The North Island Volcanic Plateau (often called the Central Plateau and occasionally the Waimarino Plateau) is a volcanic plateau covering much of central North Island of New Zealand with volcanoes, lava plateaus, and crater lakes. It contains the Taupō caldera complex, Ōkataina caldera complex and Tongariro Volcanic Centre resulting in it being currently the most frequently active and productive area of silicic volcanism on Earth. New Zealand is part of the Pacific Ring of Fire.
Location and description
The plateau is approximately east–west and the north–south distance is about .
Extensive ignimbrite sheets spread east and west from the Central Taupō Volcanic Zone, centred on the huge active supervolcanic caldera of Lake Taupō, now the largest lake in New Zealand. This last erupted less than 2000 years ago. The volcanic area includes the three active peaks of Mount Tongariro, Mount Ngauruhoe, and Mount Ruapehu in the south, and extends beyond Rotorua in the north reaching almost to the Bay of Plenty coast. North of Tongariro is the volcanic Mount Pihanga. The western boundary of the plateau forms an escarpment beside the Mamaku and Kaimai Ranges, but further south is less distinct. In the east, the plateau runs up to the foot of the Ahimanawa, Kaweka and the fertile Kaimanawa mountain ranges. West of the plateau stands the volcano Taranaki Maunga (in Egmont National Park).
Two of the country's longest rivers, the Whanganui and the Waikato have their headwaters on the plateau.
Geology
The tallest mountain, Ruapehu, is an andesite cone that was somewhat higher but has been eroded with a cone and ring-plain. It is typical of the Tongariro Volcanic Centre, whose landforms are defined by the intermediate properties of its andesite magma, having a moderate amount of silica, making it more viscous than basalt, but much less viscous than rhyolite. Andesite magma in effusive eruptions cools to form dark grey lava if gas-poor or scoria if gas-rich. The explosive eruptions tend to be small or moderate sized phreatomagmatic eruptions where the hot magma turns water into steam and the magma into ash. Northwest of Ruapehu is Hauhungatahi, the oldest recorded volcano in the south of the plateau. The other two most prominent volcanic mountains are Tongariro and Ngauruhoe, part of a single composite stratovolcano, like Ruapehu, made up of layers of lava and tephra.
The ring plain around the central volcanoes is formed from numerous volcanic deposits from slope failure or eruptions, and lahars or mudflows that occur when either the crater lake water is ejected from Ruapehu during an eruption, or snow melt is released by failure of the crater walls such as failure of an ice dam, separate from an eruptive event. In the case of lahars, they run down the mountain, picking up ash, lapilli, blocks and bombs as well as previously eroded rock, forming a thick, dense mud that is capable of picking up car-sized rocks. An old lahar flow is visible from the approach road to Whakapapa ski field. On Christmas Eve 1953 a sudden lahar at night on Mt Ruapehu knocked out the rail bridge at Tangiwai, causing six carriages of the Wellington–Auckland express to plunge into the Whangaehu River, killing 151 people in New Zealand's worst railway disaster.
The percentage of silica is the deciding factor in the thickness or viscosity of the magma and this increases to the north and east of the Tongariro Volcanic Centre, where there is a landscape of calderas and volcanic domes making up the Taupō caldera complex (Central Taupō Volcanic Zone) and Okataina caldera complex (Okataina Volcanic Centre). These produced much of the ignimbrite sheet deposits of the plateau. Ignimbrite is formed from very viscous rhyolite magma which is rich in silicon, potassium, and sodium. It is the gas-rich material ejected from the ground at high speed in an explosive caldera-forming event, creating a giant white hot fountain up to 55 kilometres high. When the ignimbrite falls it flows outwards at high speed (600–900 km/h) and can travel many kilometres, even going up and over hills and mountains before it cools, often trapping air inside. Especially around Lake Taupō the eroded ignimbrite includes pieces of quickly cooled pumice, a lightweight rock that floats on water. The top, unwelded layer of the ignimbrite sheet is relatively soft and easily eroded by streams and rivers, which transport the pumice to the lake. The lower, welded layers of the ignimbrite sheets are more dense, though still porous. These lower compressed layers often show the glint of heated silicon specks. North of the plateau this stone, called Hinuera stone (after a quarry of origin), is produced for use for building cladding. An historic eruption at the north west edge of the plateau at Mangakino about 1 million years ago produced ignimbrite that away in Auckland is up to thick.The last massive eruption in the area was the Hatepe eruption from the Lake Taupō volcanic complex 1800 years ago, which ejected lava and especially wind-blown tephra over a wide area to the north and northeast of the North Island. About of material was ejected in the space of a few minutes, in what is thought to be New Zealand's largest eruption in last 20,000 years, but the entire eruption sequence ejected over in a little longer period which might have lasted several years. As is typical after such degassing events which are separated in considerable time, dome building by effusive eruption of low gas viscous rhyolite magma then occurred resulting in this case the formation of the Horomatangi Reefs about 200 years later. Another example in Lake Taupo of such a volcanic dome is Motutaiko Island and towards the north west of the plateau is the lava dome of Mount Tarawera which was split in half by a later relatively small explosive eruption as recently as 1886 with the greatest loss of human life documented from an eruption in New Zealand history. A subsequent risk assessment suggests the Tarawera containing Okataina caldera complex as the highest risk volcanic field in New Zealand to man.
