Early in the morning of May 20, 1883,
the captain of the German warship Elizabeth reported seeing
an ~11-km-high cloud of ash and dust rising above the uninhabited
island of Krakatau, thus documenting the first eruption from this
Indonesian island in at least two centuries. Over the ensuing two
months, crews on commercial vessels and sightseers on charted
ships would experience similar spectacles, all of which were associated
with explosive noises and churning clouds of black to incandescent
ash and pumice. From a distance, the largest of these natural
fanfares impressed the local inhabitants on the coastal plains
of Java and Sumatra, creating a near-festive environment. Little
did they realize, however, that these awe-inspiring displays were
only a prelude to one of the largest eruptions in historic times.
A series of cataclysmic explosions began at mid-day on August
26, and ended on August 27 with a stupendous paroxysmal eruption.
On this day, the northern two-thirds of the island collapsed beneath
the sea, generating a series of devasting pyroclastic flows and
immense tsunamis that ravaged adjacent coastlines. The events
that began on August 26 would mark the last 24 hours on earth
for over 36,000 people, and the destruction of hundreds of coastal
villages and towns.
Indonesia
contains over 130 active volcanoes, more than any other country
on earth. They comprise the axis of the Indonesian island arc
system, which is generated by northeastward subduction of the Indo-Australian
plate. The great majority of these volcanoes lie along the topographic
crest of the arc's two largest islands - Java and Sumatra. The
islands are separated by the Sunda Straits, which
is located at a distinct bend in axis of the island arc volcanoes,
from a nearly east-west orientation in Java to a northwest-southeast
orientation in Sumatra. Krakatau is one of a several volcanic
islands in the Sunda Straits located above an active north-northeast
trending fault zone, an orientation quite distinct from the main
island-arc trend. Although relatively small when compared to the
largest volcanoes along the island arc, Krakatau and its associated
volcanoes have shown the capacity to generate highly explosive
eruptions.
The pre-eruption island of Kratatau was composed of three coalsced volcanoes aligned along a north-northwest direction, parallel to the volcanoes on Sumatra. From north to south, these were Perboewatan, Danan, and Rakata.
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During the eruption, Perboewatan, Danan, and the northern half of Rakata appear to have collapsed into the vacating magma chamber, thus forming a submarine caldera and destroying the northern two-thirds of the island. Some workers, however, have argued for an alternative to the caldron-collapse model, and instead have suggested that the caldera formed by explosive destruction of the island, and the reaming out of the pre-eruption edifice. Eruptions since 1927 have built a new cone called Anak Krakatau ("child of Krakatau") in the center of the 1883 caldera.
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.
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." |
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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 |
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:
| "Suddenly, it became pitch dark. The last thing I saw was the ash being pushed up through the cracks in the floorboards, like a fountain. I turned to my husband and heard him say in dispair ' Where is the knife?' . . . I will cut all our wrists and then we shall be released from our suffering sooner.' The knife could not be found. I felt a heavy pressure, throwing me to the ground. Then it seemed as if all the air was being sucked away and I could not breathe. . . . I felt people rolling over me . . . No sound came from my husband or children . . . I remember thinking, I want to . . . go outside . . . . but I could not straighten my back . . . I tottered, doubled up, to the door . . . I forced myself through the opening . . . I tripped and fell. I realized the ash was hot and I tried to protect my face with my hands. The hot bite of the pumice pricked like needles . . . Without thinking, I walked hopefully forward. Had I been in my right mind, I would have understood what a dangerous thing it was to . . . plunge into the hellish darkness . . . I ran up against . . . branches and did not even think of avoiding them. I entangled myself more and more . . . My hair got caught up . . . I noticed for the first time that [my] skin was hanging off everywhere, thick and moist from the ash stuck to it. Thinking it must be dirty, I wanted to pull bits of skin off, but that was still more painful . . . I did not know I had been burnt." -- From A. Scarth, 1999. |
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).
On December 29, 1927, a group of Javanese fisherman who were startled by steam and debris belching from the sea above the collapsed caldera, thus marking the reawakening of Krakatau after 44 years of calm. The activity continued, and on January 26, 1928 the rim of a basaltic scoria cone first appeared above sealevel. A year later, it had grown into a small island which was quickly dubbed Anak ("Child of") Krakatau.
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Anak Krakatau has erupted in most years since. Typically, these are rather mild strombolian to vulcanian eruptions of basaltic andesite lava flows and associated tephra deposits. Although they present little danger to surrounding islands, the eruptions from Anak Krakatau provide a constant reminder of the horror of 1883 - Lest we forget.