Mount Tongariro is part of the Tongariro volcanic centre, which consists of four massifs made of andesite: Tongariro, Kakaramea, Pihanga, and Ruapehu. The andesitic eruptions formed Tongariro, a steep stratovolcano, reaching a height of. Tongariro is composed of layers of both lava and tephra and first erupted 275,000 years ago. Tongariro consists of at least 12 cones. Ngauruhoe, while often regarded as a separate mountain, is geologically a cone of Tongariro. It is also the most active vent, having erupted more than 70 times since 1839, the last episode in 1973 to 1975. Activity has also been recorded at other vents in recent history. Te Māri Craters erupted in 2012, for the first time since 1897. Red Crater last erupted ash in 1926 and contains active fumaroles. There are many explosion craters on the massif; water has filled some of these to form the Blue Lake and the Emerald Lakes. The high altitude and severe alpine climate between March and October cause snowfall in the winter and rain can freeze, causing verglas; in contrast in the mid to late summer, the mountains can be bare apart from remnant patches of snow in south-facing gullies. Unlike nearby Mt. Ruapehu, no glaciers exist on Tongariro today. However, geomorphological evidence in the form of moraines and cirques indicates the former presence of mountain glaciers. Dating of moraines on western Tongariro show that valley glaciers were present at several times during the last glacial cycle, before melting away at the end of the Last Glacial Maximum approximately 18,000 years ago.
At 11:50 pm on 6 August 2012, Mt Tongariro had what was initially believed to be a hydrothermal eruption after a month of increased activity. The eruption occurred at the Te Māri Craters, which had been dormant since 1897. The eruption occurred in a new vent below the Upper Te Māri crater, and sent blocks as large as in size up to from the vent. An ash cloud high deposited ash into the surrounding area, especially to the east of the volcano. The ash cloud travelled in four hours. NIWA reported the ash cloud contained about of ash, and that the ash cloud was long and wide 39 minutes after the eruption. Ash and the smell of sulphur was reported in Napier and Hastings. The smell of sulphur was also reported in Wellington, Nelson and Blenheim. State Highway 1 to the east and State Highway 46 to the north of the mountain each received up to of ash cover, and were closed until the following morning due to ash and low visibility. A layer of ash thick settled on farmland east of Mount Tongariro. Particle sizes were between. The airspace within a radius of the mountain was closed after the eruption, but later reopened to visual flights only. Air New Zealand cancelled some flights in and out of Rotorua, Taupo, Gisborne, Napier, Wanganui and Palmerston North due to the risk of volcanic ash clogging the engines on their aircraft serving those airports. No injuries were reported, and the only significant property damage was to the Department of Conservation's Ketetahi Hut, which is located west of the Te Māri Craters. There was no official evacuation but 24 people living along State Highway 46 fled their homes for fear of being isolated. Mount Tongariro erupted again at 1:20 pm on 21 November, ejecting an ash cloud 4000 metres into the air. Flights in the area were cancelled, as were several the following morning. Geologists had no warning before the eruption, saying it wasn't linked to warnings the week before of elevated activity at nearby Mount Ruapehu.
