Due to the pressure occurring in the mantle, magma is forced to the surface causing an eruption . In the case of the Central North Island volcanoes, the Pacific plate is thinner than the Indo-Australian plate. The thinner Pacific Plate is forced down into the mantle. Here the earth's crust gets hotter and hotter as it is pushed further into the mantle. The crust eventually becomes so hot that it is changed to magma.
Pressure builds as more and more magma accumulates.
Eventually the pressure is so great that magma is forced to the surface. This results in an eruption like the 1995 and 1996 Ruapehu eruptions .