Auckland’s volcanoes
The city of Auckland is built on a volcanic field. There are over 53 volcanoes within an area of 360 square kilometres, forming the hills, lakes and basins of the city. Rangitoto Island was formed by the most recent volcanic eruption around 600 years ago – the blink of an eye in geological terms.
Scoria volcano
Mt Eden with Auckland city in the background. Mt Eden is a scoria volcano that has been produced by the Auckland volcanic field.
Auckland’s volcanic field
All of Auckland’s volcanoes come from one magma source. Underlying Auckland is a diffuse pool of basalt 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 centre 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 and unlikely to ever erupt again. However, the underlying magma source itself is still active – it may come through at a new place and form a new volcano next week, next year or next century.
Auckland volcanic field
Auckland is built of an active volcanic field. This map shows the volcanoes from past eruptions.
In 2014, 14 Tūpuna Maunga were returned to Ngā Mana Whenua o Tāmaki Makaurau (the mana whenua tribes of Auckland). The Tūpuna Maunga o Tāmaki Makaurau Authority (Maunga Authority) was established to co-govern these sacred sites.
Map from Volcanoes of Auckland: A field guide, by Bruce Hayward, 2019, Auckland University Press.
Eruptions in Auckland 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 to close to the volcanic vent, the fine ash particles can travel widely, impacting areas further away.
See this article Auckland’s forgotten volcano about work that scientists undertook researching and mapping Auckland’s volcanic field.
Where will the next volcano be?
Professor Jan Lindsay and other DEVORA researchers who study the Auckland volcanic field, think 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 and outwards violently, carrying fragments of ash and rocks from the magma in a spectacular eruption.
Volcanoes in Auckland
Dr Jan Lindsay talks about the Auckland volcanic field – what it is, where it is and what it means for the volcanoes that lie underneath our biggest city.
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.
Pupuke Moana (Lake Pupuke)
Pupuke Moana (Lake Pupuke) on Auckland’s North Shore is a classic example of a maar volcano. It formed through a series of phreatomagmatic eruptions that excavated a large double crater around 190,000 years ago. Before these highly explosive eruptions, Pupuke Moana was a low shield volcano, built from overlapping basaltic lava flows and possibly small scoria cones. The lava from explosions at Lake Pupuke trapped many trees, which have become fossilised. These are still visible at Takapuna beach (Takapuna Fossil Forest).
Acknowledgement: Google Earth
Phreatomagmatic eruptions leave behind a distinct shape of the resulting volcano, referred to as a maar – a roughly circular crater about 1 km in diameter that is surrounded by a ring of pyroclastic material (tuff ring). Good examples can be found at Lake Pupuke, Ōrākei 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 (vesicles) 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. The DEVORA (DEtermining VOlcanic Risk in Auckland) team often make quirky discoveries about the volcanic field – including lava caves and 28,000 year old tree remains buried under a lava flow from Maungawhau (Mt Eden)
Related content
Explore our wide range of resource on volcanoes in this introductory article. Realistic contexts connect students to authentic scientific processes and purposes. It’s all explained in Volcanoes resources – planning pathways.
Foulden Maar is a maar of geological significance in Otago. Learn about the ancient fossils of Foulden Maar and the fight to protect the site from mining.
Activity ideas
Go on a volcano hunt, which covers the length of Aoteaora. Then discover how scientists and others help us stay safe in these shaky isles with the activities Watching Rangitoto erupt, Who's on your team? and Home disaster kit.
