An uneasy quiet
Following the 1945 eruption Mt Ruapehu settled into a fitful slumber. Between 1945 and 1986, scientists reported 61 volcanic disturbances, mostly small steam eruptions that splashed water and mud within the summit crater.
A few were more substantial. Shortly after midnight on 22 June 1969, the volcano produced a lahar that destroyed the kiosk at the Whakapapa skifield. On 8 May 1971 a party surveying the crater rim saw the lake surface bulge, then burst skyward. Two of the men were drenched with acid water, blasted by choking ash and toxic gas, and bombarded with rocks. In 1975, eruptions sent lahars down the Whangaehu River and through the Whakapapa ski field.
Swimming in Ruapehu’s Crater Lake was popular during the 1950s and 1960s. The water was usually warm due to volcanic gases streaming up from the magma chamber below, and a gentle ramp of snow allowed easy access. However, volcanic gases bubbling up from below slowly began to acidify the water. Swimming was discouraged once scientists discovered the increasing acidity, and also because eruptions in the lake could occur without warning.
The 1995–96 eruptions
From January 1995, scientists monitoring Ruapehu detected a growing restlessness. On 18 and 20 September, small eruptions produced mud flows from Crater Lake. Around 5 p.m. on 23 September, hundreds of skiers were on the slopes when lake water, steam and ash were suddenly blasted into the sky. Rocks were tossed up to 1.5 kilometres from the crater. Most of the skiers dropped their gear and fled. A cloud of steam and some volcanic ash rose 12 kilometres, and lahars raced down three valleys. One narrowly missed Whakapapa’s Far West T-Bar and its queuing area, where only an hour earlier hundreds of people had thronged.
A series of explosions just a few minutes apart followed on 24 and 25 September, sending half the water in Crater Lake down the Whangaehu River. A 10-kilometre-high plume of ash rose skyward, falling to smother the Desert Road.
Skiers were quick to return to the slopes once the mountain quietened. But on 7 October they were once again threatened when earthquakes accompanied a new series of eruptions that lasted until 15 October. Crater Lake was emptied of its remaining water.
Activity dropped off and by mid-November lakes were re-forming in the crater. But in June 1996 Ruapehu once again began blasting out plumes of ash, blocks of hot rock, and lava bombs. On 20 July fountains of hot lava were seen, and sonic booms shook the area. An 11-kilometre-high cloud dispersed ash as far as Ōpōtiki, on the East Coast. By early September, Ruapehu was quiet once more.
A costly spectacle
Ruapehu’s 1995–96 eruptions were similar in size to those of 1945, but their social and economic impacts were much greater. New Zealand’s population had doubled since 1945, and visitors to the mountain had increased dramatically. In 1945 there was one ski area, and no ski lifts; by 1995 there were three ski areas and 36 ski lifts. On some days up to 10,000 skiers swarmed the slopes, and when the three ski areas closed it cost the region $100 million.
Avoiding an erupting volcano made travel difficult. The main highway and railway line connecting Wellington and Auckland skirt Ruapehu. During eruptions the central North Island also became off-limits to aircraft. Detours were lengthy and costly. Drifting ash intermittently closed airports, including Auckland and Wellington; the value of cancelled flights alone was $2.4 million.
Electricity suppliers lost $22 million as ash shorted out power pylons and later wrecked the turbines of the Rangipō power station.
Future volcanic activity
Impressive as they were, the 1995–96 eruptions were relatively minor, and scientists of the future are unlikely to find evidence of them in the long-term geological record. Existing deposits around the mountain from lava flows, pyroclastic flows (clouds of hot gas, pumice and rock fragments) and avalanches of volcanic debris suggest that Ruapehu is capable of much more violent activity.
As in 1945, the 1995–96 eruptions added unstable debris to the rim of Crater Lake, setting the scene for another breakaway flood down the Whangaehu River. However, the hazard is now continuously monitored, and systems are in place to give early warnings of lahars moving towards the Desert Road and Tangiwai. These protective measures were effective when there was a lahar from the lake on 18 March 2007, and there was little damage.