Bathymetry, heat output and convection in Ruapehu Crater Lake, New Zealand

Abstract

Bathymetric observations of Ruapehu Crater Lake show that it became shallower and its volume decreased by 3 × 10 6 m 3 between 1965 and 1970. It is likely that this was due to lava moving upwards in the region under the lake. From the regular measurements of Crater Lake temperature, outflow, and chloride and magnesium ion concentrations, the inflows of steam, cold water and chloride and magnesium ions were calculated, using the 1970 volume. Only about half the precipitation falling on and around the lake mixes with the lake water, since the cold fresh water floats on top of the denser lake water, and travels to the outlet without mixing with the main body of water in the lake. The chloride content of the input steam appears to vary with time, in a similar fashion to that of White Island fumaroles. Temperature and density profiles of the lake in 1965 and 1966 indicated that convection was occurring. The temperature profile of the top 175 m of the lake agreed with a model of turbulent convection in small cells, giving a temperature gradient at any level in the lake proportional to the heat flux at that level. The convection model predicted that the temperature at depth would exceed the local boiling point if the thermal power input reached a value which would sustain a surface temperature approaching 60°C. This prediction of instability was not inconsistent with observations. Convection in the present Crater Lake probably occurs in the form of thermal plumes from small sources.