Story: Volcanic Plateau region
Page 2 – Geology and climate
An unstable land
Beneath the Volcanic Plateau, the Pacific Plate has been sinking beneath the Australian Plate for the last 1.5 million years. This has led to intense heating, producing volcanic activity, which has found its way to the earth’s surface.
Volcanic and other zones
The eruptive part of the plateau is the Taupō Volcanic Zone. The zone is dominated by three huge calderas (basin-shaped volcanic depressions) – Taupō, Rotorua and Okataina. These were formed by infrequent but massive eruptions of rhyolite, an explosive lava. All three are currently filled with water – Taupō and Rotorua by one big lake each, Okataina by several lakes.
Ruapehu and Ngāuruhoe–Tongariro, though prominent, are much smaller volcanoes. Their cones were built up by small to moderate eruptions of andesite, a fairly stable lava, every 10 to 50 years.
Areas flanking the Taupō Volcanic Zone, including the Kāingaroa plateau, are formed from many sheets of ignimbrite, a rock formed by hot pumice, ash and gas from rhyolitic eruptions surging across the land (pyroclastic flow). Also west of the zone are older volcanic rocks, which form the Mamaku Range and adjacent areas.
The western Hauhungaroa Range, which stands out from the surrounding volcanic rocks, is greywacke, as are the North Island’s main ranges, which bound the plateau to the east.
Swamped by mud
A mud flow at Waihī, on Lake Taupō’s shores, killed over 50 people in 1846, including the Ngāti Tūwharetoa chief Mananui Te Heuheu Tūkino II. A massive lahar (volcanic mud flow) from Ruapehu’s crater lake in December 1953 destroyed a rail bridge across the Whangaehu River at Tangiwai, and 151 people died when a night express train plunged into the river. However, another lahar in March 2007 caused little damage.
Many eruptions have taken place over the last 1.5 million years. In just the last few thousand years, the whole region has been covered in ash from eruptions of the Taupō, Okataina and Rotorua volcanoes. Vegetation was destroyed many times, as buried and charred forests attest. The pyroclastic flows from the massive Taupō eruption around 200 AD would have incinerated everything within an area of 20,000 square kilometres – about a sixth of the North Island – and reached Mt Tongariro within minutes. Its effect on the atmosphere was chronicled by the Chinese and the Romans.
The Tarawera eruption around 1314 AD covered the region with a layer of ash, known as Kaharoa ash.
In 1868, violent earthquakes accompanied the eruption that formed the upper Te Maari crater on Tongariro. In 1886 Mt Tarawera erupted, killing about 150 people and destroying the famous Pink and White Terraces. The Te Maari crater erupted again in 1896–97.
Mt Ngāuruhoe is Tongariro’s youngest vent. It erupted about 70 times between 1839 and 1975, but has been dormant since then. Ruapehu’s 1945 eruptions smothered crops and farmland. Its 1995–96 eruptions closed the ski fields, which had a big economic impact on local communities. Ash fell as far away as Northland and Marlborough.
Geysers, hot springs and mud pools
Throughout the Taupō Volcanic Zone, the ground is heated by magma (molten rock) close to the surface. Water is superheated, far above the normal boiling temperature of 100°C. The most active geothermal field is at Whakarewarewa in Rotorua city, with more than 500 hot springs, and seven geysers aligned north–south along a buried fault.
All the major Rotorua lakes are products of volcanic activity. Lake Rotorua (279 m above sea level) is one caldera, and is the area’s biggest lake at 80 square kilometres. Lakes Tarawera (41 square kilometres), Okataina, Okareka and Rotomahana occupy the Okataina caldera along with part of Lake Rotoiti.
Lake Rotoiti (34 square kilometres) is the third largest Rotorua lake, and has a maximum depth of 93.5 metres. Lakes Tarawera and Rotomahana have maximum depths of 87.5 metres and 125 metres.
The Waikato River has eight lakes formed behind man-made hydroelectricity dams – Lakes Aratiatia, Ōhakuri, Ātiamuri, Whakamaru, Maraetai, Waipapa, Arapuni and Karapiro.
Of all North Island regions, the Volcanic Plateau is closest to having a continental climate. Summers are hot and winters cool. Rotorua and Taupō are the highest-altitude urban centres on the island, at 279 metres and 369 metres respectively. Taupō has 69 ground frost days a year – the most of any North Island town – and a July (winter) mean temperature of less than 7°C.
Taupō is drier than Rotorua, with an average rainfall of 960 millimetres a year compared with Rotorua’s 1,341 millimetres. But the plateau generally is dry relative to the mountains – much more rain falls on the Kaimanawa and Mamaku ranges and the central volcanoes. In winter, snow falls on the summits of the central volcanoes and the Kaimanawa Mountains, sometimes closing the Desert Road between Rangipō and Waiōuru.