Plate boundaries

Constructive boundaries occur when two plates move apart which causes new oceanic crust to be formed and mid-ocean ridges are created by the build-up of molten rock on the seafloor due to the mantle building up. This new crust pushes the old crust aside and forcing the plates apart at a very slow rate. Basaltic lava rises which means new crust is being added continuously. Rift valleys are formed at constructive boundaries, where convection current causes the crust to be pulled apart causing fractures to appear. As the crust pulls away the pieces of land fall and slot downwards. As the land has been pulled apart and has fallen down it has formed a rift valley. Destructive boundaries are between two plates that are colliding. One type is where the oceanic plate collides with a less dense continental plate. As the oceanic plate is subducted into the upper layers of the mantle, various features form. Subduction causes friction to be created by the descending slab of ocean floor, generating a lot of heat leading to a partial melting of a plate. The basaltic magma from this old ocean floor is less dense than the magma of the mantle and rises through fissures and by stoping its way through an overlying rock until it reaches the surface, where it erupts as a volcano. Deep ocean trenches are found along the seaward edge of destructive plate boundaries. They mark where one plate begins to descend under another and reach great depths. On the descending plate there is the Benioff Zone which is the location for earthquakes. Rocks scraped off the descending plate and folding of continental crust help to create young fold mountain chains on the leading edge of the continent for example the Andes on the West Coast of South America. Another type is where the oceanic plate collides with another oceanic plate. The less dense plate slides under the more dense plate creating a Subduction called a trench. For example, in the Pacific Ocean the Pacific plate is subducted under