A volcano in action
The subduction zone where the Australian plate slides below the Asian plate has produced many of the world's deadliest volcanoes. None is more famous than Krakatau.
Observations of its explosion in 1883 are regarded as the foundation stone of modern volcanology. It began on 20 May, when a German naval vessel reported an 11 km-high ash and dust cloud. In May and June, most of the summit of the mountain vanished. By August, loud explosions were being heard tens of kilometres away and ash was settling on ships and the nearby coastline. The Sunda strait in which the island lies was choked with pumice, the rock formed when hot magma is thrown straight into cold water. It later turned up floating in the ocean thousands of kilometres away. The culmination, on 26 and 27 August, ended with an explosion heard from India to Australia. The ash rose to over 30km high, four times the height of Everest.
The air wave from the explosion was observed all around the world. But it was the 40m tsunami it generated – far greater than that caused by the Indian Ocean earthquake of 2004 – that did most of the damage when it arrived at the coasts of Sumatra and Java. Over 36,000 people died, while the towns, villages and ships on which the survivors depended were destroyed and their farm animals were killed. Although most were caught by the tsunami, some were incinerated by a nuee ardente, a cloud of ash and other volcanic material that crossed 40km of ocean and arrived at the island of Sumatra still
hot enough to kill.
By the time it was all over, about two-thirds of the island of Krakatau had vanished. It had consisted of three volcanoes, Danan, Perbuwatan and Rakata. The first two had gone completely, as had much of Rakata. In 1927 the open sea produced by their destruction was broken when a new volcano, Anak Krakatau, the son of Krakatau, rose above the waves. It is an active and dangerous volcano. One of its explosions in 1993 killed a visiting tourist and injured five others.