Natural hazards

Volcanic hazards

How were they made?

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

Every now and then, a bubble of magma pinches off from the "hot spot and rises up through 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 explsion, 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 onto 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 Aucklands 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 Aucklands volcanoes erupt again?

It is unlikely that any of Aucklands 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.

What are the chances of another volcanic eruption in Auckland?

Based on the number and frequency of previous eruptions, there is once chance in 1000 (1:1000 yr return period) that an eruption could occur in Auckland in any one year.

In any 50 year period, there is a 5% probability, or 1 chance in 20 that an eruption will occur from the Auckland Volcanic Field.

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What are the likely hazards and effects of an eruption from the Auckland Volcanic Field?

Hazards:

• base surges - Hot blast of rock, gas and steam would travel outward horizontally at hundreds of kilometres per hour. This would devastate an area of around 3 km form the vent.
• volcanic ash and debris - would be deposited downwind
• fire fountaining - molten rock, or magma would erupt, affecting the area near the vent. Lava flows can form, these would be slow moving, but would eventually destroy structures in their path. Previous lava flows from the Auckland Volcanic Field have stretched 10 km from the vent
• shock waves from volcanic explosions - could flatten trees and break windows
• poisonous gases - occur near the vent or could collect in low lying areas
• small, localised earthquakes
• small tsunami caused by underwater eruptions. 

• complete devastation of any buildings or infrastructure within 3 km of the eruptive vent, or in the path of lava flows
• extreme economic losses due to clean up costs (estimated in the order of $250M), physical damage to buildings etc, damage to infrastructure, closure of businesses, damage to horticultural and agriculture products, and effects on Aucklands tourism
• risk of widespread fire from hot ash or disrupted gas supply pipe lines
• significant impacts on Aucklands infrastructure, which may force a significant number of evacuations, and will have economic and social impacts
• some disruption and possible restrictions to electricity supply and gas, water supply, wastewater and stormwater networks
• locally, volcanic ash and dust will affect air-conditioning systems and some communication networks
• operation of some roads within 10km of the vent at as little as 30% capacity, potential loss of rail service, and minor disruption of air travel
• health risk to people and animals due to irritation of lungs and eyes and poor sanitation.