Volcanic and Seismic events are major pieces of evidence towards proving that plate tectonics theory is valid (40 marks) The theory of plate tectonics states that the Earth’s lithosphere (top layer of the Earth’s crust) is split up into rigid sections called plates that are moving relative to one another as they move on top of the underlying semi-molten mantle. These plates are either continental, The North American Plate, or oceanic, The Nazca Plate.
Tectonic plates are powered by convection currents, which is the circular movement of magma within the mantle. These currents are powered by the core, which heats the magma, causing it to rise, cool and fall back down. This circular motion causes the plates, which float on the mantle, to move. The individual plates are separated by fault lines which extend from the surface all the way to the asthenosphere, at which point the fault disappears due to the plasticity of the rock that exists there. The earth’s plates are moved by convection currents in the mantle caused by localised concentrations of radiation and heat from the earth’s core. The plates are constantly in relative motion to each other, but this motion is extremely slow due to the incredible amount of mass being moved, friction, and the high viscosity of the asthenosphere. Where the different plates meet is called a plate boundary. There are three types of plate boundaries and they are; Destructive margins, Constructive margins and Conservative margins. At a destructive margin the plates are moving towards each other. This usually involves a continental plate and an oceanic plate. The oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. The point at which this happens is called the subduction zone. As the oceanic plate is forced below the continental plate it melts to form magma and earthquakes are triggered. The magma collects to form a magma chamber. This magma then rises up through cracks in the continental crust. As pressure builds up, a volcanic eruption may occur. As the plates push together, the continental crust is squashed together and forced upwards. This is called folding. The process of folding creates Fold Mountains. Fold Mountains can also be formed where two continental plates push towards each other. This is how mountain ranges such as the Himalayas and the Alps were formed.
At a constructive boundary the plates are moving apart. The plates move apart due to convection currents inside the Earth. As the plates move apart (very slowly), magma rises from the mantle. The magma erupts to the surface of the Earth. This is also accompanied by earthquakes. When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock. This process is repeated many times, over a long period of time. Eventually the new rock builds up to form a volcano. Constructive boundaries tend to be found under the sea, where the Mid Atlantic Ridge was created. Here, chains of underwater volcanoes have formed along the plate boundary. One of these volcanoes may become so large that it erupts out of the sea to form a volcanic island, eg Surtsey and the Westman Islands near Iceland.
At a conservative margin two plates try to slide past each other slowly. Quite often, the two plates stick and pressure builds up; the release of this pressure creates a severe earthquake. There are no volcanic eruptions along conservative plate margins because the crust is neither being created nor destroyed. The San Andreas Fault in California lies above the North American and Pacific plates, and is an example of a conservative plate margin.
Volcanoes can be created on the plate boundaries such as the destructive and the constructive. A volcano is created when magma rises through cracks or weaknesses in the Earth's crust. Pressure builds up inside the Earth. When this pressure is released, maybe as a result of plate movement,...
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