Imagine you are standing at the top of Sky City and you are high enough to see big picture New Zealand. You are looking south at the Kaimanawas (east of Taupo), down the Ruahines and the Tararuas. Those mountain ranges form the axial spine of the North Island. Why? Because the Pacific Plate is subducting beneath the Australian Plate. Note the direction of the crust as it plunges into the shallow mantle. It’s dragging the eroded material (detritus) from the volcano back into the melting pot. Two things happen from this. It causes drag and the result is lots of little earthquakes (earthquake swarm). It also forms quite a trench to the seaward side. Think – Hikurangi Basin. Here endeth today’s geology lesson.