Ruapehu, the largest active volcano in New Zealand, has the highest point in the North Island and has three major peaks: Tahurangi (2,797 m), Te Heuheu (2,755 m) and Paretetaitonga (2,751 m).

[5] The volcano is surrounded by a ring plain of volcanic material, made from lahar deposits, ash fall, and landslide debris.

[14] Ruapehu's glaciers are situated at the northern limit for the formation of permanent ice in New Zealand, and thus they are extremely sensitive to changes in climate.

[12][15] Ruapehu has a polar tundra climate (Köppen: ET) on the upper slopes, with average temperatures ranging from −4–15 °C in summer and −7–7 °C in winter, depending on elevation and cloudiness.

[18] Weather conditions can be changeable over the day, and mountain visitors are advised to be prepared and carry basic survival equipment.

[21] The same storm also trapped an experienced Japanese mountaineer when the weather unexpectedly closed in on him, but he built a snow cave and sheltered in it until he was rescued days later.

[23] In 2008 extreme weather resulted in about 2000 visitors being evacuated from Whakapapa skifield, with cars being led down the mountain in groups of five.

[26] Ruapehu sits on a basement of Mesozoic greywacke overlain by a thin layer of sediments of the Wanganui Basin, composed of sands, silts, shell beds, and limestone.

This prevents build-up of pressure and results in relatively small, frequent eruptions (every 20–30 years on average) at Ruapehu compared to other andesitic volcanoes around the world.

The tephra dam created by the 1945 eruptions collapsed on 24 December 1953, sending a lahar down the Whangaehu River and causing the Tangiwai disaster.

[33] The earliest known volcanic activity in Tongariro National Park was approximately 933,000 ± 46,000 years ago at Hauhungatahi, northwest of Ruapehu.

[28] Eruptions during this period are believed to have built a steep volcanic cone around a central crater, which would have been located somewhere near the present-day upper Pinnacle Ridge.

[5][28] Approximately 160,000 years ago, cone-building eruptions began again, this time from a crater that is thought to have lain northwest of present-day Mitre Peak (Ringatoto)—southeast of the original Te Herenga vent.

Parasitic eruptions also occurred at Pukeonake, a scoria cone to the north-west of Ruapehu and at several isolated craters near Ohakune.

The Mangawhero Formation can be found over most of modern Ruapehu, and it forms most of the mountain's high peaks as well as the Turoa skifield.

A second, larger lava dome appeared in May, which continued to grow over the following months and had emptied Crater Lake of water by July.

A particularly powerful eruption in the early hours of 21 August was heard in Hawkes Bay and the Tararua District, loud enough to awaken people from sleep and cause alarm.

[47] A tephra dam had formed at the lake's normal outlet during the eruptions, which eventually collapsed on 24 December 1953 causing a lahar that led to the Tangiwai disaster with the loss of 151 lives when the Tangiwai railway bridge across the Whangaehu River collapsed while the lahar was in full flood, just before an express train crossed it.

Nearly half of the water in Crater Lake was erupted into the air, which subsequently rained down onto the summit, generating lahars down several river valleys.

[45] The 1975 eruptions deepened Crater Lake from 55 to 60 m to more than 90 m.[45] Earthquake swarms to the west of Ruapehu between November 1994 and September 1995 marked the beginning of renewed heightened activity at the volcano.

[46][38] These eruptions produced more than 7 million tonnes of ash, which contaminated water supplies, destroyed crops, and led to the deaths of livestock.

[56] The eruptions also caused closures to the three ski fields on the mountain, costing the region an estimated $100 million in lost revenue.

[56] During the 1995–1996 summer period between the eruptions, Ruapehu Alpine Lifts ran its chairlifts up the mountain and organised guided tours to within 500 metres of the crater.

Hundreds of tourists visited, even though the volcano was still emitting steam and toxic sulphur gas and the Department of Conservation was warning that further eruptions were possible.

In 1997, the government proposed digging a trench through the blockage in the wall of Crater Lake, but this was plan was opposed by conservation groups and Māori.

[61] Other plans considered were building a stop bank on the Desert Road, or creating an early-warning system, which would be much more expensive than digging a trench.

The small eruption was marked by a magnitude 2.9 volcanic earthquake and sent waves 4–5 m (16 ft) tall crashing into the wall of the crater.

[31] The explosive phase of the eruption lasted for less than a minute and blasted ash, mud, and rocks northward, reaching to about 2 km from Crater Lake.

[46][70] Since then, Crater Lake has continued its regular cycle of heating and increased gas emissions, although with periods of sustained high temperatures that occurred in 2011, 2016 and 2019.

[10][9] GNS Science continuously monitors Ruapehu using a network of seismographs, GPS stations, microphones and webcams.