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ATMOSPHERIC EFFECTS

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.

stratospheric cloud of dustThe 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).

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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:

"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.

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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."

steamship Berouw
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

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THE CATACLYSMIC EVENTS OF AUGUST 26-27

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.

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The Beast Awakens
A German site on Krakatoa
A sketch of Krakatoa, most likely Rakata, months before the start of the eruption in May of 1883.

On May 20, 1883, a plume of smoke was seen rising over 10km above the island of Krakatoa. During the following summer months military and commercial vessels would also report seeing the cloud. By August, ash and pumice bars were found everywhere in the Sundra Straits. The minor eruptions on Krakatoa from May-August were from Perboewetan volcano and by the time of the main eruption Perboewetan had nearly been completely destroyed widening the caldera beneath the islands and building more pressure.

Opening Salvo, August 26, 1833

At around 1:00pm in the afternoon, Krakatoa delivered the first in a series of blasts that would continue throughout the day until the climactic eruption on August 27th. This opening blast generated and defining shockwave that alerted the nearby coastal villages on the islands of Java and Sumatra. A column of black volcanic gas rapidly ascended to a height of over 25km above Krakatoa. In the coming hours as blasts intensified it would reach at least 35km. Later in the day the coastlines were hit with a series of tsunamis generated by the ocean impact of pyroclastic flows coming off the flanks of the island. Nothing of this would compare to what the world would witness the following day.

The World Explodes, August 27, 1883

At 5:30am the first of four cataclysmic explosions began on the island of Krakatoa. Tsunamis lashed out from the island pounding nearby coastlines and ash and pumice fell in droves on surrounding islands. At 6:44am a second massive blast came from Krakatoa unleashing similar effects. Finally at 10:02am the colossal blast took place that blew the island apart. Perboewetan and Danan erupted and fell into the emptying caldera 250m below sea level. Adding to the empty chamber was Rakata as half of the erupting volcano slid into the ocean displacing large volumes of and generating a massive tsunami. In total, 23 square kilometers of the island fell into the 6km wide caldera. The ground shook in the wake of the blast which was heard over 4500km and was estimated to be equal to the detonation of over 21,000 atomic bombs.

The Horizon Vanishes: Tsunamis

"...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" - From A. Scarth, 1999

Upon the third and catastrophic explosion of Krakatoa, enormous tsunamis were generated by the displaced water as the island collapsed into the caldera. These waves moved with great speed across the Sundra Straits reaching a height of around 40m tall before slamming into the nearby coastlines. Smaller tsunamis had pounded the local villages in the previous days eruptions but nothing compared to this mammoth wave. Many small coastal islands were completely submerged and as the wave hit the mainland islands of Java and Sumatra it ravaged towns and villages while stripping away nearly all vegetation. In some cases, whole towns of several thousand people were washed away in a flash destroying and sign they had ever been there. Accounts exist of villagers scrambling up inland hills to escape the waves. Often only the small top of a hill would be spared by the enormous current leaving former neighbors in great struggle with one another, while trying to maintain a safe position from the waves.

Many ships in the Sunrda Straits at the time of the eruption met a similar fate as the villagers on the coasts. Many were caught unaware in the torrent and thrown across the sea. Once such ship, the Berouw(seen right), was carried over a mile inland and deposited on a position 10m above sea level. However, some ships were fortunate enough to be maneuvered head-first into the swells only receiving minor injury onboard.

According to Dutch authorities, 90 percent of the over 36,000 deaths were attributed to the devastating tsunamis. Bodies lined the coasts for weeks and thousands of livestock were found throughout the Sundra Straits.

Fire of the Mountain: The Pyroclastic Flows
A German site on Krakatoa
The smoldering remains of the still erupting Rakata, on Krakatoa, after the main eruption destroyed the island.

When Krakatoa exploded hot ash and tephra were sent sprawling down the flanks of the volcano and into the Sundra Straits. These pyroclastic flows, essentially avalanches of fire and rock, stormed across the straits to distances of up to 40km engulfing passing ships and coastal villages. The flows traveled at speeds of over 100km/hr leaving little time for people to evacuate from the advancing surge. The flows were able to move quickly and a great distance for two reasons. These pyroclastic flows may have reached temperatures of over 700 degrees Celsius, which overland would scorch anything in its path. However, in the open ocean the pyroclastic flow will flash boil the water as it comes in contact with it allowing the entire surge to ride on a cushion of air. This lack of friction with water or land, often compared to and air hockey table, allows the flow to move with great speed and for a long time. It was the southern coast of Sumatra that was hit hardest by the flows. Of the over 36,000 deaths, around 4500 are contributed to the deadly pyroclastic flows which would have arrived just after the tsunamis. Most likely the 4500 who met their fate with the flows had reached high ground or shelter to avoid the rushing water only to be engulfed by fire and ash.

The Skies Change

In the weeks following the eruption, fine fragments of tephra and dust that were propelled kilometers into the stratosphere began to make a ring around the equator. They would remain suspended there for years causing remarkable solar effects and atmospheric hazing as they bent the incoming light. Also the enormous volumes of sulfur dioxide gas molecules that were ejected into the atmosphere combined with water to make sulfuric acid. These acidic aerosols sufficiently blocked enough sunlight to drop the Earth's temperature by several degrees for a few years. There presence in the atmosphere also created spectacular effects over 70% of the Earth's surface. Effects such as halos around the sun and moon, and amazing sunsets and sunrises were seen. For years these particles would remain suspended in the atmosphere being the final reminder of the massive and fatal blast that occurred in Sundra Straits. At least for a time...

...And Into This World(Anak Krakatoa: The child of Krakatoa)

On December 29, 1927 debris began spouting out of the ocean surface above the collapsed caldera of Krakatoa. The debris continued to spout till finally on January 26, 1928 the rim of a new volcanic cone emerged from below sea level. Indeed, after 44 years of silence Krakatoa began erupting anew giving birth to Anak Krakatoa, the child of Krakatoa, which has been erupting steadily ever since. To this day the many eruptions of Anak Krakatoa have been mild and expanded the island to base of 2km and over 200m above sea level. Today you may go to the Sundra Straits and step foot on young volcano's rim. And perhaps there as you look across the sea to the coasts of Java and Sumatra you may be able to feel, ever so slightly, the power of the awesome events that took place there just over a century ago.