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Earthquakes
Factfile |
 Earthquakes |
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Where Earthquakes Happen
To understand why an earthquake happens, we must understand something of the inside of our Earth. If you were to peel away the layers of our Earth, you would come across 3 distinct layers: The crust, which is the land we stand upon, as well as the land underneath the oceans. The crust is broken up into a number of large pieces called tectonic plates, which are slowly, constantly moving top, liquid layer of the mantle. Beneath the crust is the mantle, and inside this layer is the core, the centre of the Earth.
The core is an incredibly hot solid ball of nickle and iron - its thought to be about 4,500ºC. and despite being so hot, it remains solid because of the intense pressure from the other layers, pushing down on it. However, heat from the core rises up through the Earth to the mantle. Here, in this layer the intense heat has caused the rocks to melt to a sticky liquid (magma) and through a process called convection, the molten rock keeps moving, just as water does when it is heated in a saucepan. It is on the molten material of the mantle that the tectonic plates float or move very slowly.
However, these plates meet at plate boundaries or margins and can slide over, under or past each other with the following results:
Diverging Plates - When two plates move away from each other - magma oozes out from the mantle and forms new plate at their edges. E.g. Mid-Atlantic Ridge
Leads to volcanic activity. For more information see the volcanoes factfile
(Earthquakes are common here but are usually very minor)
Converging Plates - When two plates move towards each other - plate destroyed - E.g. West coast of South America
Leads to Earthquakes and Volcanoes. For more information see the volcanoes factfile
Transform Plates - When two plates slide past each other (either opposite directions or at different speeds) - plate isn't lost or gained - E.g. San Andreas Fault (West coast of North America)
Leads to Earthquakes
San Francisco After Earthquake
Photograph San Francisco Museum Collection
The main risks to earthquakes are concentrated areas along the edges of the plate boundaries. However, scientists now believe that 5% of quakes happen in places that are in the middle of plates because the rocks at these places were put under pressure millions of years ago. This pressure has never gone away and over millions of years, became locked into the rocks, like a `memory'. Then, when their balance has been disturbed, this trapped pressure has been released and a mid-plate earthquake is the result. This is how scientists have explained earthquakes such as the New Madrid, Missouri, quake of 1812 in the USA.
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