Ruapehu Crater Lake: The newest theory

Ruapehu Bulletin, Volume 9, Issue 420, 21 January 1992, Page 12

Ruapehu Crater Lake: The newest theory

Crater Lake occupies the active crater near the summit of Mt Ruapehu, the highest mountain and most active volcano in the North Island. The lake is uuique due to a * combination of factors. It is: • volcanically heated yet surrounded by glacial ice and snow • situated in virtually the only high alpine landscape in the North Island, and one which has special cultural values • very accessible, with roads reaching to within four kilometres of the crater rim and ski lifts to within 300-600 metres vertical distance • part of a very active volcanic system, providing a 'window' to the workings of the volcano. Other volcanic crater lakes co-exist with glaciers in Iceland, Alaska and possible Kamchatka but they are inaccessible and considerably less active. All historic activity on Ruapehu has occurred in the centre of the western summit crater which since eruption records began in 1861 has more or less continuously contained Crater Lake. Older parts of this western crater around the lake are infilled with glacier ice, sediments and volcanic debris. The lake and its contents The lake is 500 metres wide and contains up to about 10 million cubic metres of water, with a maximum depth of about 70 metres in 1970 to more than 180 metres in

1982. The water is very acidic (pH 1-2), with high concentrations of sulphate and chloride salts to magnesium, calcium and aluminium. The composition is a result of volcanic gases venting from fumaroles beneath the lake and from breakdown of andesitic volcanic rock materials. The fumarolic activity maintains lake surface temperatures normally between 20-40 0 C in a glaciated alpirie environment where the mean annual air temperature is about -1° C The lake is often grey in colour due to suspended lake sediments stirred up by the convection resulting from fumarolic heating. Yellowish green-grey slicks composed of tiny sulphur balls can often be seen on the lake, especially in the central and northern parts of the lake above the main vents. The lake becomes a beautiful blue-green colour after several months cooling without major convection. A new eruption model A classic view of eruptions is that they are caused by blockages in a volcanic vent and explosive unblocking by Magmatic gas or new magma during eruptions. The special characteristic of Crater Lake, that it intercepts the heat and chemical outputs of Ruapehu, has enabled scientists to propose a new mechanism to better explain part of the dynamic nature of Ruapehu volcanism.

DSIR scientists with assistance from conservation staff have measured the temperature, outflow rates and chemistry of Crater Lake for the last 25 years, allowing heat and gas inputs to be calculated. The calculations suggest that heat is normally transferred by heat-pipe mechanism from a magmatic heat source under the volcano to the region immediately below the lake. Heat-pipes which are common in geothermal areas involve the simultaneous upward movement of hot vapour and downward movement of liquid from condensed vapour. This two-way movement of water appears to take place in a rubble filled vent beneath Crater Lake. The characteristic lake temperature pattern over the last few years is a result of heating phases lasting for one or two months followed by cooling for six months to a year. A recently confirmed layer of liouid sulphur underneath the lake acting as a partial barrier between the heat pipe and the lake may play a major role in this cyclic behaviour, by virtue of some unusual properties of this mineral. At temperatures between its melting point (about 116°C) and 159°C, the liquid sulphur is fluid enough to allow steam, gas and hence heat into the lake water. Gas passing through the sulphur produces the sulphur ball slicks at the lake

surface. Transfer of magmatic energy via the heat pipe eventually heats the sulphur layer above 160°C when it becomes highly viscous. This blocks the upwards passage of steam and heat into' the

lake, so a cooling phase is initiated. Heating and gas buildup continue below the layer raising the sulphur temperature still further. When the temperature has exceeded 200°C the sulphur viscosity starts to reduce and so after several months the layer becomes unstable. At

this point gases disrupt or disperse it allowing Turnpage9

Crater Lake

Frompage 12 lake water to penetrate into the porous heated region below the lake. Lake temperature rises abruptly and eruptions may occur. Meanwhile the dispersed sulphur loses heat to the lake solidifies and slowly drops to the bottom of the lake where it finally reforms a liquid layer. Lahars Crater Lake is a major contributor to lahars, the most likely serious type of volcanic hazard on Ruapehu. Lahars are floods of water and volcanic dcbris normally causcd by eruptions through the lake blasting water and mud onto the surrounding glaciers. Serious lahars may occur without eruptions however as in the Tangiwai disaster of 1953 when the collapse of an ice and debris barrier near the lake outlet released a lahar of 1.9 million cubic metres down the Whangaehu vallcy. The direction and relative size of 'normal' lahars are determined by wind direction, topography around the lake and the volume of ejected lake water. The majority of high level winds are westerlies with southerlies, northerlies and easterlies each accounting for about 15%. Therefore historical lahars have occurred most frequently in the Whangaehu valley to the east, less frequently in the Whakapapa valleys to the north even less frequently in the Mangaturuturu valley to the west, and least frequently down the distant Mangatoetoenui valley and the Wahianoa valley with its watershed ridge high above Crater Lake. No lahars have been recorded in the Turoa skifield area, although other forms of volcanic hazard, such as ash fall and erupted rock mate-. rial, can be expected every 10-30 years. Recent eruption history Ruapehu is a very active andesitic composite

volcano which has erupted over 60 times in the last 45 years. Crater Lake has a major influence on the style of this activity. Records of volcanic activity and changes in composition of erupted rocks suggest there may have been three main periods of activity since 1945. These periods encompass the 1945 eruption and following years, 1966 to about 1977 with a peak in 1975 but with consequential activity into the early 1980's and mid 1980's to the present with a possible peak in 1988. During this time the temperature of Crater Lake has fluctuatcd between about 9 0 C to 60 0 C producing a 'sawtooth' type temperature »curve of altemate heating and cooling phases.