Hot Springs, South Island
In the South Island a number of hot springs are located along the borders of the mountain ranges, mostly associated with large faults, for example, at Hanmer. Possibly the elevation of the mountains has brought rocks of higher temperature to a higher level, so that deeply circulating ground water has become heated and returned to the surface through fissured zones along the fault planes. The springs usually emerge at low altitude in or alongside rivers which have cut deeply into the mountains.
Most hot springs contain sodium chloride as the chief mineral. Others contain chiefly sulphates, produced by the oxidation of hydrogen sulphide gas, or bicarbonate (soda springs) if heated by steam. Passage through limestone or other carbonate rocks may also produce bicarbonate waters. Whether of volcanic origin or not, hot springs are usually associated with faults through impermeable rocks, and these provide a passage to the surface for hot water.