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Volcanoes of Auckland

If you look out of our window you can see two or three old volcanos and from the roof a few more, especially Rangitoto, which is featured in our Welcome Photo (Bucklands Beach in Winter). Note the last eruption was 600 years ago, that was Rangitoto.

How many volcanoes does Auckland have?

The Auckland volcanic field has about 50 (Nov 2011, now 55 see below) volcanoes, within an area of 360 square kilometres. Some take the familiar shape of hills with large craters at the top, like Rangitoto and One Tree Hill. Some have been quarried away, like Albert Park or Mount Smart, and some have always just been large holes in the ground, like Panmure Basin or Orakei Basin.

How were they made?

New Zealand sits where two slowly moving plates of the earth’s surface meet. Where one plate slips over the other, cracks can form and molten rock, or magma, from about 100 km below rises to the surface. The volcanoes of the central North Island and White Island were formed in this way. The Auckland volcanoes however, are probably the result of a “hot spot” - a concentration of magma where one of the earth’s plates is under tension.

Every now and then, a bubble of magma pinches off from the “hot spot” and rises up through solid rock, like a bubble leaving the bottom of a pan of hot water. Near the surface, pressure from the rocks above lessens, and gases within the magma are released. The bubble may also reach ground or surface water, creating enormous amounts of steam. The sudden pressure from these gases causes a huge explosion, creating a new Auckland volcano. If the magma bubble is a small one, that may be the end of the process, and only a crater remains, surrounded by a ring of ejected material. Panmure Basin is a good example of this type of structure, known as a maar.

Larger bubbles may go on to send liquid rock to the surface for years, building up rocky scoria cones like One Tree Hill. They may even produce long lava flows. About 75 square kilometres of the Auckland area is covered by such flows.

How old are they?

It is thought that Auckland’s volcanoes first began to appear between 60 000 and 140 000 years ago, starting with the eruptions of Albert Park and the Domain. The largest and most recent eruption was Rangitoto, about 600 years ago, which would have been witnessed by local Maori. The field is expected to have a total life of approximately a million years, so geologically speaking, it is still very young.

Could any of Auckland’s volcanoes erupt again?

Even though the larger volcanoes like One Tree Hill and Rangitoto may have been created by successive eruptions over a period of centuries, it is unlikely that any of Auckland’s existing volcanoes will become active again. However, bubbles could pinch off from the hot spot beneath Auckland and create new volcanoes at any time. To gather information on the field, and help provide advance warning of any volcanic activity, the Auckland Regional Council monitors seismic activity in the Auckland area.

Civil Defence and Volcanoes. If, well more likely, when another volcano is added to the 49 already in the Auckland field, the consensus of scientific opinion is that we will get a few days to a few weeks notice. Once the new site is identified everyone within a 5 Km circle will have to evacuate and everybody else prepare to live with a new neighbour. The eruption could last for weeks or years, so life in Auckland will change. When will this happen, well anytime in the next few thousand years or next week! See also New Zealand Volcanos

New volcanoes underneath Auckland (Nov 2011)

Four new volcanic craters have been discovered underneath South Auckland, bringing the total number of volcanoes in the Auckland Volcanic Field to 55. Geomarine Research identified the four previously unknown volcanic craters in the southern part of the volcanic field.

Dr Bruce Hayward said the most prominent of the new volcanoes is a 300m diameter crater surrounded by a semicircular rim of volcanic ash in Boggust Park in Favona, Mangere. “This crater was originally circular,” Dr Hayward said. “It is also one of the older volcanoes in Auckland. “The crater appears to have been invaded by the sea 130,000 years ago, during the last interglacial warm period when sea level was 5-6 m higher than today. At that time Boggust crater would have been an intertidal lagoon, like Panmure Basin today. “The tides eroded the wide open mouth that still exists between it and the nearby estuary to the east.” Dr Hayward said the other three craters are in Puhinui Reserve, Wiri. Each is about 200m across and sits on the crest of its own low tuff cone made of erupted volcanic ash.

One crater still contains a freshwater pond, while another has filled with pond and swamp sediment over thousands of years and is now drained and used as an equestrian arena. Each has been eroded on one side by water overflow from their small crater lakes - a process Dr Hayward said would have taken tens of thousands of years. He said the four craters have not been recognised before because they are not major landform features like many of Auckland's volcanoes. “Fortunately all four craters are protected within reserves and can now be cherished and managed as part of Auckland's rich volcanic heritage - unlike many of the volcanoes that have been quarried away or buried beneath subdivisions in this part of Auckland over the last 60 years.”

http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10764796

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