New Zealand lies across the junction of the Pacific plate and the Australian plate. North, the Australian plate rides over the Pacific plate. South, the Pacific plate rides over the Australian plate. In the volcanic triangle of the North Island, the country is tearing apart, creating the jutting East Cape in the process. As the Pacific Plate slides under New Zealand's North Island, it melts.
Lighter rocks float up as magma, to create the mighty volcanoes of the active volcanic zone.
These are andesite volcanoes - andesite being light, continental rock. In the active volcanic zone, New Zealand's North Island is splitting and sinking. But the rising magma pushes it up again, so that the volcanic plateau does not become a rift valley. The Hikurangi Trench off the east coast of New Zealand's North Island is only one of the faults lining up with the alpine fault system. There are many parallel faults in the North Island itself which share in the motion at the plate boundary.