Oceanic plate is subducted, plate sinks into the mantle and melts to form magma, pressure of the magma builds up and escapes through weaknesses in the rock and rises up through a composite / strato volcano, volcanic eruptions are often violent, with lots of steam, gas and ash and thick andesitic lava, sinking oceanic plate can 'stick' to the continental plate and when the plates finally snap, the energy released causes violent earthquakes, the shallower the epicentre the greater the energy