destructive

• Where convection currents push plates together, destructive plate boundaries (margins) are formed.

  Volcanoes form here in two settings where either an oceanic plate descends below another oceanic plate or an oceanic plate descends below a continental plate.
• • As the plates collide, the oceanic plate is forced beneath the continental plate. This is known as subduction. This happens because the oceanic plate is denser (heavier) than the continental plate.
• • When the plate sinks into the mantle it melts to form magma. The pressure of the magma builds up beneath the Earth's surface. 
• • The magma escapes through weaknesses in the rock and rises up through a composite volcano. The volcanic eruptions are often violent, with lots of steam, gas and ash.
• • If two continental plates collide, neither can sink and so the land buckles upwards to form fold mountains. This is called a collision margin.