The southern coast of Indonesia is one of the most active tectonic zones in the world because the Indian plate is subducting beneath the Burma microplate. This was borne out tragically in the Aceh-Andaman earthquake and Tsunami of 2004. In this week's Nature, Stein and Okal argue that the magnitude of that earthquake was 2.5 times greater than previously thought and should have been a 9.3 instead of a 9.0. Just this past Monday, another earthquake of 8.7 magnitude struck southern Sumatra. Fortunately, this one was in shallower water so much less water was displaced and thus no Tsunami was generated.
It was over a century ago, on August 27, 1883, that the volcano Krakatoa exploded nearby. The volcano was part of an island consisting of three main cones in the straight between Sumatra and Java. It stood nearly 800 metres high and had a diameter of 15 kilometres. Two of the cones were completely blown apart in the eruption. The Tsunami generated was estimated to be 40 metres high although it was of short wavelength so it didn't travel very far. It did kill 36,000 people in southern Sumatra and western Java. It carried a steamship 2 miles inland. The explosion was so loud it was heard as far away as Perth Australia and the atmospheric shock wave travelled around the world seven times. There were brilliant red sunsets for five years afterwards due to the 21 cubic kilometres of material thrown up.
The eruption was extremely well documented by the scientists of the day. The field of volcanology was invented to understand why the eruption was so violent. The current theory is that Krakatoa had been dormant for 200 years before the 1883 eruption and this allowed the magma to cool inside the volcano. About four months before the August eruption, Krakatoa became active and this cool magma escaped in a series of minor eruptions. It was then replaced by hot magma from below. The entering hot new magma heated up the remaining cool old magma releasing gases and built up tremendous pressure which was released in a massive explosion equivalent to 21,000 megatons of TNT. The amazing thing is that a new volcano has almost entirely replaced the old one. In just a hundred years, Anak Krakatau (child of Krakatoa) has risen to a height of 300 metres and is still growing. It is not expected to explode like it's parent in the near future but given recent events one never knows.