Mt Tongariro is not a single volcano, but a complex of craters that have been active at different periods. In 1868, violent earthquakes marked the eruption that formed the upper Te Maari crater, named after a Māori chieftainess. Upper Te Maari erupted again in 1896–97, dumping 50 millimetres of ash on the Desert Road, and wafting ash as far as Napier.
Mt Ngāuruhoe: young and active
Although regarded as a separate volcano, Mt Ngāuruhoe is Tongariro’s main active vent. The remarkable symmetry of Ngāuruhoe’s steep cone is the result of regular eruptions.
Ngāuruhoe frequently belches out clouds of ash. Over 70 ash eruptions have occurred between 1839 and 1975, on average about six years apart. Eruptions of lava are less common – they have been witnessed only in 1870, 1949 and 1954.
In 1867, Māori people told the scientist and explorer James Hector that in May ash from Ngāuruhoe had covered the ground with a white dust, like snow. They said that during a previous eruption, in 1865, the Taupō district – and even the water of the lake – was covered with several inches of black dust. The showers of ash that fell into Rotoaira, a small lake between the volcano and Lake Taupō, had poisoned the fish there.
The 1870 eruptions
Many eye-witness accounts describe the eruption of Ngāuruhoe on 7 July 1870, which followed about 30 years of intermittent ash eruptions. Two lava flows travelled down the north side of the mountain, and loud detonations continued for three months, heard as far as Hamilton, Rotorua, New Plymouth and Napier. The lava was also visible from distant areas. In July 1870 Colonel J. M. Roberts, at a constabulary post on the Napier–Taupō road, wrote of the event:
One night a glow was observed spreading down the cone, comparable to a tree fern fire on a distant hill, and the following day three great columns of steam were seen rising, one from the summit crater and the others from the lower slopes. 1
The 1948–49 eruptions
In 1948, explosive eruptions tossing out blocks many metres thick culminated in a three-week eruption in February 1949. The eruptions were accompanied by loud blasts, and a small lava flow. A plume of ash rose 6 kilometres into the air.
The 1954 eruptions
In 1954 Mt Ngāuruhoe produced more than six million cubic metres of lava – the largest flow ever seen in New Zealand in historic times. Spectacular lava fountains played above the summit, and a sizeable scoria cone built up within Ngāuruhoe’s crater.
During Ngāuruhoe’s 1954 eruption, D. R. Gregg of the New Zealand Geological Survey observed a lava flow on 30 June at close quarters:
‘The more rapidly moving parts of the flow were moving forward bodily. Boulders were tumbling from the face and exposing the red-hot interior of the flow as it slowly advanced over the fallen debris. As the blocks fell off, incandescent dust trickled out. The finer dust particles remained suspended in the air, producing a reddish-brown smoke most irritating to the eyes. As the flow moved, it produced continual grating and clanking noises.’ 2
The discharge of red-hot blocks of lava in January 1973 heralded the most recent major eruption of Ngāuruhoe. Sporadic activity continued through the year, building up to highly explosive eruptions of ash in January and March 1974, and in February 1975. One of these threw a 3,000-tonne block of lava 100 metres out of the crater. During the last and most violent eruption, gases streamed from the crater for several hours, producing a churning plume of ash that towered up to 13 kilometres above the crater. This column collapsed under its own weight, forming ash and scoria avalanches that swept down the flanks of Ngāuruhoe, leaving trails of rubble in their wake.
Although Ngāuruhoe’s eruptions have seldom been more than nine years apart, the mountain has been quiet since 1975 – the longest break in activity in its recorded history. Its steam vents have temporarily cooled, suggesting that the main vent has become blocked.