Mount Tongariro

The oldest recorded volcanism in the area was at 933,000 ± 46,000 years ago at Hauhungatahi, northwest of Ruapehu. There is then a gap in identified materials until a small lava inlier on the western side of Tongariro that has been dated at 512,000 ± 59,000 years ago and is essentially buried by more recent activity.[7] The 90 km³ (22 cu mi) cone and 60 km³ (14 cu mi) ring-plain of the complex has multiple eruptive centres aligned with the Taupō volcanic rift and bounded by the Waihi and Poutu fault zones. The formation of these began about 304,000 years ago in the Tama lakes area and definitely was established by 230,000 years ago.[8] The eruptive centres extend from the Te Maari craters in the northeast to the Tama Lakes in the southwest and include the more classic cone of Mount Ngauruhoe which like North Crater, another symmetrical but smaller cone, required the absence of ice after the last ice age to form. Tongariro displays evidence for extensive Quaternary glaciation in the form of moraines and lava-ice interaction textures.[8] However Pukeonake is off this axis, approximately 6 km west of the linear vent zone, but is considered to be a satellite vent. There was an intense period of large explosive eruptions around 11,000 years ago from multiple vents between Tongariro and Ruapehu (the Pahoka-Mangamate sequence).[7]

NASA satellite image of the August 2012 eruption, from Suomi NPP

After a period of volcanic unrest that had resulted in an increase in alert level on 20 July 2012,[9] at 11:50 pm (NZST, UTC+12) on 6 August 2012, Mt Tongariro had what was initially believed to be a hydrothermal eruption after this increased activity. The eruption occurred at the Te Māri Craters,[10] which had not had a major ash eruption since 1897[11] and had been dormant since September 1899.[9]

The eruption occurred in a new vent below the Upper Te Māri crater, and sent blocks as large as 1 m (3 ft) in size up to 2 km (1.2 mi) from the vent.[12]

An ash cloud 6.1 km (3.8 mi) high deposited ash into the surrounding area, especially to the east of the volcano. The ash cloud travelled 250 km (160 mi) in four hours. NIWA reported the ash cloud contained about 10,000 m³ (350,000 cu ft) of ash, and that the ash cloud was 25 km (16 mi) long and 15 km (9.3 mi) wide 39 minutes after the eruption. Ash and the smell of sulphur was reported in Napier and Hastings.[13] The smell of sulphur was also reported in Wellington, Nelson and Blenheim.[14]

State Highway 1 to the east and State Highway 46 to the north of the mountain each received up to 5 cm (2 in) of ash cover, and were closed until the following morning due to ash and low visibility.[15] A layer of ash 10–15 mm (0.4–0.6 in) thick settled on farmland 5 to 10 km (3.1 to 6.2 mi) east of Mount Tongariro. Particle sizes were between 2 and 3 mm (0.08 and 0.12 in). The airspace within a 12 km (7.5 mi) 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, Taupō, Gisborne, Napier, Wanganui and Palmerston North due to the risk of volcanic ash clogging the engines on their aircraft serving those airports.[16]

No injuries were reported, and the only significant property damage was to the Department of Conservation's Ketetahi Hut, which is located 1.5 km (0.9 mi) west of the Te Māri Craters.[16] 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.[17][18] Flights in the area were cancelled, as were several the following morning.[19] Geologists had no warning before the eruption, saying it wasn't linked to warnings the week before of elevated activity at nearby Mount Ruapehu.[20][21]