Hot water lake with many mud pots (photo by Sarah Milicich)
Mud pools form in geothermal areas where rising steam and gas from deep boiling fluids condense in shallow groundwater or rainwater. The gases make the water acidic, and attack the surface rocks, forming clay. The clay-rich soil mixes with the pond water to produce a muddy, steam-heated slurry, or mud pool. Also present are small quantities of sulphur which can be seen as black slicks on the surface.
The mud pools at Waiotapu (Maori for sacred waters) occupy the site of what was the largest mud volcano in New Zealand. The original cone, present until 1925, is now eroded and vigorously active water and mud fill the pool. The gases and steam rising to the surface mean the pools are in a constant state of movement, with mud bubbles forming at surface and erupting. At times repeated mud eruptions at a location can form small mud volcanoes.