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Remembering the 1995 Mt Ruapehu eruption

Twenty years ago in August - September 1995, Mt Ruapehu provided a timely reminder that New Zealand is vulnerable not only to earthquakes but also volcanic eruptions.

Mt Ruapehu, New Zealand’s most active volcano, which is also one of the most active in the world, has had a significant impact on this country’s landscape, mythology and history.

In Maori legend, she can often be heard sighing for her husband Taranaki. He lost the battle for her to Tongariro and retreated to the west coast, carving the Whanganui River as he went.

Sitting in the middle of one of the most active geological areas in the world, the 2797m-high Mt Ruapehu, has been the source of spectacular eruptions for 200,000 years. This has had a huge impact on the surrounding countryside.

From 900m, the impressive giant is composed predominantly of lava and ash. The surrounding Tongariro National Park is also scattered with large boulder debris and remnants of destructive mud flows, known as lahars.

Several mountains in the world are considered particularly dangerous due to the risk of lahar. Mt Ruapehu is one of them due to its continued activity and seven million cubic metres of acid water sitting in Crater Lake at 2,540m, ready to accelerate downhill whenever the barrier enclosing it ruptures.

In early 1995, scientists monitoring Ruapehu observed signs that the mountain was starting to stir again after 50 years of relative inactivity. Elevated crater lake temperatures were followed soon after by steam venting from its peak.

In late August, and again on 18 and 20 September, 1995, small eruptions occurred producing mud flows from Crater Lake. Three days later, at around 5pm, as hundreds of skiers were finishing their day on the slopes of Ruapehu’s ski areas, the volcano made its presence felt with a hiss and a roar.

Water, steam and ash were suddenly and violently blasted into the sky. Boulders were thrown up to 1.5 kilometres from the crater. Skiers fled. Those in nearby cities like Whanganui watched as a cloud of steam and volcanic ash rose 12 kilometres into the air. Lahars raced down the mountain’s valleys at speeds of up to 90km/h, with one narrowly missing the Whakapapa ski field where skiers had queued just an hour earlier.

Around 30 hours later a series of explosions just a few minutes apart propelled more water from Crater Lake down into a surging Whangaehu lahar which continued its descent for a further 15 hours. A 10-km-high plume of ash also rose skyward, falling to smother the Desert Road and spreading across communities, carried by strong winds. Smaller eruptions continued through October as the volcano expelled most of its water from Crater Lake. In early November 1995 a new lake began to form in the crater and volcanic activity ceased. . But not for long.

In the early morning of 17 June 1996, the third major eruption of this episode began with strong winds carrying ash plumes across nearby communities. By 14 August, at least 36 lahars had occurred in the Whangaehu Valley alone. Then, all was quiet once more as the giant lay down, finally content she had been heard.

The power and volatility of this country’s active volcanoes has led to intense monitoring of Ruapehu over the years, funded by EQC through the GeoNet system, and in local partnership with the Department of Conservation. For example, with the natural ash-barrier destroyed in this series of eruptions, scientists anticipated a subsequent lahar as the Crater Lake refilled. Monitoring equipment was installed to give advance warning of a lahar for road, rail, and other infrastructure. The aim was to prevent a repeat of the 1953 Tangiwai tragedy, where 151 people died after a lahar undermined a railway bridge.

The need for the monitoring was shown when the anticipated lahar occurred in March of 2007.

Through research and collaborative planning, we are better placed to manage the risks and uncertainty associated with New Zealand’s volcanism.

Along with funding research into our volcanic landscape, EQC also covers damage to residential properties as a result of a volcanic eruption. Following the 1995‐1996 Ruapehu eruptions, 203 claims were made to the Earthquake Commission due to damage following the ash fall. Almost 90% of the claims related to the claimants’ roofs, with 28 related to corrosion of metal roof surfaces.

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