One day when the mountain blows

Waimarino Bulletin, Volume 4, Issue 14, 2 September 1986, Page 23

One day when the mountain blows

The disastrous volcanic eruption and consequent lahar, or mud flow, on Mt Nevado del Ruiz in Colombia last year brings to mind the dangers of living under Mt Ruapehu. While we do not have coffee farmers pushing up the slopes as in Colombia, it is a fact that much of the high country around the mountain was formed by big lahars flowing off Ruapehu during past massive eruptions, probably much larger than the one in Colombia. A similar situation exists for Mt Taranaki (Egmont). According to Peter Otway of Geological Survey, DSIR Wairakei, eruptions and lahars of such a massive magnitude "will almost certainly happen again, but the geological time scale, dealing as it does in hundreds of thousands and millions of years, is so vast in comparison to the human life span, that we will be very unlucky to see it. "But it must be recognised that the possibility is there," said Mr Otway. Recent Ruapehu lahars in 1953, 1969 and 1975 have been relatively small, even though that of 1953 caused the loss of 151 in the Tangiwai disaster. The small lahar at Christmas Eve 1953 was not caused by an eruption at the time, but dated back to one in 1945 when the lake was drained.

By the time the lake refilled the outflow channel was blocked by ice, causing a rise in lake level eight metres beyond normal. In a rapid melt on Christmas Eve the weak dam melted through, releasing the mudflow which took out the Tangiwai railway bridge. At present the crater lake channel is flowing freely, so a repeat of the 1953 disaster is unlikely. But as the crater is weaker on the eastern side of the mountain, there is a slight possibility of a large lahar flowing towards the Desert Road, probably following the course of the Whangaehu River. According to Mr Otway, the eruptions of 1969 and 1975 came with little warning and with little preceding seismic activity. In both cases the lake temperature was decreasing, the opposite to what would normally be expected. The crater did expand slightly before the 1975 eruption and Mr Otway says that constant monitoring of the crater width might help in forecasting eruptions. "But we are watching carefully already and I suppose if we had more money and time we could always do better. "There is nothing foolproof on Mt Ruapehu, where there is a large body of water sitting on top of hot magma — the hot, liquid material from the interior of the earth — if the water gets

into the magma a sudden, very large eruption could take place,"said Mr Otway. Part of the problem with large volcanic eruptions is that while there may be warning, the specific timing of the event may be impossible to pinpoint. "In Colombia and in Mt St Helens in the USA a few years ago eruptions were anticipated but their final timing was impossible to ' predict, and Mt St Helens was much larger than expected," he said. In the case of Mt St Helens the magma forced a bulge in the crater wall which suddenly failed, like the popping of a champagne cork. Ruapehu does have lahar early-warning systems designed to give skiers sufficient time to get out of the gullies to safety. As lahars move very fast they would have to be quick. Neil Clifton, senior ranger at Tongariro National Park, said the alarms are triggered on the crater rim and sound on the skifield, giving a minimum of ten minutes warning, and at park headquarters, giving 20 minutes warning.

If the mountain did erupt suddenly and massively there would be little anyone could do and Mr Otway says people should be aware of the slight risk of skiing on, and living near, an active volcano. Mr Otway points out that Ruapehu is not the only volcanic possibility in the region. Taupo, the lake itself caused by violent past eruptions, seems to have major activity about every one or two thousands years and there is always a slight chance of an eruption. Tongariro, Tarawera, Okaitaina and even Auckland will almost certainly experience major volcanic events some time in the future. While there is considerably more chance of dying in a car accident than by a surprise volcano opening at your feet in Auckland's Queen Street, it is worth remembering that the earth we live on, seemingly stable and reliable in our lifetimes, is in a state of constant change. "There is no geological evidence of less activity now than before," said Mr Otway.