Describe the Theory of Plate Tectonics and Explain Three Prices of Evidence Which Provides Support for the Theory

Describe the theory of plate tectonics and explain three prices of evidence which provides support for the theory (40 marks)

The theory of plate tectonics was born in the late initiated by Wegeners idea of the reality of a single continent, Pangea 300 million years ago. It was developed by Wegener who published his theory of continental drift and claimed that it was supported by several pieces of evidence that Pangea was once singular. The theory of Plate tectonics includes the Location of tectonic plates, tectonic plates Characteristics, convection currents and the movement of plates. The evidence for this theory contains the processes of seafloor spreading, Paleomagnetism and Rock Sequence deposits

The first aspect of the Plate tectonic theory is to do with the Location of the plates. Before the development of the plate tectonic theory, earth scientists divided the interior of the Earth into layers; the crust, the mantle and the core. The theory has retained this simple threefold division, however new research has suggested that the crust and the upper mantle should be divided into the lithosphere and the asthenosphere. The lithosphere consists of the crust and rigid upper section of the mantle and is approximately 80-90 km thick. The upper section of the mantle is divided into seven very large plates and a number of smaller ones. The Plates are developed into two categories, oceanic and continental, depending on the type of material they are made of.

The theory expands to show that there are two types of tectonic plates; continental crust (sial) and oceanic crust (sima). However, these terms do not refer to actual continents and oceans, but to different types of rock. The continental crust has a high thickness of 35-60 km due to mountain chains building the thickness, however the oceanic crust is substantially thinner by inly being 6-10km on average. The age of the continental crust is over 1500 million years; however, the age of the oceanic crust is less than 200 million years ago. Another characteristic that sets the Sial and Sima apart is the weight of rocks. The Continental crust is a lot lighter because it is built of granite, which is rich in silica and aluminium), with an average density of 2.6 leading to an inability to sink into mantle. However, the oceanic crust is made of a much heavier rock, basalt, rich in silica and magnesium. It has a density of 3.0, and therefore sinks down into the mantle. The final dividing characteristic is the nature of the rocks that built the crusts. The Continental Crust is mainly formed by Granite; however, the Oceanic crust is mainly formed by Basalt.

Additionally, Plate tectonic theory also inhibits the cause of plate movement: convection currents. Convectional currents are a circular pattern driven by the rising hot material and/or sinking material. The hot material has a low density, therefore rising, and the cold material has a high density, therefore sinking. The direction of the convectional currents will circulate the different ways of tectonic movement, constantly driven by heat and always creating equilibrium.

The fourth part of the theory includes the type of plate boundaries, that occur when 2 tectonic plates meet. These are where mountains, volcanoes and oceanic trenches are formed and earthquakes are likely to form there. The first type of plate boundary is called ‘constructive’. This is when 2 plates (often the same types) are moving in opposite directions (diverging), due to the high pressure magma underneath causing the plates to move apart as the magma moves up and seals the gap – oozing out of the top. This then cools and dries forming more land, an example of which is Iceland. The destructive plate boundary is opposite to this, and is formed by 2 different types of plates converging (oceanic and continental). The oceanic plate is heavier, causing it to sink underneath the continental plate, whilst heating up the plate and the material underground due to friction. A magma chamber is then formed between the 2 plates, eventually filling and pushing its way up through the cracks. The magma can be there for years building up, and will then begin to bulge before erupting its volcano. These volcanoes will have ‘sticky’ magma, making it more powerful and meaning the magma sticks to the side of the volcano. They will often blow themselves up, then reform, then blow themselves up again. A conservative type of plate boundary is made when 2 plates are moving side by side, in the same or different directions. When they are moving, they can get stuck on each other, which leads to the force building up until the energy is released as an earthquake. They cause no volcanoes, just earthquakes when they move. The final type of plate boundary is called a collision plate boundary. This is where two plates of the same type collide e.g. oceanic. Neither one sinks, as they have the same density. When they meet, they both push up at the same time, forming mountains e.g. Himalayas/ M ount Everest. They are also known as fold mountains. This movement is continuous, meaning they continue to grow.