A geyser is formed when a column of water becomes heated until boiling-point pressure is exceeded and steam is flashed off. This expels the water above, reducing the pressure further and causing more steam to form. When all the water is expelled the eruption ceases, but if more hot water can flow in the eruption may continue for some time, as it does at Pohutu Geyser at Whakarewarewa. Drilled steam wells draw on large supplies of hot water which keep discharging continuously.
by James Healy, M.SC., Volcanologist, New Zealand Geological Survey, Rotorua.