Scenario: Volcanic eruption — Ruapehu

Understanding what could happen in a large volcanic eruption provides information on how to be prepared and reduce risk.

Average frequency of similar sized eruptions	Estimated damage from 1995–96 eruption	Estimated number of people affected	Months of volcanic activity	Land affected by over 1cm of ashfall
50 years	>$253 m	100,000	12	30,000 km²

Average frequency of similar sized eruptions: 50 years
Estimated damage from 1995–96 eruption: >$253 m
Estimated number of people affected: 100,000
Months of volcanic activity: 12
Land affected by over 1cm of ashfall: 30,000 km²

What could happen?

This scenario explores what could happen if Mount Ruapehu erupts in the Tongariro National Park. Mount Ruapehu is an active volcano and the North Islands highest peak. It last erupted in 2007. In this scenario the volcano would likely produce a series of large eruptions that continue for around a year. During this time, there would be small earthquakes, volcanic mudflows (lahars), and widespread ashfall. Some eruptions send ash high into the sky, reaching up to 12 km, and spreading up to 300 km away from the volcano.

Extent and thickness of the two largest ash falls from the Ruapehu eruptions in 1995–96. Ash thickness is measured in millimetres.

Map created from data digitised from Te Ara, The Encyclopaedia of New Zealand

Mount Ruapehu is one of New Zealand’s most active volcanoes, and minor eruptions occur frequently. There have been about 60 minor eruptions since 1945 alone. The last eruption occurred on 25 September 2007 causing ash, rocks and water to be ejected across the summit. The event lasted seven minutes and generated two lahars. Current research suggests large eruptions such as those in 1995–96 have occurred approximately every 50 years.

Read more about volcanic hazards and how they can affect people and places.

Volcanic eruptions cause a range of impacts near and far away from Mt Ruapehu. On the volcano, the ski field will close, and people will not be allowed on the mountain. During the eruption, fast-moving clouds of hot steam and ash called pyroclastic flows may run down the side of the mountain, causing severe damage. Iwikau and Whakapapa Village may be exposed to pyroclastic flows, which can travel for many kilometres. Lahars may travel a long way down valleys and damage anything in their path. These can happen anytime, even after the eruption has finished. People, property and infrastructure close to rivers and streams flowing from Ruapehu are exposed to lahars, particularly the towns of Tūrangi and Taumarunui.

Most homes will not be badly damaged because there aren’t many buildings close to, or on, the volcano. The main impact to people and property will be ash, which can travel for hundreds of kilometres. Ashfall can damage and corrode property, including roofs, guttering, downpipes, cars, and air conditioning units. Power, water, and sanitation services may be disrupted. Ash falling onto roads may make driving unsafe, and some roads may need to close. Ash can also impact the health of humans and animals, and some may experience irritated eyes and throats if they are exposed to ash for a long time.

By exploring the potential impacts and damage from volcanic activity this information can be used to increase preparedness and understand ways to reduce risk. Councils and planners can use this information to identify the most at-risk areas to guide decisions on building consents, land use, and emergency response plans.

For residents, it is important to know what natural hazards your property is exposed to and take steps to prepare:

Learn how to make your home safer from natural hazardsopen_in_new
Check that you have the right insurance in placeopen_in_new. A home insurance policy may qualify you for natural hazards cover.
Check your local council website for information about local hazards and emergency planning in your area.

Volcanic ash on Iwikau Village and Whakapapa Ski Field, Mount Ruapehu erupting in the distance, 1995.

Photo: GNS Science. Used with permission.

In March 2007, a lahar came within meters of the bridge adjacent to Tirorangi Marae, near Ohakune.

Photo: GNS Science. Used with permission.