New Zealand lies at the south-west end of a vast horseshoe-shaped zone of intense volcanism and earthquakes. This zone extends, essentially unbroken, around the margins of the Pacific Ocean – the so-called Pacific Ring of Fire.
This immense belt of volcanic and earthquake activity corresponds closely with the edge of the Pacific tectonic plate, and also coincides with some of the most densely populated regions on the planet. More than half of the world’s active volcanoes above sea level are found in this zone.
New Zealand sits astride the colliding edges of the Pacific and Australian plates. The occurrence of earthquakes, jagged mountain ranges and volcanoes through New Zealand, and the contrasting geology and landscapes of the North and South islands, can all be explained by the different ways in which the edges of the two colliding plates are interacting along the length of the country.
When tectonic plates collide generally one of two things happen: either one plate slides beneath the other, curving back down into the mantle (a process called subduction), or the edges of the plates are crumpled and forced up, forming a wide zone of mountain uplift. Subduction occurs where one of the interacting plates is thin and dense and is forced beneath thicker and more buoyant crust.
As the subducting plate slides down into the earth’s hot mantle, the crustal rocks are heated and water and other volatile elements are boiled off. The chemical effect of this water is to lower the melting point of rocks in the solid mantle above the subducting plate, allowing magma to form. This buoyant molten rock rises to the surface and erupts to form volcanoes such as Taranaki (Mt Egmont) and Ruapehu.
Kermadec Ridge and Taupō Volcanic Zone
To the north-east of New Zealand, the Kermadec Ridge is a chain of mainly andesitic submarine volcanoes formed above the subduction zone by melting of the oceanic Australian Plate.
Close to New Zealand, the Australian Plate changes from oceanic to continental crust and the Kermadec Ridge merges into the Taupō Volcanic Zone. This is a narrow zone of rhyolite calderas, with groups of andesite volcanoes at its southern end – Ruapehu, Ngāuruhoe and Tongariro – and at its northern end – Mt Edgecumbe (Pūtauaki), Whale Island (Moutohorā) and White Island (Whakaari). Rhyolite magmas form from melting of the continental crust.
Mayor Island – a volcanological oddity
Mayor Island (Tūhua) lies 60 kilometres west of the Taupō Volcanic Zone. It is separate from all the other volcanoes and it ejects a rare magma type, unusually high in alkali elements (peralkaline), not found elsewhere in New Zealand. The magma chemistry causes some unusual minerals to form, and tephras from Mayor Island can be readily identified by their distinctive mineralogy.
The high alkali content is responsible for the widespread occurrence of obsidian, a glassy rock, which forms when viscous lava is rapidly chilled. Mayor Island obsidian was collected and widely traded by early Māori settlers. Because of its distinctive composition, obsidian from Mayor Island can be readily identified by chemical analysis.
Volcanoes are classified according to their state of activity, which changes as magma rises towards the surface.
- An active volcano is one that is actually erupting, or is showing signs of unrest (e.g., ground movement, earthquakes) that may lead to an eruption. New zealand scientists use a scale of alert levels ranging from 1 (signs of unrest) to 5 (major hazardous eruption under way) to indicate the level of activity.
- A dormant volcano is currently inactive, but may erupt again in the future.
- An extinct volcano is one that is unlikely to erupt again, usually because millions of years have elapsed since the last eruption. For example, the Dunedin volcano, which last erupted about 11 million years ago, is regarded as extinct.