Tongariro Volcano

Tongariro is a complex andesite structure located NNE of Ruapehu volcano in the southern section of the Taupo Volcanic Zone, New Zealand. Tongariro (and Ruapehu) are distinct from other volcanoes in the Taupo subduction zone (where the Pacific plate moves under the Indian-Australasian plate), since they are primarily andesitic, in contrast to the primarily rhyolitic Taupo, Maroa, Okataina and Rotorua volcanic centers further north. The most prominent feature of Tongariro is Ngauruhoe stratocone (2291m) which is often (incorrectly) regarded as a distinct volcano. Whilst Ngauruhoe is the historically most active part of the complex, eruptions have occurred in historic times from Red Crater (1886m) and the upper Te Mari crater as recently 1926 and 1896, respectively.

Overview of Tongariro Volcano from NE. Te Mari Crater in foreground.	North Crater, Ngauruhoe Cone, Ruapehu (from front to back)

Overview of Tongariro Volcano from NE. Red Crater bottom left.	Ngauruhoe Cone N flank, 1954 Lava flows.	Blue Lake Crater, North Crater behind.

The Tongariro complex has been built over the last 275000 years. In the last 25000 years eruptions occurred from numerous vents in the complex. These included eruptions at the North crater, Blue Lake crater and the Tama lake craters, but also at a number of other sites spread mainly in a SW-NW orientation over a length of 15km. The Te Mari craters mark the N end of this activity, whereas the S end is marked by the Tama Lake craters. The complex extends further S to the Saddle Cone area which is however now buried under Ruapehu. Activity at Tongariro is though to have reached its climax over a period of several hundred years about 10000 years ago in an eruptive sequence now referred to as the Pahoka-Mangamate (PM) sequence. At least 6 major events involving multiple vents each emplaced over a cubic kilometer of pyroclastics and a number of smaller similar events occurred during this period which is thought to have involved a number of massive plinian eruptions. This period was associated with increased extension of the regional fault zone running under the volcano and rapid rising of several distinct magma bodies to the surface. In contrast, most volcanism at Tongariro involves slow-rising magma bodies which stagnate at shallow depths yielding chemically distinct eruptates. The solidified lava lake that forms the flat top of the massive N crater was probably formed during this period. The small pit in the top is a small explosion crater that formed subsequently.

Tongariro´s N Crater and Ngauruhoe Cone.	North Crater, explosion pit and old viscous lava flows.

Red Crater	Red Crater with Ngauruhoe behind.	Red Crater

Tama Lake Craters with Ngauruhoe behind.	Lower Tama Lake Crater	Upper Tama Lake Crater with Ngauruhoe behind.

Nguaruhoe cone rises about 900m from a base consisting of remnants of several older cones, including the Pukekaikiore cone, the rim of which is still partially visible on the N flank. The cone is thought to have grown over the last 2500 years by a succession of eruptions which appear to have included effusive, strombolian, vulcanian and sub-plinian activities. These appear to be fed by a succession of small magma batches, rather than a large magma reservoir. Ngauruhoe has erupted on average every 2-3 years between 1840 and the most recent eruption in 1975 (if the 2 small ash clouds erupted on 4. July 1977 are not counted). The eruptive history of Ngauruhoe is extensively documented (see Hobden, Houghton and Nairn, Bulletin Volcanology 64, p.392-409 (2002)) and unusual in that individual eruptions may involve several different types of activity in relatively short succession and that lava composition varies between eruptions or even eruption phases. Five main groups of lavas and pyroclastics can however be defined which have contributed to growth of the cone.

Ngauruhoe Cone Summit Crater	Ngauruhoe Cone Summit Crater

Lava has flowed down the flanks of Ngauruhoe in 1870, 1949 and 1954. Most eruptions have however been predominantly explosive. The two most recent major eruptions were quite different and deserve mention. The 1954-55 eruption was predominantly strombolian and emplaced a 150m wide scoria cone in the 400m wide summit crater which grew further during subsequent eruptions. During vigorous strombolian episodes over a period of 4 months, 10 distinct lava flows were emplaced on the NW flank. These flows, which can still be seen today, had an aa morphology near the summit yet assumed a more blocky morphology as they progressed to the valley floor. The 1974-1975 eruption was predominantly vulcanian. The eruption involved a number of explosive vulcanian episodes seperated by periods of relative calm. In January and March 1974, a number of vulcanian eruptions resulted in 3000m high eruption columns which partially collapsed, resulting in pyroclastic (block and ash) flows on the flanks of the cone. Such pyroclastic flows had last been observed during the 1949 eruption only hours before the effusive phase of that eruption. On 19 February 1975, a 7km eruption column was observed, soon followed by several about 4km high columns. Column collapse pyroclastic flows were again observed which mostly flowed down two distinct chutes on the N flank of the cone. Approximately 2 million cubic meters of material are thought to have been erupted on this day alone. Ballistics up to a meter in size were thrown up to 2.8km from the cone.

View of Ngauruhoe from WSW whilst on Tama Lakes Trail.	Ngauruhoe Cone with Upper Tama Lake.

Statistically, an eruption of Ngauruhoe is considered long overdue, yet fumarolic activity is presently low. The cone can be climbed as an add-on to the popular "tongariro crossing" hike which also passes the Red crater. The Tama lakes trail is an easier alternative and provides good views of both the Tongariro complex and nearby Ruapehu. Touristic flight over the complex can also easily be arranged.

For further info on New Zealands volcanoes, see sections on Ruapehu, Tarawera, White Island and Wai-O-Tapu geothermal area.

Further Photos

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