Geology of New Zealand

New Zealand straddles the boundary between two tectonic plates. The subduction of the Pacific plate under the Indo-Australian Plate results in volcanism, especially in the North Island's Taupo Volcanic Zone. The associated geothermal energy is used in numerous hydrothermal power plants. Some volcanic places are also famous tourist destinations, such as the Rotorua geysers.

The collision between the two plates causes regular earthquakes, though severe ones are infrequent. These have uplifted the Southern Alps along the Alpine Fault, and the resulting orographic rainfall enables the hydroelectric generation of most of the country's electricity. New Zealand experiences around 14,000 earthquakes a year, some in excess of magnitude 7.

There are karst sedimentary rock formations, and the Waitomo Caves and the Pancake Rocks are recognised tourist attractions.

The geology of New Zealand is noted for its volcanic activity, earthquakes and geothermal areas because of its position on the boundary of the Australian Plate and Pacific Plates.

Gondwana origins
South America, Africa, India, Antarctica, Australia, and New Zealand formed the super-continent of Gondwana. The rocks that now form New Zealand were nestled between Eastern Australia and Western Antarctica.

Separation from Gondwana
New Zealand and New Caledonia represent the visible part of a portion of continental crust, usually referred to as Zealandia. The rest of Zealandia includes the Challenger Plateau and Lord Howe Rise, which stretch from Northwest of New Zealand almost to Northern Australia, and the Campbell Plateau and Chatham Rise, to the southeast of New Zealand.

The Australia-New Zealand continental fragment of Gondwana split from the rest of Gondwana in the late Cretaceous time (95-90 Ma). Then around 83 Ma, Zealandia started to separate from Australia forming the Tasman Sea, initially separating from the south. By 75 Ma, Zealandia was essentially separate from Australia and Antarctica, although only shallow seas might have separated Zealandia and Australia in the north. The landmasses continued to separate until early Eocene times (53 Ma). The Tasman Sea, and part of Zealandia then locked together with Australia to form the Australian Plate (40 Ma), and a new plate boundary was created between the Australian Plate and Pacific Plate. Zealandia ended up at a pivot point between the Pacific and Australian Plates, with spreading in the south, and convergence in the north, where the Pacific Plate was subducted beneath the Australian Plate. A precursor to the Kermadec Arc was created. The convergent part of the plate boundary propagated through Zealandia from the north, eventually forming a proto-Alpine Fault in Miocene times (23 Ma). The various ridges and basins north of New Zealand relate to previous positions of the plate boundary.

Land movement
Because New Zealand straddles a plate boundary, it is tectonically active. The Pacific Plate is colliding with the Australian Plate at a rate of about 40 mm/yr.

The East coast of the North Island is being compressed and lifted by this collision, producing the North Island and Marlborough fault systems. The East Coast of the North Island is also rotating clockwise, relative to Northland, Auckland and Taranaki, stretching the Bay of Plenty, and producing the Hauraki Rift (Hauraki Plains and Hauraki Gulf) and Taupo Volcanic Zone.

The East Coast of the South Island is sliding obliquely towards the Alpine Fault, relative to Westland, causing the Southern Alps to rise about 10 mm/yr (although they are also worn down at a similar rate).

The Hauraki Plains, Hamilton, Bay of Plenty, Marlborough Sounds, and Christchurch are sinking. The Marlborough Sounds are well known for their sunken mountain ranges. As Wellington rises, and Marlborough sinks, Cook Strait is being shifted further south.

A Map showing the distribution of earthquakes in New Zealand can be obtained from Te Ara Encyclopedia of New Zealand.

Because the Pacific Plate is subducting under the eastern side of the North Island, there are frequent deep earthquakes east of a line from the Bay of Plenty to Nelson (the approximate edge of the subducted plate), with the earthquakes being deeper to the west, and shallower to the east.

Because the Australian Plate is subducting under the Pacific Plate in Fiordland, there are frequent deep earthquakes near Fiordland, with the earthquakes being deeper to the east and shallower near the west.

There are few deep earthquakes along the Southern Alps, because the two sides largely slide past each other.

Shallow earthquakes are caused by stresses in the top plate. Shallow earthquakes are more widespread, occurring almost everywhere throughout New Zealand (especially the Bay of Plenty, East Cape to Marlborough, and Alpine Fault). However, Northland, Waikato, Canterbury and Otago are very stable.

Geothermal activity
The Taupo Volcanic Zone is well known for its geothermal activity. For example, Rotorua and the surrounding area have many areas with geysers, silica terraces, fumaroles, mudpools, hot springs, etc. Notable geothermal areas include Whakarewarewa, Tikitere, Waimangu, Waiotapu, Craters of the Moon and Orakei Korako. Geothermal energy is used to generate electricity at Wairakei, near Taupo. Hot pools abound throughout New Zealand.

Geological hazards
New Zealand suffers from many natural hazards, including earthquakes, volcanic and hydrothermal eruptions, tsunamis, landslides, and floods.

The most severe earthquake occurred in a M8.2 earthquake in the Wairarapa, in 1855, and the most deaths (261) in a M7.8 earthquake in Hawkes Bay in 1931. So far New Zealand has been lucky to not have a major earthquake in a large city.

Earthquakes along the East coast of the North Island have the potential to cause tsunami, as do earthquakes in South America.

The most severe eruption since European times is the Tarawera eruption in 1886. However, there are many potentially dangerous volcanoes in the Taupo Volcanic Zone. Even a minor eruption at Ruapehu could cause the loss of electricity for Auckland, due to ash on the power lines, and in the Waikato River (stopping the generation of hydroelectric power).

Much of the North Island is steep, and composed of soft mudstone, that easily generates landslides.

Parts of New Zealand, especially Northland, Coromandel Peninsula, Bay of Plenty, Gisborne, and the Manawatu can have high rainfall that can generate flooding of farmland.
Geological resources
The only area in New Zealand with significant known oil and gas deposits is the Taranaki area, but many other offshore areas have the potential for deposits.

Iron sand is plentiful on the west coast around the coast from Taranaki to Auckland.

Gold has been mined in the Coromandel and Kaimai Ranges (especially the Martha Mine at Waihi), Westland, Central Otago, and Eastern Otago (especially Macraes Mine).

Geothermal energy is used to generate electricity in the Taupo Volcanic Zone.

Coal has been mined in Northland, the Waikato, Taranaki, Nelson and Westland, Canterbury, Otago, and Southland. The West Coast contains some of New Zealand’s best bituminous coal. The largest coal deposits occur in Southland.