Speaking of disasters (before global warming).. 1883 - Eruption of Krakatoa (Sunda Strait), Indonesia
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Date
27th August 1883
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The Sunda Strait is a major shipping corridor situated between the Indonesia islands of Sumatera and Java. Barely 22 kilometres wide at its narrowest point and averaging a depth of 200 metres, it was the scene of the worst volcanic disaster in modern history. Prior to 1883, four small islands; Krakatoa, Verlatan, Lang and Polish Hat were situated in this strait. Krakatoa was the largest and was known to be volcanic. Seismic activiy increased in the 1880's and soon thereafter Perbuwatan, the largest volcanic cone on Krakatoa, erupted on May 20th, 1883 destroying much of the island's vegetation. The damage was surveyed by a Dutchman, Capt. Ferzenaar who also noted two new volcanic vents in the central part of the island.
The eruptions on Krakatoa began dying down, just after 4am, on the morning of August 27th, 1833, after weeks of intense activity. But only an hour and a half later, the first great eruption occurred. Examination of the pressure gauges at the gas-works at Batavia (now Jakarta) indicated 4 major blasts; 05:30, 06:44, 10:02 and 10:52, the third being the most powerful. The explosion was loud enough to be heard thousands of miles away. At Elsey Creek, in South Australia, 3224 kilometres from Krakatoa, sleeping people were awakened. At Diego Garcia, in the Chagos Archipelago, in the Indian Ocean, 3647 kilometres from the volcano, the explosions were first thought to be from ships in distress, firing its guns to attract attention and so people ran to various vantage points to hear it. Rodriguez Island, near Mauritius, at a distance of 4811 kilometres was the farthest point where the explosions were heard. The climatic explosions generated a huge explosion column. Tephra raining down and ash fall was reported as far away as Cocos Islands, 1850 kilometres. In Batavia, ash fall began at 10:15 and the sky was completely blackened by 11:20 and remained so till 13:00 that afternoon. But closer to the volcano darkness persisted for nearly two days after the eruption.
In the Sunda Straits, huge tsunamis lashed the shores wiping out dozens of towns and villages with a short time after the commencement of the eruption. The waves were so powerful that coral blocks weighing as much as 600 tons were thrown ashore. A warship in the area was carried 3 kilometres inland by the waves and was deposited at a 10-meter elevation above sea level. The huge tsunami was well documented in terms of visual observations of heights reached along the coasts of Java and Sumatra as well from a recording at a tide gauge at Batavia (Jakarta). The tsunami travel time to the closest villages of Sumatra was about 1 hour after the explosion of Krakatoa. At Telok Batong, tsunami waves up to 22 meters, completely submerged the village. At Vlakke Hook, the maximum tsunami wave height was 15 meters. Waves also reached the Western coast of Java within an hour after the explosion of Krakatoa. The village of Sirik was almost entirely swept away by these waves. It took also about one hour for the destructive tsunami waves to reach Anjer, where a 10-meter wave completely overwhelmed the lower part of town. At Tyringen, waves ranged from 15 - 20 meters in height, while at Merak, the waves reached a maximum of 35 meters. It took approximately 2.5 hours for the tsunami waves to refract around the western end of the island of Java and to reach Batavia (Jakarta), the capital of Indonesia on. Waves of 2.4 meters were reported there with a very long period of 122 minutes. By the time the tsunami reached Surabaya, at the eastern part of Java, the reported wave was only 0.2 meters. Low-lying areas were devastated. The port of Anjer (Anyer Kidul, Java) simply ceased to exist as great waves washed over it, carrying away the flimsy wooden buildings that made up the town. One of the tsunamis, which arrived in the Tandjong Priok, Batavia (Jakarta) at 12:36 on the 27th of August, was far larger than any of the others. These waves had reported heights of more than 37 meters (120 feet).
They spread out into the Indian Ocean and struck the coast of India. The tsunami is reported to have been about 1.5 meters at Chennai (formerly Madras). In Sri Lanka (then known as Ceylon), the waves were big enough to leave small boats in harbours stranded and then re-floated them. Small sea level oscillations from Krakatoa's tsunami were recorded by tide gauges as far away as Hawaii, the American West Coast, South America, and even as far away as the English Channel, in France and England. It took 12 hours for the tsunami to reach Aden on the southern tip of the Arabian Peninsula, some 3800 nautical miles away. The travel time of a little over 300 nautical miles per hour to Aden appears reasonable. There were no land boundaries on the Indian Ocean side of Krakatoa to prevent the tsunami energy from spreading in that direction.
However, it is doubtful that the waves, which were reported at distant locations in the Pacific or in the Atlantic Ocean, represented the actual tsunami generated in the Sunda Strait. Very little, if any at all, of this energy could have escaped the surrounding inland seas to the east of the Sunda Strait. Most probably, the small waves that were observed in the Pacific as well as in the Atlantic were generated by the atmospheric pressure waves, which resulted from the major Krakatoa explosion, and not from the actual tsunami generated in the Sunda Strait. In Japan, a small sea level oscillation was recorded at Honshu-Sagami and at Shikoku-Satsuma. In Australia, a trace of the tsunami was recorded being less than 0.1 meter. In New Zealand, a 0.3-meter change in water level was reported. The Honolulu tide gauge on the island of Oahu recorded a small oscillation of only 0.24 meters, 17 hours after the explosion of Krakatoa. In Alaska's Kodiak Island a small oscillation of 0.1 meter was recorded. In San Francisco, California, a 0.1-meter sea level oscillation was recorded 20 hours after the explosion.
The massive tsunami was probably triggered by the collapse of half of the Rakata cone into sea, in an avalanche similar to that of Mount Saint Helens in 1980. Rafts of pumice, upto 3 feet thick were clogging the waters of the strait. The Royal Dutch Navy was among the first to survey the devastated area. They found that the entire northern part of Krakatoa, (the entire Perbuwatan cone) had disappeared, with the exception of a bank of pumice and an isolated rock, 10 meters square. About two thirds of Krakatoa had disappeared. Half of the cone of Rakata lay perched on the southern rim of the caldera, its northern face virtually unsupported. This side then slumped into the sea, leaving behind a spectacularly bisected volcanic cone, which is all that remains of Krakatoa today. Two brand new island, Steers and Calmeyer were formed. The islands of Verlaten and Lang ended up larger, buried and surrounded by enormous amounts of pyroclastic deposits while the island of Polish Hat has simply disappeared. A new volcano has built up on the island. Its called Anak Krakatoa, which means child of Krakatoa.
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REFERENCES
1) Francis, P., "Volcanoes - A Planetary Prospective", Oxford University Press, 1992.
2) International Tsunami Symposium
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