Caldera
Krakatau (Krakatoa) Volcano lies in Sunda Strait between Sumatra and Java Islands. It belongs to Lampung Province. Krakatau Volcanic Complex consists of Rakata, Sertung, Panjang and Anak Krakatau Islands. Anak Krakatau is the currently active centre at the volcano and emerged from the sea in 1930.
Warning: climbing Anak Krakatau is currently very dangerous and should not be attempted.
Krakatau became famous after the paroxysmal eruption on August 27, 1883. The volcano erupted more than 18 cubic km of ash to a height of 80 km, and produced a tsunami as high as 30 m along the west coast of Banten and south coast of Lampung. The tsunami hit 295 villages and killed over 36,000 people.
2010 Eruptions
On 29th October 2010 there were 136 eruptions at the volcano, compared to a recent average of 10 per day. Krakatau volcano was raised to level 2 alert.
2009 Eruptions
November 2009
Alert level at Krakatau volcano in Indonesia has been lowered from Siaga (Level III) to Waspada (Level II). Explosion earthquakes measured over the past few months were August 4311, September 541, and October 34. There has been a significant decline in eruptive activity at the volcano since August 2009. People are still advised to avoid landing on Anak Krakatau. Communities in the coastal areas of Banten and Lampung have been warned not to believe runours that the eruption Anak Krakatau will cause a tsunami. Fishing boats are permitted around Anak Krakatau.
May 2009
Eruptions are continuing at Krakatau volcano in Indonesia from a crater on the SW side of the cone in May 2009. A volcanic ash advisory from Darwin VAAC reported ash to 10,000 drifting NW of the volcano. On 6th May 2009 the level of alert at Krakatau was raised from Waspada (Level 2) to Siaga (Level 3), out of a maximum level 4. A renewed period of eruptions began at Krakatau on 19th March 2009, when 19 explosions were recorded. Between 1-25 April 2009 there have been 4060 explosions. Number of explosion earthquakes measured are 30 April 229, 1 May 324, 2 May 318, 3 May 250, 4 May 403, 5 May 371, and 6 May 132 (until midday). Shallow volcanic earthquakes, and tremor have been recorded. Visual observations from Rajabasa District in Sumatra in April reported ash emissions from 50-1000 m above the summit. From 1-24 April booming noises were heard from Sumatra 174 times, 83 times from 25-29 April. No sounds were heard from Krakatau between 30th April and 6th May. Incandescent material and ash are being emitted to a radius of 500 m from the crater. Pyroclastic flows may reach a distance of 700 m from the crater. Ash is falling 5 km from the volcano. Requests from the Indonesian Directorate of Volcanology and Geological Hazard Mitigation request people stay at least 2 km from the volcano. Local communities are still allowed to fish outside the 2 km danger zone radius. Communities in the coastal province of Banten and Lampung are requested to remain calm and not fear a tsunami.
KRAKATAU, INDONESIA ERUPTION STEPS
OVERVIEW
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 centures. 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.
LOCATION AND PLATE-TECTONIC SETTING
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.
PHYSIOGRAPHY BEFORE AND AFTER THE 1883 ERUPTION
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 calderaand 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.
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 bypyroclastic 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 steamshipBerouw 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, theLouden (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. |
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.
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).
AFTERMATH: the Child of Krakatau
On December 29, 1927, a group of Javanese fisherman who were startled by steam and debris bleching 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.
KRAKATAU NOW...
Mount Anak Krakatau in the Sunda Strait waters was raised to alert status since last few days.Head of the Observation Post Mount Anak Krakatau (GAK) in the Village District Hargopancuran Rajabasa, South Lampung, Andi Suardi said, in Rajabasa, Friday (29/10/2010), volcanic activity has increased since four days ago."The intensity of the shallow and deep volcanic earthquake which accompanied the eruption continues to increase," he said.He explains, based on monitoring, eruption volume recorded reached 138 times on this Friday along with bursts of volcanic ash and earlier on Thursday only 30 times the eruption.Meanwhile, volcanic earthquakes, he said, there were 21 and shallow volcanic earthquake 27 times today."The volume of the eruption is on the rise, while volcanic earthquake is more likely to decline," he explained.He said the increased activity is a natural phenomenon because it is an active volcano and the increase in activity can occur at any time."If the normal time of eruption of a maximum of only 10 times in one day," said Andi.He added, it will continue to improve the monitoring of the volcano because of Volcanology and Geological Hazard Mitigation (PVMBG) has raised its status to be vigilant.
Mount Anak Krakatau in the Sunda Strait is still busy backfire. Until around 24.00 pm, heading observers Mount Anak Krakatau eruption reported that there were 251.
On Monday, November 1 until around 24.00 pm yesterday, there were 4 times in a volcanic earthquakes, 79 shallow volcanic earthquake times, 251 times the eruption, 86 times the blast, and 229 times the tremor.
