Describe the type of tectonic and seismic activity which occurs at each plate boundary
(25 mark)
In the theory of plate tectonics, the earth's crust is broken into plates that move around relative to each other. As a result of this movement, three types of plate boundaries are formed: Divergent, Convergent- Oceanic/Continental & Continental/Continental, and Transform boundaries. Not all plate boundaries fit into the main groups shown above. In some parts of the world the boundaries are not easily defined because the deformation caused by the plates covers a wide area rather than having clearly defined plate edges. This deformation is called a plate-boundary zone. The smaller fragments of plates are called microplates. The destruction of the earth’s crust occurs along plate boundaries, where plates are moving towards each other and the creation of the crust occurs where plates are moving away from each other.
A divergent plate boundary, also knows as an constructive or extensional boundary) is a tectonic boundary where two plates are moving away from one another forming new crust as magma rises from the earth’s surface between the two plates. The boundaries are found along spreading centres -The Mid-Atlantic ridge being an example of a ‘textbook’ slow spreading centre and in contrast the East Pacific Rise is an example of a fast spreading ridge.
Divergent boundaries can be Continental or Oceanic. When a divergent boundary occurs underneath ocean lithosphere, the rising convection currents below lift the lithosphere upwards producing a mid- ocean ridge. Extensional forces stretch the lithosphere and creates fissures which open reducing pressure on the mantle below. It responds by melting and magma flows into the fissure which then solidifies and the process is repeated. The most common example of this Oceanic Divergent boundary is the Mid- Atlantic ridge which