Auckland’s volcanoes

The city of Auckland is built on a volcanic field. There are 50 volcanoes within an area of 1,000 square kilometres, forming the hills, lakes and basins of the city. Rangitoto Island was formed by the most recent volcanic eruption 600 years ago – the blink of an eye in geological terms.

Auckland’s volcanic field

All of Auckland’s volcanoes come from one magma source. Underlying Auckland is a diffuse pool of magma that occasionally finds its way to the surface. Unlike a ‘classic’ volcano – such as Mt Taranaki or Mt Ngāuruhoe with a single vent through the crust – in Auckland, the magma finds different routes through the crust and erupts in a different place each time.

Each volcanic cone in Auckland stems from a separate eruption from the pool of magma that lies under the city. It’s unlikely that the magma will push through in the same place twice, so each volcano that can be seen on today’s landscape can be thought of as dormant. However, the underlying magma is still active – it may come through at a new place and form a new cone next week, next year or next century.

The explosions of Auckland scoria cone volcanoes would have only affected areas of 5–20 square kilometres (the size of an average suburb), but while the fallout of rock and scoria is limited for this type of volcano, the fine ash particles can travel widely.

Where will the next volcano be?

Dr Jan Lindsay, a scientist at The University of Auckland who studies the Auckland volcanic field, thinks the most likely scenario is for an eruption to form in a location that is currently under water. This would cause a phreatomagmatic explosion – an eruption where the superhot magma interacts with groundwater or seawater at the surface. The heat of the magma instantly changes the water from liquid to gas, forming a lot of steam. At the same time, the hot magma is instantly changed from a liquid to a solid, shattering in the process. The steam explodes upwards violently, carrying fragments of ash and rocks from the magma in a spectacular eruption.

These types of eruptions have happened many times in Auckland’s past. It is estimated that 73% of all Auckland eruptions involved phreatomagmatic eruptions – magma would have met water under the harbour at the beginning of the Rangitoto eruption.

Phreatomagmatic eruptions leave behind a distinct shape in the cone of the resulting volcano, referred to as a maar – a roughly circular crater about 1km in diameter that is surrounded by a ring of pyroclastic material. Good examples can be found at Lake Pupuke, Orakei Basin and Crater Hill.

What type of rock is involved?

The Auckland volcanic field mainly produces scoria cone volcanoes. Scoria is a type of andesite or basalt rock (depending on the exact mineral composition). These rocks are quite dark in colour and, in the case of scoria, can contain holes where gas was once trapped in the rock as it cooled. The scoria and basalt from previous Auckland eruptions has been widely used for building roads, pavements and buildings. Some volcanoes have been mined to the extent that they no longer exist as hills – they are now just holes in the ground.

Scientists continue to study the Auckland volcanic field, read about some unexpected discoveries in the article Auckland's forgotten volcanoes.