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The Krakatoa Eruption

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A computer-generated image of Krakatoa before the eruption, taken from the 2005 BBC documentary Krakatoa Revealed.

Anak Krakatau is an island in the Pacific Ocean approximately 40 kilometres from Java. It has an explosive history and an explosive nature. The original volcano of Krakatau - Krakatau is the Indonesian name for the volcano westerners call Krakatoa - gave birth to the current island of Anak Krakatau. What follows is an explanation of one of the most powerful volcanic eruptions in recorded history1.

The History of the Islands

In the year 416 AD, the island of Krakatau (Krakatoa) was estimated to be two kilometres high, with a radius of some nine kilometres. It was created by debris buildup from the volcano, Krakatau. Its first recorded eruption was during this same year and broke up the island, forming three smaller islands, Rakata, Sertung and Penjang, approximately 40 kilometres from Java. The volcano was located on Rakata.

Before the eruption in 1883, the Island of Rakata was approximately 47 square kilometres and actually had three volcanoes: Perboewatan, Danan, and Krakatau.

The Eruption

On 20 May, 1883, an 11-kilometre-high cloud of ash was seen rising from the uninhabited island of Rakata. This would be the beginning of two months of unstable volcanic activity. During this time, many ships travelling near the island reported seeing similar spectacles accompanied by explosions. Nearby islanders celebrated the spectacle; nobody guessed at the ferocity of the explosion that was to happen on 26 August and would continue until 27 August of that year.

At midday on 26 August, Krakatau began to erupt, throwing pumice2 and ash some 25 kilometres into the sky. So forceful were the explosions, the sound carried 4,800 kilometres, as far as Australia. The eruption was so forceful and destructive the volcano blew itself apart, taking the other two volcanoes with it. Only one third of the island of Rakata survived the explosion. The rest of the island fell 300 metres below sea level.

The Cataclysmic Events That Followed

The initial blast formed a cloud of volcanic ash and rock that rose 25 kilometres above the island. A few hours later, the cloud widened dramatically to the northeast and rose even higher, to around 36 kilometres, and entered the earth’s stratosphere. Frightened, the coastal communities of western Sumatra, western Java and other islands close by began to panic. A few hours later, these villages were devastated by tsunamis.

The Tsunamis

The final blast from Krakatau happened on the following day, 27 August, 1883. This blast collapsed the island and sent giant waves, some up to 40 metres in height, across the Sunda Straits. Most of the islands that were closest to Krakatau were completely submerged. No life survived from these islands; they were even stripped bare of vegetation. Some 80 kilometres away, the Thousand Islands were submerged under two metres of water. The villagers saved themselves by climbing up the trees to get clear of the water.

A ship called the Loudon was anchored in Lampong Bay near the village of Telok Betong. The captain of the ship spotted the tsunami coming broadside to the ship. With skill born of desperate fear, Captain Lindermann managed to turn the ship head on into the oncoming wave just in time. Had the ship been broadside to the wave it would certainly have capsized, killing all those on board.

For many months after the eruption, the Sunda Straits were clogged with thick pumice banks, which often held fifty or more bodies. To this day nobody knows how many people were killed by the eruption, though Dutch authorities there at the time calculated the death toll at some 36,417. Nearly 90% of this death toll was attributed to the tsunami.

Some tsunami waves travelled as far as 13,000 kilometres. So strong were the tsunami waves, they were even felt in the seas around Great Britain.

Pyroclastic Flows and Debris

If 90% of the deaths were due to the tsunamis, 10% of the death toll, roughly 4,500 deaths, were caused by pyroclastic flows and falling debris. To give you an idea of the total amount of ash and debris thrown into the air, Krakatau generated some 20 cubic kilometres of volcanic debris, more than 20 times what Mount St Helens produced when it erupted in 1980.

The Sunda Straits were congested with so much debris that it appeared to be solid ground, preventing relief ships from reaching coastal communities for weeks. Over the coming months, storms and high-tides would scatter the debris. Ships thousands of kilometres from Krakatau reported seeing this floating debris for months after the eruption. One such accumulation floated until it reached the coast of South Africa in September of the following year.

A pyroclastic flow consists of super hot gases and rock that are spewed from an erupting volcano. The flow begins when the huge mass of erupted gas can no longer hold itself in the air. It collapses in on itself forming a flow of hot gases and rocks that resemble a landslide, or a river bursting its banks, but on a much larger scale. (It differs from a lava flow, which is hot, liquid rock that has escaped from an underground chamber and flows down the side of a volcano, usually along the path of least resistance.)

The pyroclastic flow travelled 40 kilometres over water to reach Sumatra. Approximately 2,000 severely burned bodies were found there. It is believed that as the flow travelled over the water, the water turned to steam, allowing the flow to travel further than it normally would be able to.

Atmospheric Ramifications

Fine particles from the eruption entered the stratosphere before spreading out along the equatorial belt in just two weeks. These particles stayed in the atmosphere for years while dispersing further north and south.

Sulphur dioxide gas within the particles mixed with water to create sulphuric acid. The resulting acidic water and dust created a shield that reflected enough of the suns rays to cause a global drop in temperatures of several degrees. The same vapour also created spectacular effects over 70% of the earth’s surface: for several years after the eruption, people saw halos around the sun and moon, and morning and evening skies were tinged with bright, almost pyrotechnic, colours.

So rich and vibrant were colours that New York City firefighters were frequently called out for false alarms; the glow of sunset often looked like a catastrophic fire taking place on the horizon.

Barometers monitoring the air pressures at the time of the explosion picked up the pressure waves generated by the eruption. In some cases they were picked up seven times as the shock waves bounced around the earth.

The Child of Krakatau

On 29 December, 1927, some Javanese fisherman were startled by steam and debris belching from the sea where Krakatau once stood. This marked the rebirth of Krakatau. Within one month of the activity beginning, the tip of a new volcano appeared from out of the sea. The Anak, or child of, Krakatau had been born. A year later, Anak Krakatau became a small island.

Anak Krakatau has erupted in most years since. Typically, these are rather mild eruptions of basaltic andesine3 lava flows. Though they present little danger to surrounding islands, the eruptions from Anak Krakatau provide a constant reminder of the dreadfulness of 1883, and the remains of Rakata have been evacuated.

Anak Krakatau is now an island that hosts plant and animal life. No mammals have been found on the island, but a huge array of plant and insect life now inhabit it.

1The title of the most powerful volcanic eruption in recorded history of course belongs to Tambora on the island of Sumbawa in Indonesia in 1815.2A light, porous form of lava.3A gray, fine-grained, igneous, obviously volcanic, rock.

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