15-May

The city of Auckland is built on a volcanic field. There are 50 Auckland 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 the Auckland 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.

Auckland is built of an active volcanic field. This map shows the Auckland volcanoes from past eruptions.
Auckland is built of an active volcanic field. This map shows the Auckland volcanoes from past eruptions.

Evolvement

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.

The field ranges from Lake Pupuke and Rangitoto Island in the north to Matukutururu (Wiri Mountain) in the south, and from Mount Albert in the west to Pigeon Mountain in the east.

The first vent erupted at Onepoto 248,000 ± 28,000 years ago. The most recent eruption (about 600 years ago and within historical memory of the local Māori iwi) was of Rangitoto, an island shield volcano just east of the city, erupting 2.3 cubic kilometres of lava. The eruptions have tended to become bigger over time, with Rangitoto making up almost 60 per cent of the field’s entire volume of erupted material. The field’s volcanoes are relatively small, with most less than 150 metres (490 ft) in height.

Lake Pupuke, on the North Shore near Takapuna, is a volcanic explosion crater. A few similar craters such as Orakei Basin are open to the sea.

The field has produced voluminous lava flows that cover much of the Auckland isthmus. One of the longest runs from Mt Saint John northward, almost crossing the Waitematā Harbour to form Meola Reef. More than 50 lava tubes and other lava caves have been discovered, including the 290-metre (950 ft)-long Wiri Lava Cave. The second-longest individual cave in the Auckland field, some 270 metres (890 ft) in total length, is the Cave of a Thousand Press-ups to the east of Maungakiekie/One Tree Hill. Two impressive depressions caused by lava cave collapses are the Puka Street Grotto and the nearby Hochstetter Pond, also known as Grotto Street Pond, in Onehunga.

For most of the 250,000 years that the field has been erupting, the planet has been in glacial periods (ice ages) where sea levels were much lower due to water being locked up as ice, and the Waitemata and Manukau Harbours were dry land. All the volcanoes probably erupted on land except for Rangitoto, which erupted during the current interglacial (warmer) period.

Settlements

Many of the volcanic cones were occupied by substantial Māori pā before European settlement, and many terraces and other archeological remnants are still visible. Many of the cones have been leveled or strongly altered—in small part due to the historical Māori use, but mostly through relatively recent quarrying of construction materials (especially scoria). However several of the remaining volcanoes are now preserved as landmarks and parks. The cones are also protected by a 1915 law, the Reserves and Other Lands Disposal and Public Bodies Empowering Act 1915, which was passed due to early concern that the distinctive landscape was being eroded, especially by quarrying. While often ignored until the late 20th century, it has amongst other things minimised severe changes to Mount Roskill proposed by Transit New Zealand for the Southwestern Motorway.

In March 2007, New Zealand submitted the volcanic field, with several specifically named features, as a World Heritage Site candidate based on its unique combination of natural and cultural features. At that time, only 2 per cent of more than 800 World Heritage Sites worldwide were in this “mixed” category.


That’s a lot of history about Auckland.  A great place to live and enjoy.

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