Story: Hot springs, mud pools and geysers
Page 3 – Geysers
Geysers are a rare and spectacular phenomenon: hot springs which intermittently eject jets of boiling water and steam into the air. Their name is related to the Icelandic word geysa, meaning ‘to gush or spout’.
Geysers are rare and precious features – there are only about 1,000 active around the world. Of these, about half are in Yellowstone National Park, in Wyoming, USA. The only other major geyser fields are in Chile, Iceland, Russia’s Kamchatka peninsula, Alaska and New Zealand.
New Zealand’s geysers
In the 19th century there were five major geyser fields in the North Island: Rotomahana, Whakarewarewa, Ōrākei Kōrako, Wairākei and Taupō Spa. The Rotomahana geyser field was destroyed by the 1886 eruption of Mt Tarawera, and most of the remaining geysers have been damaged or affected by human activity, especially withdrawing steam or hot water for heating. Whakarewarewa is now the only major remaining geyser field.
Running out of steam
In the 19th century about 220 geysers were recorded in New Zealand. By 2004 only 58 geysers survived, some very small.
How geysers work
Geysers can form when hot water welling up from an underground reservoir has to pass through a narrow or constricted vent to the surface. The vent acts like the safety valve of a pressure cooker. The water in the reservoir becomes highly pressurised and superheated above its boiling point. Eventually, bubbles of steam form and rise through the constricted vent to the surface. This displaces some of the overlying water at the surface, which in turn relieves the pressure in the reservoir. The heated reservoir water flashes to steam, and the frothy mixture of expanding steam and boiling water is sprayed out of the vent. After this eruptive phase, the reservoir begins to fill up from below, and the cycle starts over again.
Geysers at Whakarewarewa
Although there are more than 500 hot springs at Whakarewarewa in Rotorua, the most obvious features are the seven geysers on Geyser Flat, where there is almost always some activity. The geysers are aligned north–south along a buried fault, through which hot water escapes to the surface.
Pōhutu, the largest geyser, regularly erupts to a height of 15–20 metres, and sometimes much higher. Prince of Wales Feathers, a few metres north of Pōhutu, began spurting in June 1886, after the Mt Tarawera eruption – probably triggered by earthquakes. Originally it was known as the Indicator, as it normally played shortly before Pōhutu erupted, but in 1901 it was renamed in honour of the royal visit that year. Since 1992 it has played almost continuously.
Although visitors to Whakarewarewa will almost always see geysers playing on Geyser Flat, most of the others that made the area famous are no longer active. In particular, the Waikite and Wairoa have disappeared, although you can see their vents, surrounded by sinter. Waikite was one of the highest above sea level (315 metres), built on a prominent sinter mound. The southern end of Fenton Street (the main street of Rotorua) was designed to offer travellers a view of the geyser.
Lady Knox geyser, Waiotapu
Lady Knox geyser erupts every morning at 10.15, when it is deliberately activated by pouring soap into its vent. The area close by was originally part of a prison camp, and it is said that the prisoners discovered the action of the geyser when they were washing their clothes.
The spectacular Wairoa geyser spurted up to 50 metres high. Eruptions could be triggered by soap, which appears to have two effects: it reduces the surface tension of the water, and a reaction between soap and mineralised water creates nuclei for soap bubbles to form on. The geyser was sometimes artificially induced for important visitors.