Alpine flora is so subtle you barely register that it forms part of the landscape. This little orchid is typical growing in an upright habit between 10 and 30 cm. It can have variably one to three leaves and one to four green hooded flowers which are only about 1.5cm long with browny blotches on the hood. Its preferred habitat is wet peaty areas of tussock grassland, herbfields, open scrub and forest margins at 800 to 1200 metres altitude. It is found in Tongariro National Park in the North Island but more widely in the South Island.
In the novel this little bush features a lot. From the distance it looks like sheep on the hillside. Cushion-like white masses grow up to 60cm high by 2m long and 1m across. Its branchlets are so tightly packed it is impossible to part them. The interior of the plants contain a dense peaty mass of branchlets with old leaves. Amazingly, during winter the plant may be covered by several metres of snow, whilst in summer it will be baked during the day and blanketed in frost at night. A very cool plant indeed.
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.