The largest and hottest thermal springs, like the active volcanoes, lie within the Taupo Volcanic Zone, emphasising their volcanic association. There are two main theories as to their origin. The first is that the water is dominantly meteoric, that is, it is ground water which originated as rainfall, but has been heated either by coming into contact with hot rock or by steam rising from magma (molten rock).
According to the first theory, injection of magma, which is the molten material erupted by volcanoes, heats up the rock and the ground water in the joints and pores. The hot water then rises, colder water coming in from the side to replace it and in turn becoming heated, so that a regular convective circulation is set up, some of the hot water being discharged at the surface as hot springs. By the second theory the magma, as it cools and crystallises, concentrates steam and other gases in the remaining liquid portion. This causes the pressure to rise until the steam forces its way out into the overlying rocks, where it may escape directly to the surface as volcanic steam or, more usually, be condensed in the ground water, setting up circulation as described for the first theory.
Whatever the origin of the heat, geothermal investigations have shown that the hot-spring areas of the Taupo Volcanic Zone are fed by water at temperatures higher than 200°C. The water at depth is at a pressure corresponding to the weight of ground water above it, but as it rises this gradually decreases. When the pressure becomes less than boiling-point pressure the water boils and gives off steam, eventually reaching the surface as a mixture of steam and boiling water. It is this steam which supplies the innumerable steam vents in each thermal area, while the hot water flows away as springs. In drilled wells the same process occurs, and to make use of the steam for power or other purposes it is separated from the hot water.