I have been involved in conservation in and around Tongariro National Park as a member of the Tongariro Natural History Society. Members have a wealth of knowledge and experience in the park. Some of the information below comes from Society publications as well as DOC journals and GEONET. The Society also organises excursions to Crater Lake with a member volcanologist.
My story takes place a little north east of the glacier and lake but they are pivotal to the action.
Tuwharetoa Glacier is the most active glacier on Mt Ruapehu. It grows through an accumulation of wind-blown and fallen snow onto the surface and is the only glacier to grow rather than recede. The adjoining Crater Lake limits the extension of the glacier by undercutting the end of it and promoting crevasses, ice avalanches, calving and melting. The glacier adds 100,000 to 200,000 cubic metres of snow and ice annually to the lake. The collapse of snow and ice from the glacier into the lake can generate waves which help erode the tephra.
Changes in Crater Lake are monitored annually. What we know about the lake prior to the 18 March 2007 lahar are this: there was a significant increase in the concentrations of sulphate, flouride, chloride and metal ions in the lake water and an increased acidity. This suggests an increased flux of magmatic gases mixing with hydrothermal fluids in the vent system and moving up the lake. Then the lake cooled which suggests that something other than the sulphur layer which sits at the bottom of the lake impeded the heat flow. This may have helped to build pressure. Then there was a rapid influx of geothermal water into the lake causing a wave run up of four to five metres horizontally at the snow covered tephra dam.