At a compressional or destructive boundary 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 [magma chamber: A region under the surface of the Earth where hot molten magma collects. ]. This magma then rises up through cracks in the continental crust. As pressure builds up, a volcanic eruption may occur.
The diagram below shows how the oceanic plate is pushed underneath the continental plate, causing mountains and possibly volcanoes to form along the destructive plate boundary.
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.