Mt Ruapehu is the largest mountain on the North Island of New Zealand. As well as being a popular ski resort, Ruapehu is an active andesitic stratovolcano. Formed approximately 200,000 years ago, activity is currently confined to the Crater Lake vent; this deep depression fills with water from snow melt between eruptive episodes.
Similarly to the recent eruption of Mount Ontake in Japan, Ruapehu has been known to erupt without warning. In September 2007, the volcano produced a sudden blast of steam and debris in a minor phreatic eruption, trapping two climbers who were near the vent at the time and generating mud and ice slurries from the resultant meltwater.
On this snow-capped peak, the biggest threat to skiers from such unexpected phreatic events is the production of large lahars by collapse of Crater Lake. Understandably, GNS (the country’s geological survey) have taken a number of measures to try and protect the tourists that flock to Ruapehu during the ski season.
The volcano is monitored using 2 web cameras, 10 seismographs, 6 microphones and 9 continuous GPS stations. The temperature of Crater Lake is regularly measured, and GNS conduct airborne gas surveys. Most importantly for skiers, the eastern flanks of the volcano are guarded by ERLAWS, a lahar warning system which gives skiers up to five minutes warning of an impending lahar.
Yet, notwithstanding this battery of monitoring equipment, we should remember that Ruapehu has the potential to catch us all by surprise.