Natural hazards and emergencies

Volcanic hazards

Auckland is vulnerable to volcanic eruptions. Much of Auckland is built on the Auckland Volcanic Field (AVF), which covers 360km2 and contains at least 50 volcanoes. It is expected that these volcanoes won’t erupt again and any future eruptions will occur in new, unknown locations. Auckland has experienced volcanic ash falls from Mount Taranaki/Egmont, Taupo, Okataina, Tongariro and Mayor Island.

 


The Auckland Volcanic Field (AVF)

Auckland’s volcanoes vary in shape, size and character. The earliest volcanic eruption in the AVF was an estimated 250,000 years ago. The last occurred about 600 years ago and formed Rangitoto. Māori living on Motutapu Island witnessed it. 

Past eruptions have sometimes started with a large explosion because of either ground or sea water coming in contact with rising magma. An eruption of this type is more likely to recur in Auckland due to the close proximity of many water sources. These eruptions can form large craters, which can subsequently fill with water such as Lake Pupuke, Orakei Basin and Onepoto Reserve. 

Continued eruptions often create volcanic or scoria cones such as Mt Wellington, Mount Eden, Browns Island and Three Kings. As the volcano continues to erupt it may produce extensive lava flows. Many have been mapped within the city, extending up to 10km from the source.

Likely hazards and effects of the next Auckland eruption depend on what type of eruption occurs and for how long. 

When and where future eruptions will occur is unknown. Based on the number and frequency of past eruptions it is estimated there is about a 1 in 1000 (0.001 per cent) chance an eruption could occur in any one year.

This means that there is an 8 per cent probability (1 in 12.5 chance) an eruption will occur in the AVF field over any 80 year period. This is based on the life expectancy of a New Zealand child born in 2008 being 82.4 years for females, and 78.4 years for males (Births and Deaths: December 2009 quarter - Statistics New Zealand). 

Here is a summary of the possible resulting hazards:

Hazard

Description

Base surge

Base surges are a hot blast of rock, gas and steam that travels across the land at hundreds of kilometres an hour. These surges flow outward from the vent and may significantly devastate an area up to 3km from the eruption location. 

Ash and debris

For the duration of the eruption, ash and debris will accumulate down wind of the volcano. The depth of ash and debris deposited will decrease further from the vent.

Fire fountaining

Fire fountaining includes molten rock and magma erupting from the volcano’s vent. Continued fire fountaining can lead to the formation of lava flows that will be slow moving and destroy most things in their path.

Shockwaves from explosions

Shockwaves from violent explosions will be stronger closer to the vent and can flatten trees and break windows.

Poisonous gases 

Gases are likely to be released closer to the volcanoes vent. As these gases can be much denser than the surrounding air, they often pool and become concentrated in low-lying areas.

Earthquakes

As rising magma moves upwards through the earth’s surface, it can generate earthquakes. Earthquake activity is likely to increase as an eruption becomes more likely and will continue for the duration of the eruption.

Tsunami

Volcanic eruptions occurring beneath the ocean or large water bodies can displace water creating a tsunami.

Impacts could include:

  • devastation of buildings and infrastructure within a 3km radius of the volcanic vent or in the direct path of lava flow
  • large economic losses due to clean-up costs, physical damage to buildings and infrastructure, closure of businesses, damage to horticultural and agricultural products and a decline in the region’s tourism
  • an increased risk of widespread fires from hot ash or disrupted gas supply lines
  • significant impacts on Auckland’s infrastructure from any of the listed hazards which may result in mass evacuations
  • disruption and restrictions to lifeline networks such as electricity, gas and water supplies, and waste and stormwater networks from any of the listed hazards
  • ash and dust will affect air-conditioning systems and some communication networks
  • roading networks within a 10km radius of the vent may be impassable or greatly affected
  • rail and air services stopping or greatly reduced
  • eye and lung irritation and poor sanitation causing Increased health risk.


 

Central North Island volcanic sources

Auckland is at risk from ash fall from several large and frequently active volcanoes in the central North Island and the possible reawakening of volcanic activity in Northland. 

In contrast to the Auckland field, eruptions from New Zealand stratovolcanoes such as Mount Ruapehu and Mount Taranaki/Egmont occur, on average, every 50 to 300 years. Over the past 80,000 years, eruptions from distant volcanoes have deposited at least 82 different ash layers, greater than 0.5m thick, in Auckland. 

Here is a summary of the source of these layers compared to those identified from the AVF over the same period.

Source

Number of identified tephra layers

Mount Egmont/Taranaki

52

Okataina and Taupo

21

Mount Tongariro

7

Mayor Island

2

Auckland Volcanic Field (AVF) 

24


Whether ash from a distant eruption will reach Auckland, and the thickness of any associated ash deposit, depends on factors such as the type of eruption, its location, duration and wind direction. 

Some of the impacts that may occur include:

  • eye and lung irritation increasing health risks
  • high economic costs due to clean-up, damage to infrastructure, temporary business closures and adverse effects on tourism
  • economic losses due to damage of horticultural and agricultural industries
  • disrupted electricity supply with power outages if the ash is wet
  • widespread disruption to transport infrastructure including the closure of roads and airport facilities
  • disruption of fresh and waste water systems
  • disrupted communication systems due to interference, overloading or direct damage.


 

Monitoring volcanic hazards in Auckland

GNS Science, through the GeoNet Project, monitors volcanic hazards across New Zealand. It studies changes in volcanic activity through earthquake and gas monitoring, ground deformation and the composition, including chemical make-up, of crater lakes. 


 

More information 

To learn more about the monitoring methods used and the location of Auckland seismographs, visit the GeoNet website.  

Visit the GNS Science for more general information on volcanoes.