Between a Rock and a Hard Place


Lakes and lahars at Mt Ruapehu

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.

Skiing on Mt Ruapehu, North Island, New Zealand. Photo credit: Airflore

Skiing on Mt Ruapehu, North Island, New Zealand. The name ‘Ruapehu’ is Māori for ‘exploding pit’. Photo credit: Airflore

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.

Phreatic eruptions – the silent assasins


Mt Ontake, Japan, in a more placid mood.
Photo credit: Tetusya Kanakubo

The recent eruption of Mt Ontake, Japan tragically killed at least 50 hikers who were on the volcano at the time. Within hours of the eruption taking place, social media was flooded with first-hand video footage illustrating just how close many survivors came to perishing in an onrushing pyroclastic flow.

Despite having a sophisticated seismic and geodetic monitoring system, many news reports stated that Ontake erupted seemingly without warning. Based on the evidence available at this time, it is probable that the event at Ontake was phreatic.

Phreatic eruptions occur when water enters a magmatic system and is heated to form steam. As these volcanic events do not involve the movement of magma, they are not accompanied by any of the normal eruption precursors such as ground inflation, seismic swarms or increased gaseous emissions.

As the Ontake hikers found out, a volcano which can erupt without warning is a pretty dangerous prospect.