Subduction is the process in which one plate is pushed downward beneath another plate into the
underlying mantle when plates move towards each other. The plate that is denser will slide under
the thicker, less dense plate. Faulting occurs in the process. It is the process in which rocks break and move or are displaced along the fractures. The subducted plate usually moves in jerks, resulting in earthquakes. The area where the subduction occurs is the subduction zone. A long, narrow, deep depression forms in this area. It is called an oceanic trench.
(Fig 1.12) Subduction forms
The jerky movement, as well as the friction between the plates causes much heat, and together with
the heat from the mantle and from radioactive decay, causes the subducted plate to melt. Magma is produced by the melting plate. It rises through fractures in the crust and reaches the surface to form
volcanoes. In the end, the heat may be so intense that large areas of the crust are melted, forming granites just below the surface. The boundary of such plate movements is 'destructive' as part of the
plate melts during subduction and is destroyed.
As plates move towards each other, the opposing force between them is so great that the massive folding or the bending of rocks also occurs at the edges of the less dense plate. A range of fold mountains, such as the Himalayas, is formed.