Besides Whakaari / White Island, the Tongariro Volcanic Centre shows the highest volcanic activity in New Zealand. Nevertheless, with more than 130,000 tourists per year the eight-hour, 19.4 km (12.1 mi) Tongariro Alpine Crossing is probably the most popular day hike of the country. Since not only volcanic craters, but also crater springs, hot springs and fumaroles line the trail, I have decided that it falls within the scope of this website and include it here.

The giantic Mount Tongariro Volcano south of Taupō encompasses at least 12 cones, four of them host major craters, and it covers an area of approximately 10 km² (3.9 mi²). In contast to the hot crater lake of the adjacent Mount Ruapehu, for which over 40 steam and ash eruptions were recorded in the last 120 years, the crater lakes of Mount Tongariro are cold ones. But this would not mean that Tongariro is much less active than Ruapehu. Ngauruhoe, with 2,500 years the youngest of Tongariro's vents, underwent about 60 pyroclastic (explosive ash) eruptions during the last 150 years, on average one every six years. The last eruptions occurred between 1973 and 1975, one of those threw a 3,000-tonne block of lava 100 metres out of the crater.

On 23^rd November 2019, the earliest bus took us from the Discovery Lodge to Mangatepopo Carpark, were a start of the hike is recommended because of the favourable ratio of ascent to descent. The first point of interest was Soda Springs. Even though the name may suggest true geothermal springs, the Soda Springs are nice but ordinary mountain springs whose cold water cascades down a steep slope.

The Soda Springs mark the beginning of a somewhat steeper ascent to the Tongariro saddle. Although this section of the trail is known as Devil's Staircase, the hike was more enjoyment rather than hell. We passed many spots with great views, but we did not come across the first geothermal feature until at an altitude of 1,886 m (6,188 feet) the highest point of the hike was reached. All the more overwhelming was the view into Tongariro's Red Crater. It was formed about 3000 years ago and the last eruptions were witnessed between 1855 and 1890. Iron oxides tint the inner slopes of the crater dark red, a kind of reverberation from eruptions long gone. The two vertically aligned rock slabs, called dike, are the remains of a giant lava protrusion, whose skin solidified before the inner molten material was pulled back into the vent and left the structure partially hollow.

On the subsequent descent we had the privilege to enjoy the Emerald Lakes deserted and in nearly perfect conditions. The early morning cirrostratus clouds were almost gone, and since we started the hike at the earliest possible time, later or slower hikers had not yet arrived while the faster hikers of our group had already passed the lakes.

The Emerald Lakes, whose Māori name is Ngarotopounamu (Greenstone-hued Lakes), are filling smaller explosion craters of the Tongariro system and their main water supply is by rain or melting water. Quite obvious are the high number of fumaroles around the lakes.

Fumarolic activity and eluted minerals from the rocks enrich the water of the lakes with chemical compounds and are responsible for the colour. Mainly the sulfurous emissions of the fumaroles turn the water into a weak acid with a pH of 3.2, comparable to orange juice but definitely not as tasty. Furthermore, it is reported that the greenish tint of the springs is caused by polysulfide ions, converted from fumarolic sulfur (Waikato Regional Council Technical Report 2011/29).

The fumaroles do not only determine the chemistry of the lakes, but they also pose a hazard for visitors who leave the trail that should not be underestimated. The last incident happened at Upper Emerald Lake's east shore in March 2020 when a tourist broke through thin surface crust near a fumarole and suffered second-degree burns up to the knees.

Then, Lower Emerald Lake follows at some distance. Quite noticeable is the intensely orange coloured inflow stream from the southwest into the lake. Even if a first impression may imply that this could be a hot stream colonised by thermophilic microorganisms, most probably the precipitated iron ore limonite is responsible for the bright colour.

In contrast to the other lakes, Lower Emerald Lake appears transparent clear with a vivid blue-green hue. This is the result of mineral deposits on the bottom combined with water almost free of suspended matter. Depending on the viewing angle, yellowish deposits, light scattering, and reflections of the sky make colours change between light green and deep blue.

Just as the upper lakes also Lower Emerald Lake is lined by fumaroles. They are very prominent on its southeast side, where a side path permits closer access. A particular high number of fumaroles occupies the slope down into the adjacent Oturere Valley.

After traversing the Central Crater with its wide spread lava flow the trail climbs up to Blue Lake (Te Wai-whakaata-o-te Rangihiroa, The Mirroring Water of Rangihiroa). It is even weaker acidic than the Emerald Lakes and displays a steel blue colour. This lake is sacred (tapu) to the Māori and visitors should avoid to touch the water or to eat or drink in its vicinity.

Blue Lake also marks the spot where the long descent begins. Wandering the serpentines downwards, the steaming Te Maari (also written Te Māri) on the northeast slope of Mount Tongariro come into view. The upper Te Maari Crater was formed during an explosive eruption in 1869, while the larger lower Te Maari Crater is much older. A series of eruptions in the 1890s reshaped the upper crater significantly.

The youngest events were recorded on 6^th August and 21^st November 2012, when vents at the upper Te Maari Crater sent a three kilometre ash plume for about five minutes into the air and large blocks were thrown out, reportedly up to an altitude of 1 km (0,6 mi). Nowadays, the upper crater still displays pronounced fumarolic activity which can be easily spotted from the trail.

The 2012 eruptions were accompanied by the formation of a new crater and eruption fissure. Additionally, sections of the Tongariro trail and the Ketetahi Hut were damaged by ejected blocks up to 1 m (3 feet) in diameter.

Across the trail from Te Maari an elongated ravine with bare, multicoloured ground attracts attention. This Māori-owned area on the north slope of Mount Tongariro is called Ketetahi Hot Springs and it was free to visit until 1996, when the owners closed it due to abuse by hikers. Allegedly a tourist was scalded to death at Ketetahi, but I could find no evidence for this rumour. Since 1996 access is only permitted together with a Māori guide.

G. Rennison and J.L. Brock mapped the muddy, acidic, and sulfate rich springs and the fumaroles in 1968 (G. Rennison, J.L. Brock, A survey of the Ketetahi Hot Springs, New Zealand Journal of Science 24, 1981). The most active and largest vent on location was the noisy Great Ketetahi Blowhole which released high pressure steam of up to 138 °C (280 °F) from an opening 4.5 m (15 feet) in diameter. Several smaller features were found to erupt water jets permanently up to 1,5 m (5 feet) high or up to "15 feet radius over rocks". They observed no true geysers at Ketetahi, whereas Ashley D. Cody reported two cyclic geysers spouting to 2 m (6.5 feet) from visits in 1978, 1983 and 2000 (Waikato Regional Council Technical Report 2020/04, Sinter-forming springs and geysers of the Waikato region). All features discharge into the hot Mangatipua Stream, which crosses the trail at some distance down the slope.

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