Meanwhile, on 31 October, recorded 31 times in a volcanic earthquake, 221 shallow volcanic earthquake, 178 times the eruption, 148 times the blast, and 151 times the tremor.
"Volcanic earthquake in that means going on inside or more than 3 km, while the superficial means happening near the surface, and the tremors of an earthquake which means continuous," said an observer of Mount Anak Krakatau, Jumono, when contacted by AFP on Tuesday (11/02/2010 .)
Although the number of seismicity on Monday less than on Sunday (31/10), but the number of eruptions increased. Alert status that has been pinned on the mountain since October 31, 2009 has not changed.
"It's still alert, and please keep keep a safe distance of 2 km around the mountain, because there are throw-throw material from the mountain," said Jumono.
Incandescent material that was brought up not only the gravel, but also rocks. Materials that have a hot incandescent 600-1000 degrees Celsius.
According Jumono, mountain seismic activity does not trigger a tsunami wave that threatens the citizens of Banten and Lampung."For an earthquake because of (activity) of this mountain does not trigger a tsunami, volcanic and tectonic because it is different," he continued.
He reminded people inedible issues tsunami. Moreover, the earthquake that occurred just around the mountain and only felt by a seismograph.
Alert status of Mount Anak Krakatau has been embedded since October 31, 2009. Since October 25, the number of eruptions over 100 times. "October 31 to be cautious after the previous mode. Then the number of eruptions decreased. But the October 10, 2010 back again there are eruptions," said Head Volcano Observation Hendrasto West region when contacted by AFP on Friday (29/10).
On 17 October, in a day is 45 times the eruption. Then for a few days the eruption had disappeared. Until October 23, appearing twice on 34 October eruption, and there 89 times.
"On the 25th there are 126, dated 26 no 182, dated 27 there are 101, 117 and 28 yesterday there is an eruption," said Hendrasto.
Status of the dangers of the mountain there are four levels. Danger level I or active normal means based on visual observations, seismicity and other volcanic phenomena showed no abnormalities. Level II or alert means there is increased activity in the form of abnormalities that appear visually or the results of the crater, seismicity and other volcanic phenomena.
At level III or standby, there was an increase in visual observation of the crater, seismicity and other metide support each other.While the level IV or alert, initial eruption began to occur in the form of ash / smoke. This will be followed by main eruption. (Vit / NRL)
On Monday, November 1 until around 24.00 pm yesterday, there were 4 times in a volcanic earthquakes, 79 shallow volcanic earthquake times, 251 times the eruption, 86 times the blast, and 229 times the tremor.
Meanwhile, on 31 October, recorded 31 times in a volcanic earthquake, 221 shallow volcanic earthquake, 178 times the eruption, 148 times the blast, and 151 times the tremor.
"Volcanic earthquake in that means going on inside or more than 3 km, while the superficial means happening near the surface, and the tremors of an earthquake which means continuous," said an observer of Mount Anak Krakatau, Jumono, when contacted by AFP on Tuesday (11/02/2010 .)
Although the number of seismicity on Monday less than on Sunday (31/10), but the number of eruptions increased. Alert status that has been pinned on the mountain since October 31, 2009 has not changed.
"It's still alert, and please keep keep a safe distance of 2 km around the mountain, because there are throw-throw material from the mountain," said Jumono.
Incandescent material that was brought up not only the gravel, but also rocks. Materials that have a hot incandescent 600-1000 degrees Celsius.
According Jumono, mountain seismic activity does not trigger a tsunami wave that threatens the citizens of Banten and Lampung."For an earthquake because of (activity) of this mountain does not trigger a tsunami, volcanic and tectonic because it is different," he continued.
He reminded people inedible issues tsunami. Moreover, the earthquake that occurred just around the mountain and only felt by a seismograph.
Alert status of Mount Anak Krakatau has been embedded since October 31, 2009. Since October 25, the number of eruptions over 100 times. "October 31 to be cautious after the previous mode. Then the number of eruptions decreased. But the October 10, 2010 back again there are eruptions," said Head Volcano Observation Hendrasto West region when contacted by AFP on Friday (29/10).
On 17 October, in a day is 45 times the eruption. Then for a few days the eruption had disappeared. Until October 23, appearing twice on 34 October eruption, and there 89 times.
"On the 25th there are 126, dated 26 no 182, dated 27 there are 101, 117 and 28 yesterday there is an eruption," said Hendrasto.
Status of the dangers of the mountain there are four levels. Danger level I or active normal means based on visual observations, seismicity and other volcanic phenomena showed no abnormalities. Level II or alert means there is increased activity in the form of abnormalities that appear visually or the results of the crater, seismicity and other volcanic phenomena.
At level III or standby, there was an increase in visual observation of the crater, seismicity and other metide support each other.While the level IV or alert, initial eruption began to occur in the form of ash / smoke. This will be followed by main eruption. (Vit / NRL)
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