Active Volcanoes

New Zealand's active volcanoes are confined to the Taupo Volcanic Zone, a belt which extends from Tongariro National Park to White Island. Ruapehu (9,175 ft) is the highest peak in the North Island and carries its only permanent snowfield, within which is a hot crater lake. Since 1889 more than 12 steam eruptions and four ash eruptions have been recorded. That of 1945 began with the appearance of a lava dome which displaced the lake, after which intermittent eruptions continued until 1947, leaving a deep crater. The lake had previously overflowed into the Whangaehu River through a tunnel beneath the surrounding ice, but it refilled to a higher level because of a barrier of scoria and debris from the eruption. On 24 December 1953 this carried away, precipitating a flood or lahar down the Whangaehu, which destroyed the railway bridge at Tangiwai and engulfed an express train with the loss of 151 lives. Smaller floods occurred previously in 1861, 1895, 1889, and 1925.

At the north-west foot of Ruapehu are a tourist hotel, the Chateau Tongariro, and numerous accommodation buildings erected by tramping and ski clubs. A ski lift takes passengers to the foot of the Whakapapa Glacier.

Ngauruhoe (7,504 ft) is a subsidiary cone of Tongariro. There have been more than 60 eruptions since it was first seen by Europeans in 1839; there was a lava flow in 1870, another in 1949, and 17 in 1954. Some of these were preceded by hot avalanches. It has a perfectly shaped cone which is 3,000 ft high, with a truncated summit a quarter of a mile across.

Tongariro (6,458 ft) is a large multiple volcano with several craters within a summit 3 miles in diameter. Four of these have erupted within historic time, the largest eruption being that of Te Mari in 1896.

The above three volcanoes and their surroundings were constituted as Tongariro National Park in 1894 following a gift to the New Zealand Government of 6,500 acres in 1887 by Te Heuheu Horonuku. Ngauruhoe was first climbed by J. C. Bidwill in 1839, and Ruapehu crater lake was first seen by J. P. Maxwell and G. Beetham in 1879.

On 10 June 1886 Mount Tarawera suddenly erupted violently. A line of craters was blasted across its three domes, ejecting blocks, scoria, and ash over a wide area, the noise of the detonations being heard in Auckland. The eruption extended progressively along the line into Rotomahana, where steam-blast eruptions destroyed the Pink and White Terraces and showered debris over the surrounding country. About 100 people lost their lives. It is now known that Tarawera had lain dormant for about 900 years since a previous eruption had spread pumice around.

White Island, in the Bay of Plenty, is an active volcano on the summit of a large submarine volcanic edifice about 15 miles in diameter. It has been continuously active since first seen and named by Captain Cook in 1769. Several craters emit large quantities of steam and, occasionally, ash. In 1914 a landslide from the crater rim produced a mud flow which destroyed a number of buildings and killed the sulphur workers on the island.

In addition to the active volcanoes, there is geological and radiocarbon-dating evidence that the following districts have experienced volcanic activity within the past few hundreds or thousands of years, and further eruptions there may be expected. In the Auckland district 63 points of eruption have been recognised, and the youngest of these, Rangitoto, is only about 750 years old. They include basaltic scoria cones, flows, tuff rings, and maars. Basaltic cones of age comparable with some of the younger Auckland ones are found in the Kaikohe – Bay of Islands and Whangarei districts. In Taranaki the last eruption from Mount Egmont occurred about 350 years ago.

The most violent of all the eruptions of the past few thousand years took place at Taupo about 1,830 years ago, when pumice was erupted over several thousand square miles. Previous eruptions have occurred in the same locality. Other pumice eruptions have occurred in the Maroa and Okataina volcanic centres, the latter including Tarawera. These places remain as potential danger spots.

Thermal Springs

The largest and hottest thermal springs, like the active volcanoes, lie within the Taupo Volcanic Zone, emphasising their volcanic association. There are two main theories as to their origin. The first is that the water is dominantly meteoric, that is, it is ground water which originated as rainfall, but has been heated either by coming into contact with hot rock or by steam rising from magma (molten rock).

According to the first theory, injection of magma, which is the molten material erupted by volcanoes, heats up the rock and the ground water in the joints and pores. The hot water then rises, colder water coming in from the side to replace it and in turn becoming heated, so that a regular convective circulation is set up, some of the hot water being discharged at the surface as hot springs. By the second theory the magma, as it cools and crystallises, concentrates steam and other gases in the remaining liquid portion. This causes the pressure to rise until the steam forces its way out into the overlying rocks, where it may escape directly to the surface as volcanic steam or, more usually, be condensed in the ground water, setting up circulation as described for the first theory.

Whatever the origin of the heat, geothermal investigations have shown that the hot-spring areas of the Taupo Volcanic Zone are fed by water at temperatures higher than 200°C. The water at depth is at a pressure corresponding to the weight of ground water above it, but as it rises this gradually decreases. When the pressure becomes less than boiling-point pressure the water boils and gives off steam, eventually reaching the surface as a mixture of steam and boiling water. It is this steam which supplies the innumerable steam vents in each thermal area, while the hot water flows away as springs. In drilled wells the same process occurs, and to make use of the steam for power or other purposes it is separated from the hot water.

Hot Springs, North Island

Tokaanu hot springs, at the south end of Lake Taupo, are located on flat ground beside the Tokaanu Stream. The active area is about 20 acres in extent, and includes two geysers. There are small hot springs on the lake shore at Waihi, and a number of strong fumaroles on the scarp above.

Geyser Valley at Wairakei includes many boiling springs and several geysers. Together with Waiora springs and Karapiti fumarole, the total area which includes warm spots is 7 sq. miles. Nearly 100 bores have been drilled and steam from these is used to generate 150 mW of electricity.

Rotokawa occupies about 400 acres between Lake Rotokawa and the Waikato River. It has hot springs, steaming ground, and a number of large holes formed by collapse due to underground chemical sapping. Sulphur has been mined here.

Orakeikorako hot springs, including a number of geysers, are scattered along the banks of the Waikato River for 2 miles. Many were drowned, and some higher ones have become more active with the filling of Lake Ohakuri. They are related to the Paeroa Fault, on which are located also Te Kopia and Waikite thermal areas.

Waiotapu hot springs spread over about 6 sq. miles, and include a number of lake-filled craters formed by steam explosions, some of which are on the flank of Rainbow Mountain. Investigation steam wells have been drilled there.

Rotomahana was the best known thermal area because of the famous Pink and White Terraces, destroyed in 1886 by the eruption of Tarawera. Hot springs remain in some of the other craters to the south-west. Waimangu Geyser erupted intermittently to a height of 1,000 ft between 1900 and 1917, when an eruption at the adjoining Frying Pan Flat opened up a new crater, now occupied by a hot lake known as the Waimangu boiling lake.

In Rotorua there are hot springs at Whakarewarewa and Ohinemutu, but all are related and more than 400 wells have been drilled for hot water in the city over an area of 3 sq. miles. Tikitere, from which some sulphur has been mined, occupies about 1 sq. mile.

Onepu springs near Kawerau, on the banks of the Tarawera River, have been drilled for steam for the Tasman Pulp Mill.

Hot springs associated with the Coromandel Ranges are related to older volcanic activity and include no boiling springs. The springs of South and North Auckland probably also have volcanic association. Isolated hot springs occur at Tarawera, Morere, Te Puia, and Puketitiri, east of the volcanic zone.

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.

Geysers

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.