Tsunami claim more lives than any other natural disaster, with masses of water moving shoreward as fast as an airliner, writes Mike Williams.
Tsunami is a Japanese word meaning "harbour wave", so-called because it is only when such waves approach the shore that they become visible - out in the deep oceans they are barely noticeable. They differ from storm waves in that they involve the mass transfer towards the shore of large masses of water whereas in ordinary waves the water particles simply move in circular orbits and do not move shoreward to any great extent.
As we have seen in the recent tragedy around the Indian Ocean, the height of the wave does not need to be significant to cause massive amounts of damage; it is the pure volume of the water mass travelling inexorably in one direction that creates the destruction.
Such waves can be generated by earthquakes (relative vertical movement of the earth's crust), by down-slope collapse of submarine sediment, collapse of oceanic volcanoes or some combination of these. The tsunami generated can travel across oceans with the speed of an airliner. We can estimate, for example, that the tsunami generated by the Lisbon earthquake of 1755 took about three to four hours to reach the west coast of Ireland where they were reported to have destroyed the castle at Coranroe on the north coast of Co Clare. These waves also travelled the other way across the Atlantic, causing waves as high as houses to strike the Caribbean islands.
In the case of the recent earthquake and its devastating effects around the Indian Ocean, this is the result of activity in a very young part of the Earth's crust. Here the crustal plate containing the Australian continent is being pulled downward under another plate of the crust containing the Indonesian volcanic islands. This crust is only two million years old in places.
This interaction takes place at a great gash in the ocean crust known as the Java Trench, where water reaches depths of over seven kilometres. Movement along this trench was probably the cause of the earthquakes that generated the tsunami here, with the consequent catastrophic loss of life. Of all types of natural disasters, tsunami have been responsible for the greatest loss of human life on the planet.
Much has been made in some media reports of the recent earthquake shifting islands upwards and causing the earth to wobble on its spin axis. There is nothing strange in seeing the effects of earthquakes being felt on the surface of the earth even though they may be generated at some depth in the crust.
We are all familiar with film clips of bridges swaying and buildings collapsing. In California, for instance, we can visualise the actual movement of the crust on the surface as regimental linear lines of orange trees can be seen to have been displaced along a recent fault line on the surface.
According the some scientists, the downward movement of one plate beneath another would cause a change in the rate of spin of the earth. The theory of plate tectonics, however, shows that these processes are cyclic in nature. As the descending plate goes down it gets hotter until it finally melts, releasing magma which rises back towards the surface generating volcanoes. So, over the long term, there is no addition of material to the deeper parts of the crust. New, cold ocean crust is created by the moving apart of plates.
The Indian Ocean Ridge, for example, stretches from the Red Sea in the north across the whole of the Indian Ocean and passes south of Australia. The plates on either side of this ridge are moving apart, allowing magma to rise through the crust onto the ocean floor, cooling quickly and forming new ocean crust. So as crust is destroyed at the Java Trench it is created along the Indian Ridge.
In the same way new crust is being created in the middle of the Atlantic Ocean, moving Ireland away from North America at the rate of about two centimetres a year.
There is, therefore, a long-term balance between the creation and destruction of crust. This explains why the earth neither expands nor contracts significantly.
Since this process has been going on for over 4,000 million years without sending the planet into a catastrophic wobble, it's unlikely this event will have a dramatic effect on the spin of the earth.
* Mike Williams is a professor in the department of earth & ocean sciences, at the National University of Ireland, Galway.