It seems like a phenomenon. The 2004 Indian Ocean Earthquake saw a tsunami up to 30 metres high hitting coastal regions around the Indian Ocean, leading to more than 230,000 deaths. But what is a tsunami? Could such a disaster be caused by a single earthquake?
What is a tsunami?
www.thefreedictionary.com defines a tsunami as ‘a very large ocean wave that is caused by an underwater earthquake or volcanic eruption and often causes extreme destruction when it strikes land’. Contrary to popular belief, a tsunami doesn’t consist of a single wave, rather it is a succession of large waves (a ‘wave train’). Tsunamis have such a big wavelength that waves are often between 15-60 minutes apart – resulting in more deaths in some areas where residents believe the ‘tsunami’ to have ended, causing them to return before the tsunami has actually finished which leads to a larger loss of life. Tsunamis are often caused by a single earthquake, but they never consist of a single wave.
How are these earthquakes caused?
The Earth’s crust is broken up into plates, of which there are two types – ‘continental plates’ and ‘oceanic plates’. Convection currents in the mantle* cause these plates to move – these movements are called ‘plate tectonics’ and they can cause earthquakes and volcanoes at plate boundaries.
This diagram from geobytesgcse.blogspot.com shows the different plates and the boundary types. The destructive boundaries, indicated on the diagram using lines with triangles attached, are where earthquakes occur.
How do tsunamis occur?
1.) When convection currents cause an oceanic plate and a continental plate to collide, the denser oceanic plate is usually forced underneath the continental plate, causing subduction*.
2.) When the oceanic plate subducts, it pulls the continental plate down with it.
3.) When the continental plate can’t be pulled down any further, it ‘snaps back’. This causes an earthquake; the enormous release of pressure then moves the sea water upwards, causing the creation of a ‘wall of water’.
4.) This ‘wall’ is forced downwards by gravity, causing it to split in half and lead to the creation of two colossal waves, travelling in opposite directions.
These waves can reach up to 98ft in height and travel in excess of 550 mph, which can cause widespread destruction such as that seen in the Indonesian islands after the 2004 tsunami struck. The biggest tsunami ever recorded occurred in Lituya Bay, Alaska on July 9, 1958. It measured 1,719ft (524m) in height – almost 300ft higher than the Empire State Building.
Where do tsunamis normally occur?
According to National Geographic, 80% of tsunamis occur around the Pacific Plate (the ‘Ring of Fire’) – an area where tectonic activity occurs more frequently than other areas, resulting in a higher volume of earthquakes, volcanoes and tsunamis. Tsunamis usually occur at ‘destructive’ plate boundaries where earthquakes are common and plates move towards each other, leading to subduction.
(*Mantle – The region of the earth’s interior between the crust and the core, believed to consist of hot, dense silicate rocks
Subduction – The sideways and downward movement of the edge of a plate of the earth’s crust into the mantle beneath another plate
Source: google.co.uk [search terms ‘mantle definition’, ‘subduction definition’])
