Geology And Geophysics

Plate Tectonics

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The theory of plate tectonics and continental drift was first put forward by Alfred Wegner in 1912. Previous to this there had been various paradigms, such as the idea that the earth was flat and the belief that earthquakes and related tectonic activity were “acts of God”. Wegner however, concluded that the earth is in fact made up of different layers and that above these layers you have continental sized pieces of rock (layman’s guide to plates) that move slowly but surely around the surface. He stated that the earth was once home to a giant “supercontinent”, which he called Pangaea, comprising of what we know today as continents such as Africa and America and all the other continents. It was after this, he said, that Pangaea split into two, forming Laurasia in the north and Gondwanaland in the south before subsequently splitting again and drifting to form the land masses we are so familiar with today.

Wegner was originally slated for this theory, hearing himself called a “madman” and a renegade of lies. However, it wasn’t until after his death that his theory (and it is important to remember that, even though it is spoken of as fact, it is still a theory) was re-established as the most credible available with regards to explaining tectonics features. This is because of the obvious scientific and geological evidence available to us. For example, the fact that fossil remains of the same reptile the messosaurus have been found in both Africa and South America says it all. It is highly unlikely that the same reptile developed separately or that the crossing of the Atlantic ocean was included in its migration patterns.

The main problem came for Wegner when he was actually asked how the plates came to move. Unlike Alfred, science has come to what it think is a viable hypothesis. It is thought that hot spots (areas of intense heat) around the earth’s core generate convection currents which rise and spread, before cooling and eventually sinking, much like the heat waves from a radiator travel around a room. This produces a circular motion that moves the plates away from and towards each other. The process is continuous and explains the way in which many of our planet’s greatest features are formed. For example, the formation of fold mountains as two continental plates collide and the creation of the mariana trench where two oceanic plates met.

The theory was taken further when science embarked on explaining why certain plates will usually behave in a particular way. For instance, if an oceanic plate and a continental plate collide, the oceanic will always be forced beneath (subducted) the continental. In years gone by it wasn’t know what caused this. Now however, we have worked out that oceanic plates are a lot denser than continental ones, meaning that, even though the continental plates are a lot thicker (at around 30-70 km to the oceanics’ 6-10 km), oceanic plates are always the ones who are forced downwards. Another example of scientific advances with regards to plate tectonics is the understanding of vulcanicity and how it is related to tectonic activity. We now know why you get volcanoes at some plate boundaries and none at others. It is relatively simple; where there is no subduction (the downwards movement of a plate), there are no volcanoes. This is due to the fact that as a plate moves closer to the earth’s core during subduction the surroundings become and hotter and localised melting of the plate into magma occurs in an area of the subduction zone named the “benioff zone”. From this, magma rises to the surface, eventually breaking through and forming volcanoes. Take two continental plates colliding for example. Continental plates have a much lower density than the underlying layers, meaning that they cannot be pushed under. Because of this, neither plate is subducted, resulting in no volcanoes.

There are many who are under the impression that the theory of plate tectonics is fact. It is not. However, in reality, although still a theory, it is a theory that is taken as fact, so much to the extent that land use planning and building construction in many countries often takes into account plate boundaries and how they can affect the local population. Although unproven, it is a theory that only an imbecile would disregard. With more and more megacities being created each decade, it is only a matter of time before an earthquake causes more destruction than has ever been recorded and we start to appreciate the awesome power of plate tectonics.  

More about this author: John Stephenson

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