Krakatau Volcano eruption 1883
Krakatoa (Indonesian: Krakatau), also spelled Krakatau, is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island (also called Rakata), and the volcano as a whole.
Although there are earlier descriptions of an island in the Sunda Strait with a "pointed mountain", the earliest mention of Krakatoa by name in the Western world was on a 1611 map by Lucas Janszoon Waghenaer, who labeled the island "Pulo Carcata". (Pulo is a form of pulau, the Indonesian word for "island".) About two dozen variants have been found, including Crackatouw, Cracatoa, and Krakatao (in an older Portuguese-based spelling). The first known appearance of the spelling Krakatau was by Wouter Schouten, who passed by "the high tree-covered island of Krakatau" in October 1658.
There are two generally accepted modern spellings, Krakatoa and Krakatau. The origin of the English spelling Krakatoa is unclear but may have been the result of a typographical error made in a British source reporting on the massive eruption of 1883. Also, like Egypt a couple of decades earlier, Polynesia (South Pacific) was in vogue in the late 19th century, and the Polynesian-like suffix -oa (as in Samoa) may have caught on as a result. While Krakatoa is more common in the English-speaking world, the Indonesian Krakatau tends to be favored by others, including geologists. Verbeek seems to have started the modern convention of using Krakatau for the island proper and reserving Rakata for the main cone.
The best-known eruption of Krakatoa culminated in a series of massive explosions on August 26–27, 1883, which was among the most violent volcanic events in modern and recorded history.
The 1883 eruption of Krakatoa began in May 1883 culminating with the destruction of Krakatoa in August 1883. Minor seismic activity continued to be reported until February 1884, though reports after October 1883 were later dismissed by Rogier Verbeek's (a Dutch geologist and nature scientist) investigation.
In the years before the 1883 eruption, seismic activity around the volcano was intense, with some earthquakes felt as far distant as Australia. Beginning 20 May 1883, three months before the final explosion, steam venting began to occur regularly from (Perboewatan, the northernmost of the island's three cones. Eruptions of ash reached an altitude of 6 km (20,000 ft) and explosions could be heard in Batavia (Jakarta) 160 km (100 miles) away. Activity died down by the end of May, with no records of activity until mid June.
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.
Eruptions started again around 16 June, when loud explosions were heard and a thick black cloud covered the islands for five days. On 24 June an east wind blew this cloud away and two ash columns were seen issuing from Krakatoa. The new seat of the eruption is believed to have been a new vent or vents which formed between Perboewatan and Danan, near the location of the volcanic cone of Anak Krakatau. The violence of the eruption caused tides in the vicinity to be unusually high, and ships at anchor had to be moored with chains as a result. Earthquake shocks began to be felt at Anjer (Java), and large pumice masses started to be reported by ships in the Indian Ocean to the west.
On 11 August, H.J.G. Ferzenaar investigated the islands. He noted three major ash columns (the newer from Danan), which obscured the western part of the island (the wind blows primarily from the east at this time of year), and steam plumes from at least eleven other vents, mostly between Danan and Rakata. Where he landed, he found an ash layer about 0.5 m (1 ft 8 in) thick; all vegetation had been destroyed, with only tree stumps left. He advised against any further landings. The next day, a ship passing to the north reported a new vent "only a few meters above sea level" (this may the most northerly spot indicated on Ferzenaar's map). Activity continued through mid August.
By 25 August, eruptions further intensified. At about 1 pm (local time) on 26 August, the volcano went into its paroxysmal phase, and by 2 pm observers could see a black cloud of ash 27 km (17 miles) high. At this point, the eruption was virtually continuous and explosions could be heard every ten minutes or so. Ships within 20 km (11 nautical miles) of the volcano reported heavy ash fall, with pieces of hot pumice up to 10 cm in diameter landing on their decks. A small tsunami hit the shores of Java and Sumatra some 40 km (28 miles) away between the time of 18:00 and 19:00 hours.
