Processes that produce an extreme natural event
A process is a term that is used to describe something (it is usually a series of inter-related
actions) that causes a change in a system. These words usually end in -ing or -tion.
There are five processes that combine together to produce a tsunami. Any individual process
on its own will not cause a tsunami, all need to come together to create this phenomena.
Subduction
The Earth's crust is made up of tectonic plates. These are basically giant slabs of rock that
float on top of magma (superheated rock that is a liquid). The heat from this magma causes
currents and this causes the plates to move. It's a little like at a swimming pool that has its
surface covered in foam mats, the mats move against each other as the water below moves.
Where the plates meet is called a plate boundary. At the plate boundary several types of
movement can occur:
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the plates can slide along each other (transform boundary)
the plates can move away from each other (divergent boundary)
one plate is forced below the other plate (subduction)
The last boundary is the one we are interested in.
In the Boxing Day tsunami there are two tectonic plates that collide; the Indo-Australian plate
and the Burma plate. The Burma plate is the lighter of the two, so moves over the top of the
heavier, denser Indo-Australian plate. This plate boundary is located about 250 km off the
coast of Sumatra, Indonesia. For the last 150 years there has been no movement between
these two plates and so pressure has been building up. On average these plates should move
about 6cm a year.
Subduction Zone
In this diagram the plate that is diving below the other is the Indo-Australian plate.
Earthquake Formation
When two tectonic plates are pushing against each other pressure builds up. Eventually one
plate moves against the other and pressure is released. This movement is called an
earthquake. Where this occurs is known as the epicentre and shock waves move out from this
point.
On 26 December, 2004 the Indo-Australian plate slipped 90cm under the Burma plate,
resulting in a 9.3 magnitude earthquake. The epicentre was 30 kilometres below the surface
of the Indian Ocean. The earth shook for 8 minutes.
Shock waves
This image shows the epicentre for the Boxing Day tsunami and the resulting shock waves
that radiated out from it.
Tsunami Formation
When there is a sudden upheaval on the sea floor water is displaced (just like throwing a
stone into a pond), this water creates large waves that can travel huge distances at speed. This
is the tsunami.
During the earthquake that triggered the Boxing Day tsunami the Burma Plate was shunted
up almost a metre. This displaced billions of tonnes of water vertically. This radiated out and
formed a series of waves. Out in the middle of the Indian Ocean the waves were barely
noticeable, even though they were travelling at over 800km per hour. When they reach
shallow water is when people realised the devastation that was about to be unleashed.
Water displacement
Think of this like doing a bomb in a swimming
pool at the deep end and the resulting wave that
radiates to the shallow end of the pool. If the
pool is shallow enough the 'waves' that are
created get bigger.
Wave Formation
When you are out at sea and the weather is calm you do not sea waves like you do at the
beach, all you might notice is a gentle swell. As the water reaches the shallower water near
the coast the wave crest starts to develop. The wave energy starts to concentrate and the wave
starts to increase in height. This is the reason that during a tsunami the water at the coast gets
sucked out as the wave sucks in more and more water before moving ashore.
Those places closer to the epicentre get hit by the tsunami first and then as it radiates out
further away places get struck. The waves in the Boxing Day tsunami took just 30 minutes to
reach Sumatra. What had been a 30cm wave out in the ocean suddenly became a towering 20
metres as they crashed to shore. Next to be hit was Thailand, it is estimated that a thousand
tonnes of water came ashore in Thailand. Next came Sri Lanka and then India, the Maldives,
Somalia and Kenya.
Wave Creation
While this diagram uses the Samoan tsunami to illustrate the point, it is exactly the same for
all tsunamis.
Cultural Processes
This process is about the presence of people who live near coastal areas that are prone to
tsunamis. This is not a blame situation - people have to live somewhere and there is virtually
nowhere that is not exposed to some for of extreme natural event.
Population
While this map does not show the entire region hit by the tsunami, it does give you an idea of
how populated some of the countries are.
Construct a flow diagram to show the steps that led to the Boxing Day Tsunami