Fumaroles, steaming ground and hydrothermal eruptions
Fumaroles are steam and gas vents. They are common on the flanks of active volcanoes as well as in geothermal fields, where temperatures are generally close to the boiling point of water.
If the steam diffuses generally upwards through the soil rather than following well-defined pathways, areas of steaming ground result. The vividly coloured rocks and soil in many geothermal areas are the result of hot, acidic gases and fluids interacting with the rock. This produces clay minerals tinted by trace amounts of minerals. Some of the more unusual shades include purple (from cinnabar – mercury sulfide), orange (from realgar – arsenic sulfide), and yellow to grey (from sulfur).
The gases escaping from fumaroles can be rich in hydrogen sulfide. When the hot gases come into contact with the atmosphere they cool and oxidise, and can form yellow crystals of pure sulfur around fumarole vents. The extensive sulfur deposits around the fumaroles of White Island (Whakaari) were mined until the 1930s.
Mud pools are an icon of New Zealand scenery. They form where steam and gas rise to the surface under rainwater ponds. The acidic gases attack surface rocks, forming clay. The clay-rich soil mixes with the pond water to produce a muddy, steam-heated slurry, or mud pool.
Rainfall affects the appearance of mud pools. In dry conditions, the mud is thick and sticky, and small mud volcanoes may form. When rainfall is high, the mud is much more fluid and the pool may look more like dark boiling water.
Large holes in geothermal areas are known as craters. The more common type is the collapse crater. This is formed when the rock beneath the surface has been dissolved away by acid waters so that the ground collapses. In such areas the ground resonates, and the possibility of collapse is a good reason not to wander off defined tracks.
If excess steam pressure builds up under the ground, a violent hydrothermal eruption will blow out debris and create an eruption crater. At Waiotapu thermal area there are more than 20 eruption craters. Radiocarbon dating indicates that these were formed 700–800 years ago, and were possibly triggered by a major eruption of Mt Tarawera, about 15 kilometres to the north-east. Eruption craters are sometimes difficult to distinguish from collapse craters, but it is usually possible to find a surrounding layer of material that has erupted from the crater.
Small hydrothermal eruptions have occurred at Kuirau Park, near the centre of Rotorua, for over 100 years. As a result the area has been made a reserve: after an eruption the affected area is fenced off, and the vegetation grows back again within a few years.
Life in a thermal town
Old newspapers in Rotorua and Taupō have many accounts of thermal activity breaking out in unexpected fashion. There are stories of cold taps running from hot to boiling, and steaming cracks opening up on concrete floors. This is the price of living in an active geothermal area.
Spectacular hydrothermal eruptions occur regularly when the steel casing of neglected geothermal bores corrodes or is uncovered during excavations, allowing high-pressure steam to escape. A few new buildings have been unknowingly constructed over old vents and uncapped wells which have later reactivated.