Tsunami: on boxing day 2004, nature reminded us of its awesome power.

The world's land masses sit on top of plates that float on the molten core of our planet. These plates move and where they meet up with one another earthquakes and volcanoes are common.

About 160 kilometres off the west coast of the Indonesian island of Sumatra, the Indian plate meets the Burma plate. The two plates rub together along a zone that is 1,200 kilometres long and 480 kilometres wide.

The Indian plate is sliding under the Burma plate. But, the movement is jerky, as the submerging plate tends to drag the upper plate down with it. As pressure builds, the upper plate breaks free, springing back to its original position. That's what happened on 26 December 2004.

The amount of energy released was vast. (One estimate puts it at 30 percent greater than all the energy used in the United States in a year.) It was enough to cause Earth to wobble on its axis by about 2.5 centimetres and to speed up its rotation by a few tiny fractions of a second. The plate on which Sumatra sits lurched back about 15 metres in a few seconds. At the same time, the seabed lifted displacing a vast quantity of ocean water.

Dr. Roger Musson, is an earthquake expert with the British Geological Survey in Edinburgh, Scotland. He told The Times newspaper: "The effect of the earthquake is like throwing a stone in a pond, except that you are throwing it from below. You get the equivalent of a splash and water is displaced with waves spreading outwards."

The Japanese, who have a lot of experience with earthquake generated waves, call them "tsunamis" (tsu = harbour + nami = wave). The word was coined by fishers who returned to port to find their homes devastated, although they hadn't been aware of any wave in the open water.

Out in the ocean a tsunami is often barely noticeable; it may cause the ocean surface to rise by only a few centimetres but it can travel at the speed of a jet aircraft. Despite their apparent insignifance, tsumanis contain phenominal amounts of energy.

Tad Murty of the Tsunami Society has estimated the Boxing Day "seismic sea waves," as the oceanographers call them, to have contained the energy equivalent of about five megatons of TNT. That's more than twice the explosive energy used during the whole of World War II, including the atomic bombs dropped on Hiroshima and Nagasaki. He also estimated that tsunamis represent only a quarter of the energy of the initial earthquake.

Eventually, a tsunami approaches land. As the ocean floor rises towards the coast, the forward speed of the wave drops but the water begins to pile up. When the tsunami crashes ashore the effect is catastrophic. No wonder the most frequently heard description of the destruction caused by the recent tsunami was "Unimaginable."

THE BIG ONE

Scientists say there are several geological time-bombs Scattered across the world's oceans; one of them is in the volcanic Canary Islands. A huge piece (estimated to weigh 500 billion tones) of the western flank of the island of La Palma is in danger of falling into the Atlantic Ocean. This may happen at any time during the next few thousand years triggering a mega-tsunami. The wave, measuring several hundred metres in height, would reach the eastern seaboard of the American continent in a few hours.