On 27 August four enormous explosions took place at 05:30, 06:44, 10:02, and 10:41 local time. The explosions were so violent that they were heard 3,500 km (2,200 mi) away in Perth, Western Australia and the Indian Ocean island of Rodrigues near Mauritius, 4,800 km (3,000 mi) away, where they were thought to be cannonfire from a nearby ship.:22 Each was accompanied by very large tsunamis, which are believed to have been over 30 meters (100 ft) high in places. A large area of the Sunda Strait and a number of places on the Sumatran coast were affected by pyroclastic flows from the volcano.
The pressure wave generated by the colossal final explosion radiated from Krakatoa at 1,086 km (675 mi) . It was so powerful that it shattered the eardrums of sailors on ships in the Sunda Strait and caused a spike of more than two and half inches of mercury in pressure gauges attached to gasometers in the Jakarta gasworks, sending them off the scale. The pressure wave radiated across the globe and was recorded on barographs all over the world, which continued to register it up to 5 days after the explosion. Barograph recordings show that the shockwave from the final explosion reverberated around the globe 7 times in total. Ash was propelled to a height of 80 km (50 mi).
The eruptions diminished rapidly after that point, and by the morning of August 28 Krakatoa was quiet. Small eruptions, mostly of mud, continued through October, though further reports continued through February 1884. These reports were discounted by Verbeek.
Ships as far away as South Africa rocked as tsunamis hit them, and the bodies of victims were found floating in the ocean for weeks after the event. The tsunamis which accompanied the eruption are believed to have been caused by gigantic pyroclastic flows entering the sea; each of the four great explosions was accompanied by a massive pyroclastic flow resulting from the gravitational collapse of the eruption column. This caused several cubic kilometers of material to enter the sea, displacing an equally huge volume of seawater. The town of Merak was destroyed by a 46 metre-high tsunami. Some of the pyroclastic flows reached the Sumatran coast as much as 25 miles (40 km) away, having apparently moved across the water on a "cushion" of superheated steam. There are also indications of submarine pyroclastic flows reaching 10 miles (15 km) from the volcano.
A recent documentary film showed tests made by a research team at Kiel University, Germany of pyroclastic flows moving over water. The tests revealed that hot ash traveled 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.
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, and may have been caused by concussive air waves from the eruption. These air waves circled the globe several times and were still detectable using barographs five days later.
With a Volcanic Explosivity Index (VEI) of 6, the eruption was equivalent to 200 megatons (MT) of TNT—about 13,000 times the nuclear yield of the Little Boy bomb (13 to 16 kT) that devastated Hiroshima, Japan during World War II and four times the yield of the Tsar Bomba (50 MT), the largest nuclear device ever detonated.
Volcanic Explosivity Index (VEI) is a scale that describes the size of an explosive volcanic eruption. VEI is analogous to the Richter magnitude scale for earthquakes. In the 0 to 8 scale of VEI, each interval represents an increase of a factor of ten. An eruption of VEI 4 is 10 times larger than a 3 and one hundred times larger than a 2. Key characteristics that define VEI include:
Volume of ash produced
Height of eruption cloud above the vent
Duration of eruption
Krakatau, Indonesia, 1883
VEI 6 (Very Large)
Bulk Volume of Erupted Products: 4 cubic miles
The 1883 the Krakatoa eruption ejected more than 25 cubic kilometres of rock, ash, and pumice and generated the loudest sound historically reported at 180 Decibels: the cataclysmic explosion was distinctly heard as far away as Perth in Australia approx. 1,930 miles (3,110 km), and the island of Rodrigues near Mauritius approx. 3,000 miles (5,000 km).
Nine tsunamis were generated, some of the tsunami waves are believed to have been over 100 ft (30 meters) high in places.
Around noon on August 27, a rain of hot ash fell around Ketimbang in Sumatra. Around a thousand people were killed, the only large number of victims killed by Krakatoa itself, and not the waves or after-effects.
Darkness covered the Sunda Straits from 11 a.m. on the 27th until dawn the next day.
Every recording barograph in the world ducumented the passage of the airwave created by the explosion, some as many as 7 times as the wave bounced back and forth between the eruption site and its antipodes for 5 days after the explosion.
Blue and green suns were observed as fine ash and aerosol, erupted perhaps 50 km into the stratosphere, circled the equator in 13 days.
