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Krakatau
At some point in the distant past, Krakatau consisted of a single, large volcanic island. This island was destroyed in eruptions presumably of great violence, leaving three fragments of the original volcanic walls in a broken ring, or caldera, around the edge of this original island. The three islands are now named Rakata, Sertung and Panjang. Subsequently, further eruptions began, building up the largest island (Rakata) back into the center of the caldera. As of 1883, the only previous recorded eruptions had come from this big island in 1680. The islands of Panjang and Sertung, then as now, remained dormant. Then in May of 1883, eruptions began again on the big island, and one by one, the three peaks of the island -- Perboewatan, Danan and Rakata -- each came into action as the cycle of building and destruction reached its peak. Finally, on August 27th, the sequence ended in catastrophe, as huge volumes of ejecta were hurled into the sky, plunging the surrounding region for a radius of 80 km into 57 hours of darkness. Relatively few people appear to have died as a direct result of the ejecta, but huge numbers died because of an indirect consequence of the eruption. As the magma chamber emptied, the outer walls of the volcano failed, and collapsed -- repeating the pre-historic caldera collapse -- displacing two-thirds of the island.
These events generated a series of giant waves, or tsunami, which steepened as they reached shallow waters. Lava forming islandsThese waves swept across the coastal lowlands of Java and Sumatra on either side of the Sunda Strait, killing an estimated 36,000 people and destroying many settlements. As an illustration of the forces involved, a government gunboat, the Berouw, was carried nearly 3 km inland and stranded behind a small hill 9 m above sea-level (the crew of 28 were amongst those who died). The violence ended abruptly, leaving a greatly re-shaped archipelago. In the centre of the caldera, where there had once been a substantial island, the sea-floor was reached at a depth of over 250 m. The three remaining islands were greatly re-shaped, and in places extensive new land surfaces had been created by the deposition of great thicknesses of pyroclastic ashes where once had been nothing but the sea. On the persisting areas of land, an average of 60-80 m of these ashes had been emplaced. The resulting landscapes were completely barren, and as far as can be established, no life survived. Plants and animals soon colonized, and the ecosystem re-building began -- a story we will come back to. Down in the depths of the earth, the emptied magma chamber will, once again, have begun to fill, eventually creating sufficient pressure to begin the construction phase of the cycle once again, naturally, pretty much in the center of the caldera
The Child Grows
Eruption plumesVolcanoes typically build up in fits and starts. After 1883, Krakatau was dormant until 1927, when a new cone began to build at the edge of the 1883 caldera. In 1928 alone, 22,358 earth tremors were registered by a temporary volcanological observatory on the island of Panjang, while eruption plumes were seen to reach 1,200 meters high. However, the first three attempts to break the surface formed only temporary ash-cone islands, which were destroyed by marine erosion and submarine sliding on the steep slope of the submerged caldera. The second of these islands was visited by a party of scientists in May 1929, shortly before it slipped back into the sea. They collected several insects, including a large black cricket and a brown ant. The present island, the fourth, emerged in August 1930, and over the next few years, it alternated between periods of quiet and rapid growth. At times, particularly in the early 1930s, the eruptions produced huge quantities of fine ash, which, aided by the alternating monsoon winds, have been deposited on the developing forests of Panjang and Sertung islands, a few kilometers distant (in opposite directions), causing extensive damage. By September 1932, the island was 47 meters high and had a typical low wide crater some 700 meters in diameter. By 1933, the height had increased to 67 meters, and by 1941, it had doubled again to reach 132 meters. The 1940s was a period of relatively little activity, but in 1952 and 1953, the volcano sprang back into action, producing large quantities of ejecta. This was produced in such quantities and to such heights (clouds reaching 4,000 meters at times) that once again it was carried across to Panjang and Sertung islands, damaging as much of 90 percent of their forests to a degree visible from boats passing through the group. On Anak Krakatau itself, the colonization of plants and animals was abruptly halted by these eruptions, as the vegetation was more or less eliminated. A new, high inner cone formed, that was about 500 meters in diameter, enclosing a new crater lake. The records then indicate a further period of inactivity, until 1958, with further ash eruptions occurring over the period to 1963, during which some traces of ash reached sites on the coast of Java, some 40 kilometers distant.
Anak KrakatauThe Volcano Today: Will It Blow Again?
By the mid-1990s the island had overtopped both Panjang and Sertung islands, reaching a height of about 300 meters, with a diameter varying between 3 and 4 kilometers. It has some way to go before it reaches the height of the last fragment of the 1883 volcano Rakata, at some 780 meters. What are the chances of the cycle of growth and violent destruction repeating itself? Volcanologists have a number of means of assessing the likelihood of catastrophic eruptive activity. One important measure is the silica content of the ejecta. As the magma increases in silica content its viscosity increases, making it more difficult for contained gases to escape. When they do, there is a cataclysmic eruption, and silica content then falls to begin the cycle again. The Krakatau data broadly fit this Magmamodel, and do not suggest that another caldera collapse event is imminent. But you can never be certain with volcanoes. In any event, it is not a volcano to take lightly. Over the last 70 years, it has deposited something like a meter of volcanic ashes on Panjang and Sertung islands, and has repeatedly cut-back the development of its own ecosystems. When in full eruption, it is an awesome sight, with huge volcanic bombs thrown hundreds of meters from the eruption center, and great plumes of ash ascending many hundreds and sometimes thousands of metres. Recent outpourings of lava have also been spectacular, for instance, in November 1992, a four day period of activity saw 35,650 square meters covered by lava of about 5 meters in thickness -- an estimated volume of over 2 million cubic meters.
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