The last two million years (Quarternary Period) have been marked by violent volcanic activity in the North Island, where pockets of molten rock have welled up to the surface.
The largest and most violent volcanoes in New Zealand are not cone-shaped mountains; they are huge basin-shaped volcanic depressions known as calderas. New Zealand has a number of these super-volcanoes, including the Taupō, Rotorua, and Okataina calderas. Created by repeated catastrophic eruptions during the last 1.6 million years, some calderas are now occupied by lakes, such as at Taupō and Rotorua.
During caldera eruptions, magma is blasted out largely in rapid flows of incandescent ash, pumice and gases. When the material comes to rest, it forms a rock known as ignimbrite. Plateaus of ignimbrite hundreds of metres thick surround the calderas. Ash from their eruptions has spread for thousands of kilometres – for example, ash from the Taupō caldera is found in the Chatham Islands.
Most of New Zealand’s geothermal areas, such as Rotorua and Waimangu, lie within the calderas. Deep molten magma provides the heat that keeps the geysers, hot springs and mud pools bubbling.
Andesitic volcanic cones
The major active volcanoes of the North Island include Mt Taranaki (Mt Egmont), the peaks of Tongariro National Park (Tongariro, Ngāuruhoe, Ruapehu) and White Island. Eruptions began in the Taranaki area around 1.7 million years ago. The volcanoes of Tongariro National Park have been built largely during the last 260,000 years.
Auckland – a city on volcanoes
Auckland is built on a volcanic field that has been active as recently as 600 years ago. Scattered through the city are dozens of volcanic cones. The oldest, Maungataketake, is about 50,000 years old, and the most recent volcano is Rangitoto Island. The eruptions that built each cone have been short-lived, spanning perhaps as little as 10 years.
The numerous eruptions that have spread volcanic ash far and wide over the New Zealand landscape during the Quaternary period have created unique time markers. Volcanic ash (tephra) from individual eruptions can be identified by their distinctive compositions and dated by radiometric methods. The ash layers can often be found within beds of other sediment – any sediment on top of the ash must have been deposited after the ash was erupted. Ash layers have been used to date landscape features such as deposits left by ice age glaciers.