Tuesday, 4 September 2012

KRAKATAU



KRAKATAU
 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.

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:
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.

 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).

KRAKATAU CHILDREN
Starting in 1927 or about 40 years after the eruption of Mount Krakatau, the volcano emerged known as the volcano's caldera of the ancient region that is still active and still growing in height. High growth rate of about 20 inches per month. Every year it becomes more about 20 feet tall and over 40 feet wide. Another note to mention the addition of a height of about 4 cm per year and if calculated, then within 25 years the addition of high-achieving children Rakata 7500 inches or 500 feet higher than 25 years earlier. The cause of the high mountain was caused by material that came out of the belly of the mountain new. Currently the volcano reaches a height of around 230 meters above sea level, while Mount Krakatau previously had high 813 meters above sea level.

According to Simon Winchester, despite what happens in the life of Krakatau which used to be very scary realities of geological, seismic and tectonic in Java and Sumatra, the strange will ensure that what used to happen at some point will happen again. No one knows exactly when the volcano will erupt. Some geologists predict eruptions will occur between 2015-2083. However, the effect of the earthquake on the Indian Ocean on December 26, 2004 also can not be ignored
previous eruptions.
According to Professor Ueda Nakayama vulcanologist one Japanese national, Anak Krakatau is still an active and relatively safe although there is often a small explosion, there are only certain times when the tourists are prohibited from approaching the area because of the danger that spewed lava volcano. Other experts said there was no plausible theory of Anak Krakatau will be re-erupted. Even if there are at least 3 or more centuries after 2325 AD But clearly, the number of victims affected more powerful than the previous eruptions. Anak Krakatau is now by the general public better known as "Krakatoa" also, though the mountain is actually growing new post previous eruptions
 


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