Ruapehu is one of the world’s most active volcanoes. The deep crater lies between its peaks and fills with a lake between big eruptions

The crater lake of New Zealand’s Mount Ruapehu is brimming and could burst at any time, releasing at least one million cubic metres of water and sending a mudflow – or lahar – gushing down the volcano.

The last major Mt Ruapehu lahar, in 1953, was on a similar scale. It swept away a railway bridge, killing 151 people travelling across it. Some local newspapers are raising the prospect of another potential disaster – but scientists say they are scare-mongering.

A new automatic monitoring and alarm system, in combination with alterations to bridges and roads, and detailed predictions of the track of a lahar, mean the public should not be at risk, says Harry Keys, a geologist with the New Zealand Department of Conservation (DoC) in Tongariro/Taupo, the area that encompasses Mt Ruapehu.

Local claims that a simple engineering fix was rejected because of protests from Maori, who consider the mountain sacred, are also wrong, Keys says. Bulldozing a trench through the dam to let water escape slowly would have been a short-term solution to only this lahar threat. “What we have in place now is the best solution for managing lahars in the long term,” he says.


Alpine sports

Mt Ruapehu, which is 2797 metres high, is in the North Island, about 40 km southwest of Lake Taupo, and is hugely popular for skiing, kayaking and hiking.

Its 500-metre wide crater lake has been filling up since spectacular volcanic explosions in 1995 and 1996, which deposited ash and rocks around the crater. The lake’s level jumped by more than a metre after a minor eruption on 4 October 2006, and is now 2.8 metres below the crest of the tephra barrier.

This natural barrier – made up of rock ash produced during eruptions – is eroding as the water rises. While a breach is not inevitable, it is probable, and the most likely time for that to happen is February or March, says the DoC.

If the dam bursts, the DoC says it knows where the water will go. “It will flow down the channel where the Whangaehu river flows,” says Keys, following the route of the 1953 lahar. “There’s no doubt about that.”

Stronger bridge

The river channel route is believed to be the most active lahar path in the world. It tracks east down the mountain and then south, before emptying into the ocean.

There are no human settlements in the route, but at Tangiwai, 40 kilometres (25 miles) downstream and at least a 1.5 hour journey for a lahar, there is a rail-bridge – a much stronger replacement for the bridge that failed in 1953. The road bridge at the same location has been raised.

Furthermore, a barrier has been erected alongside another major road, which is at the most likely spot where the lahar could breach the natural channel, says Keys. A lahar would take 20-minutes to reach this site. The alarm system would prevent trains and cars from crossing the bridges.

Prime opportunity

The monitoring and alarm system is the result of an 8-year review following the 1996 eruption. Critics argue that a drainage trench would have been much simpler, cheaper and safer, and that the idea was abandoned because of spiritual objections by Maori. “Nothing is further from the truth,” maintains Keys.

Digging a trench would have been dangerous and could have dealt with only this lahar threat, he says. Lahars are often caused suddenly by eruptions, which are largely unpredictable. Making better-protected roads and bridges, as well as an alarm system, is a better option, he argues.

If a lahar does happen, it will give scientists a prime opportunity to observe one up close. International researchers are currently setting up additional sensors, including flood height monitors and cameras. “We will get the best scientific data on a lahar, which will have utility [for hazard management] around the world,” says Keys.