The plateau is entirely above 200 metres, with much of the part south of Lake Taupō more than 800 metres above sea level. This reflects that the underlying marine sedimentary deposits and subsequent volcanic deposits related to the Taupō Rift have been raised by up to a maximum of during the last 5 million years in the central North Island. Such a process of surface uplift in an extensional regime is believed to be related to the continental nature of the underlying plate.
Climate The plateau has a mild climate although the temperature regularly falls below freezing in winter and snow can fall between March and October. Of all North Island regions, the Volcanic Plateau is closest to having a continental climate. The period of maximum snow is July to September. Sudden blizzard conditions can be experienced on the volcanoes in winter. Such conditions have killed climbers as recently as 2019. The Desert Road on State Highway 1 is often closed by snow in winter for brief periods.
Flora
The natural vegetation of the area includes the podocarp conifers including rimu (Dacrydium cupressinum), miro (Prumnopitys ferruginea), mataī (Prumnopitys taxifolia), tōtara (Podocarpus totara) and kahikatea (Dacrycarpus dacrydioides) and hardwoods that once covered this whole area before the eruption of Taupō and other volcanoes. To the west of the three large volcanoes, the land is rough forested hill country, while the north close to Lake Taupō is more fertile and undulating and much has been cleared for farming. The forest used to extend to the west coast and there are still patches of natural vegetation in wetlands including the Horowhenua floodplains.
However to the south of Taupō the barren, ash-laden soils and harsh alpine climate leave the high land largely bare and unprofitable, capable of growing only scrubby plants. This area is known as the Rangipo Desert although it is not a true desert as the annual rainfall is over 1,000mm PA and the real reason for the low and sparse vegetation was the mass sterilisation of seeds caused by the 26,000-year-old Taupō eruption, which swept white-hot ignimbrite through this valley. Prior to this cataclysmic event this area was forested and stumps of charcoalised trees can be seen where soil has eroded. Around Taupō itself there are geothermal areas with specialised plant life such as the prostrate kanuka (a subspecies of kunzea ericoides).
Further to the east of the plateau lies more rough hill country in the Kaimanawa Ranges, a popular hunting area for wild red and sika deer. The tops of the mountains are open and tussock covered. Helicopters and light aircraft can land on a rough air strip. Hunting range in the bush is often 50 m or less but in the open tops the range can be much further. The area to the south, around Lake Moawhango is richer in plant life. Finally there are areas of beech forest and alpine vegetation at higher elevations throughout the region.
Fauna The northern side of the plateau especially is home to wildlife including birds such as the North Island kōkako wattlebird (Callaeas wilsoni), New Zealand kaka, and large populations of brown kiwi (Apteryx australis) and blue duck (Hymenolaimus malacorhynchos). The area to the south of the plateau and down to Wellington is home to a variety of lizards including Whitaker's skink (Cyclodina whitakeri), forest gecko (Hoplodactylus granulatus) and Wellington green gecko (Naultinus elegans punctatus). Wetlands throughout the region support waterbirds and fish including the endemic brown mudfish.
Threats and preservation The lowland areas are quite highly populated and therefore urban development, agriculture and forestry are a major threat to natural habitats there. One block of lowland forest remains in and around Whanganui National Park and large areas of the uplands are protected either by Maori tradition or in national parks of which Te Urewera is the largest. The four North Island national parks, which surround the plateau, are Te Urewera, Whanganui, Egmont and New Zealand's first national park, the World Heritage listed Tongariro National Park. Introduced species and livestock grazing are now major threats to some endemic species. Rats, cats and stoats prey on native reptiles and birds (including the kokako, blue duck and brown kiwi), while imported plants such as old man's beard, banana passionfruit and gorse can take over from indigenous species. Finally grazing by goats and possums can eradicate others such as kamahi, kaikawaka (Libocedrus bidwillii) and mountain tōtara. Therefore, smaller islands off the coast of Wellington have been used as a refuge for North Island birds and reptiles whose habitats may be threatened now or in the future.