Rumblings of discontent

Something is stirring under the central North Island and the question is, should we be worried? Our largest land volcano and the North Island’s highest mountain, Mt Ruapehu, is clearly not happy. In the past month, the water in Crater Lake, which fills the active vent on the summit, has been steadily warming and is now at 37degC.

There have also been elevated levels of volcanic tremor, in fact the longest spell of such tremors in about 20 years.

Scientists have been quick to reassure those living around the Central Plateau and others beyond that these happenings are not necessarily cause for alarm. They point out that in fact the most likely consequence of these rumblings is that no eruption occurs, given there have been none after other periods of unrest in the past 15 years.

However, eruptions do occur. GNS Science volcanologist Brad Scott communicated the uncertainty of the situation and how maverick volcanoes can be by summing it up on Twitter this week: "Will it, won’t it?".

Aotearoa’s incredible landscape has been built by violent events, paroxysms of the ground which might last just minutes, a couple of days or a few weeks.

Our position teetering on the Pacific Ring of Fire ensures frequent significant earthquakes and volcanic eruptions. The volcanoes of the Central Plateau are at the southern end of a zone which extends roughly north-northeast to Rotorua and the Bay of Plenty, through the Kermadec Islands and up to Tonga.

Every New Zealander knows a large earthquake can cause a huge amount of damage that takes decades to recover from. We need only look at Christchurch as an example of that.

Earthquakes and other natural events such as floods and gales can be devastating, affecting the lives and livelihoods of tens of thousands to hundreds of thousands of people. However, a major volcanic eruption is on another scale entirely.

If there were a large volcanic explosion in the centre of the North Island, it is possible a million or more people might be affected. Thick ash could cover a large swathe of the middle part of the island and the effects on transport networks, especially aviation corridors, would be significant.

Mt Ruapehu is currently categorised as being in a state of moderate to heightened volcanic unrest, at alert level 2. This is the highest level of alert without an actual eruption.

One of the biggest worries is the threat of lahars, the streams of hot and cold rocks mixed with water and mud that gush down river valleys. A lahar from Ruapehu’s Crater Lake caused the Tangiwai rail disaster in December 1953, when the collapse of the Whangaehu River bridge resulted in the deaths of 151 people.

The mountain’s most recent eruption in September 2007 also produced lahars in two valleys, and spread ash and rocks across the summit. However, unlike in the 1995-96 eruptions, there was no plume of ash blasted high into the atmosphere.

The experts believe if an eruption were to occur, it might be like the 2007 event, or even the slightly bigger April 1975 or June 1969 eruptions.

In recent years we have seen how even small, localised eruptions can have fatal consequences. The December 9, 2019 explosion on Whakaari-White Island killed 22 visitors and badly burnt another 25. In August 2012, Mt Tongariro erupted for the first time in 115 years, and did so again a few months later.

If all that weren’t enough, one of the world’s few supervolcanoes lies below Lake Taupo. It has erupted 28 times in the past 26,000 years, with the Oruanui eruption the world’s largest in the past 70,000 years. Ash from this event even fell 18cm deep on the Chatham Islands.

The Central Plateau volcanoes are a long way from our homes in the South. But it is worth remembering that they pose, arguably, the biggest threat of any natural hazard to life in New Zealand.

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