Three months later, the eruption fallout had spread to higher latitudes causing such vivid red sunset afterglows that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration. Unusual sunsets continued for 3 years.
The volcanic dust veil that created such spectacular atmospheric effects also acted as a solar radiation filter, lowering global temperatures as much as 1.2 degree C in the year after the eruption. Temperatures did not return to normal until 1888.
After the eruption of Krakatoa, on the 21st of August 1883, unusual observations were reported. For example:
On the 3rd of September: During the past few days, there has been a fairly even, gray Cloud mass, normally covering the entire sky, above the cumulus and Stratus clouds;
On the 3rd of September: At midday hazy gray air. Hazy, gray air condensing into Dew towards evening;
On the 5th of September: Air appears yellow and watery;
On the 7th of September: The atmosphere appears to be filled with very small, evenly distributed clouds of vapor;
On the 13th of September: The yellowish "haze" continues in the upper atmosphere;
On the 11th of October: Fiery atmosphere, cloudless sky;
On the 5th of November: Pale atmosphere;
On the 10th of December: The air was very clear and looked like the air in the Southern Indian Ocean during the typhoon season;
On the 13th of December: Lead-colored sky.
These early observations could possibly have been dismissed as coincidence if the period until 1886 had not been accompanied by a permanent phenomenon, a "hazy fog", a strange, smoky cloudiness in the atmosphere, which was observed everywhere around the globe, in the tropical as well as in sub-polar areas. One of the descriptions given was: "The hazy fog appears as a constant companion of the extraordinary optical phenomena in the atmosphere during the entire period of the atmospheric-optical disturbance". How the young science viz. meteorology could not be concerned with what was going on? Had the oceans been recognized as stabilizers, the greenhouse effect would be understood much better today. The explanation is easy.
The “hazy fog” was a compound of volcanic dust and oceanic water vapor. This “extra stuff” from the atmosphere wrapped the earth like in a blanket. This blanket protected the earth from losing heat too quickly and thus compensated for the deficiency of blocked-out sunrays (10-15%) for a few years. The interdependence is evident:
In the wake of the eruption over 36,000 lay dead. In addition, almost 5000 were killed as a result of falling debris and hot pyroclastic flows.
Numerous volcanic outpours have happened both before and after the 1883 explosion. Since 1927, small eruptions have been frequent and the top of a new volcanic island rose from the waters, Anak Krakatau (Child of Krakatau), and it has been growing ever since. In 1960, Anak Krakatau had a diameter of about 1.5 km and was more than 160m tall.
Krakatoa is located along "The Ring of Fire" which is a zone of frequent earthquakes and volcanic eruptions that encircles the Pacific Ocean. It is associated with a nearly continuous series of oceanic trenches, island arcs, and volcanic mountain ranges.
Verbeek, in his report on the eruption, predicted that any new activity would manifest itself in the region which had been between Perboewatan and Danan. This prediction came true on 29 December 1927, when evidence of a submarine eruption was seen in this area (an earlier event in the same area had been reported in June 1927). A few days later, a new island volcano rose above the waterline, named Anak Krakatau ("Child of Krakatoa"). Initially, the eruptions were of pumice and ash, and that island and the two islands that followed were quickly eroded away by the sea. Eventually a fourth island named Anak Krakatau broke water in August 1930 and produced lava flows faster than the waves could erode them. Of considerable interest to volcanologists, this has been the subject of extensive study.
Since the 1950s, Anak Krakatau has grown at an average rate of five inches (13 cm) per week. The island is still active, with its most recent eruptive episode having begun in 1994. Since then, quiet periods of a few days have alternated with almost continuous Strombolian eruptions, with occasional much larger explosions.
The most recent eruption began in April 2008, when hot gases, rocks, and lava were released. Scientists monitoring the volcano have warned people to stay out of a 3 km zone around the island
Very good website about Krakatau with actual newspaper records of the time of the explosion (But in Dutch) : : In the Realm of the volcano Astronomical Sleuths Link Krakatoa to Edvard Munch's Painting The Scream : click Here !