly contrained by direct observations, the evidence suggests that they can travel great distances over open water. One compeling feature of the Krakatau eruption is that the pyroclastic flows appear to have travelled an incredible 40 km across the Sunda Straits, where they remained hot enough to cause the burn-related fatalities on Sumatra. These same flows, however, were also recorded by several ships located at greater distances. On August 27, the Louden (see above) was located ~65 km north-northeast of Krakatau when it was struck by severe winds and tephra, and the W.H. Besse was located at ~80 km east-northeast of Krakatau when it was hit by hurricane-force winds, heavy tephra, and the strong smell of sulfur. At these greater distances, the pyroclastic flows were at lower temperatures so that the ships and crew survived.
How is it possible for pyroclastic flows to travel such great distances? Pyroclastic flows are hot mixtures of solid particles and expanding volcanic gases. While advancing over water, the base of the flow will conert the water to steam. The rapid expansion of water to vapor greatly enhances flow fluidization and inhibits the deposition of particles, particularly the low-density pumiceous particles, thus allowing the flow to travel tens of kilometers over flat oceanic waters. This mobility was first recognized during the 1902 eruption of a pyroclastic flow from Mt. Pelée, which destroyed the coastal city of St. Pierre, only to continue across open waters to overturn and burn ships anchored several kilometers offshore.
After travelling 40 km over the Sunda Straits, pyroclastic flows struck southern Sumatra with a vengence, remaining hot enough to incinerate entire villages and burn all vegetation before loosing impetus on the highly forested mountainsides. The wife of Controller Beyerinck from the Sumatra village of Ketimbang described her expereince on the morning of August 27, when the outermost edges of a pyroclastic flow enveloped her family and their acquantainces, killing some and sparing others:
Tephra from the eruption fell as far as 2,500 km downwind in the days following the eruption. However, the finest fragments were propelled high into the stratosphere, spreading outward as a broad cloud across the entire equatorial belt in only two weeks. These particles would remain suspended in the atmosphere for years, propogating farther to the north and south before finally dissipating.
The stratospheric cloud of dust also contained large volumes of sulfur dioxide gas emitted from Krakatau. These gas molecules rapidly combined with water vapor to generate sulfuric acid droplets in the high atmosphere. The resulting veil of acidic areosols and volcanic dust provided an atmospheric shield capable of reflected enough sunlight to cause global temperatures to drop by several degrees. This aerosol-rich veil also generated spectacular optical effects over 70% of the earth's surface. For several years after the 1883 eruption, the earth experienced exotic colors in the sky, halos around the sun and moon, and a spectacular array of anomalous sunsets and sunrises. Artists were fascinated by these aerial displays and captured them on canvas. The painting shown here is one such sunset captured by the artitst William Ascroft on the banks of the River Thames in London, on November 26, 1883 (Courtesy of Peter Francis).