New Zealand’s long, thin, drowned continent forms an unusual-shaped seabed. Even more striking is the manner in which this continent and the surrounding ocean basins are being torn and crushed along the boundary where two of the earth’s largest crustal plates meet. Along this boundary, the Pacific Plate to the east and the Australian Plate to the west grind into and past one another. This has created a line of extremely deep trenches, volcanic ridges and, above the sea, snow-capped mountains.
Kermadec Trench–Ridge system
The Kermadec Trench forms a great gash in the surface of the earth. From the 5-kilometre-deep Southwest Pacific Basin to the east, the trench gradually drops to more than 10 kilometres below the water’s surface. The Kermadec Trench marks where the Pacific Plate meets the Australian Plate. The Pacific Plate bends and cracks as it plunges under the Australian Plate – a process known as subduction.
Lying to the west, and towering higher than Mt Everest above the trench, the Kermadec Ridge rises to 1 kilometre below the ocean’s surface. This ridge marks the edge of the Australian Plate. It has been pushed upward by the Pacific Plate driving beneath it, and has been built up by volcanism.
Hikurangi Trough and Plateau
The Kermadec Trench shoals southward and merges with the 3-kilometre-deep, mud-filled Hikurangi Trough east of the North Island. This is where the unusually shallow oceanic floor of the Hikurangi Plateau is subducting beneath the continental crust of the Australian Plate.
South-west of Fiordland, the Puysegur Trench marks the zone where the oceanic crust of the Australian Plate plunges beneath the Pacific Plate. The topography is not as dramatic as the Kermadec Trench, as the plates largely slide north–south past one another, rather than over and under. The north–south sliding of the Australian and Pacific plates continues to the South Island, where the movement is taken up by the Alpine Fault.
Along the western side of the Kermadec Ridge is a line of mainly submerged volcanic cones, with names like Rumble I, II and III. The volcanoes run from the Kermadec Islands to White Island and ultimately Mt Ruapehu.
Between the volcanoes of the Kermadec Ridge and the Colville Ridge, 100 kilometres to the west, is the Havre Trough. Three kilometres deep, the trough is where the earth is being torn apart between the Kermadec and Colville ridges by molten rock rising above the deeply diving Pacific Plate. The trough continues ashore into the Rotorua–Taupō area of New Zealand, where the land is also being stretched. The sea floor of the Havre Trough is particularly complex, with many ridges and lines of extinct volcanoes serving to record the trough’s formation.
South of New Zealand, volcanic activity is rare. Only the extinct volcanic Solander Island is located near present volatile plate-boundary processes. The Solander Trough is smooth and old, and unrelated to the spreading movement found in basin sea floors.