Potential impacts of a widespread subplinian andesitic eruption from Tongariro volcano, based on a study of the Poutu Lapilli
Recent eruptions from Tongariro volcano, particularly from Ngauruhoe, have been small (Volcanic Explosivity Index or VEI ≤ 3), but between 11,000-12,000 calendar years BP there was a series of larger subplinian eruptions (VEI 4) from Tongariro volcano (Mangamate Formation), the last of which formed the Poutu Lapilli. This unit is considered to have had a volume of 1.1 km³, distributed in three main lobes (NW, NE, E) around a possible source or sources close to the present site of Ngauruhoe. Column height during the eruption ranged from 16 to 23 km. If such an eruption occurred today it would have a significant impact on the surrounding area. In the proximal zone (<25 km from source) >20 cm of ash/lapilli would be deposited, sufficient to cause collapse of some buildings in Tokaanu and the surrounding communities and damage to the Tokaanu Power Station. Much of the other infrastructure in the area (e.g electricity transmission lines, water supply and distribution, roads) would also be affected, as would the tourist industry. Evacuation may be necessary from this proximal area, and plans for this should be drawn up well before the event. Adequate provision also has to be made for disposal of ash from towns/communities and from key roads. Medial and distal areas (up to 100 km from source) would be affected, but to a decreasing extent with distance from source. An eruption like that forming the Poutu Lapilli is possible at any time in the future from Tongariro, and it is important that an effective management system is put in place to deal with such an event.