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 60 km (37 mi) east–west and the north–south distance is about 125 km (78 mi). 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 150 km3 (36 cu mi) cone and 150 km3 (36 cu mi) 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.
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