About 80 kilometres below the central North Island, the crust of the descending Pacific plate melts, and hot rock pushes up towards the surface. The result is the chain of volcanoes and thermal areas stretching from Ruapehu and Lake Taupo to Rotorua and out to White Island at sea. Recent sea-floor surveys have shown that there are undersea volcanic mounts and thermal vents roughly in a line to the Kermadec Islands and beyond towards Tonga.



Mt Ruapehu is the largest and most southerly in this chain. Built up over hundreds of thousands of years, it is also the highest point in the North Island, at 2,797 metres in elevation. It has a lake in its summit crater which was once a nice spot for a hot soak, but is now dangerously acidic thanks to the most recent eruptions, in September/October 1995 and June 1996, which billowed towering clouds of ash over the surrounding countryside and sent glowing boulders bouncing down the slopes. It completely ruined the ski season in 1996 and jets had to make detours out to sea to avoid the clouds of ash. Although it is capable of cataclysmic eruptions, the most dangerous aspect of Ruapehu's volcanic nature has been the flows of water, mud, rocks and ice, called lahars, which have swept down its slopes from the crater lake. On Christmas Eve of 1953, a lahar burst out through an ice cave under a glacier, swept down the Whangaehu River, and took out a railway bridge at Tangiwai. Only a short time later, a train carrying holiday makers to Auckland plunged off the broken bridge into the raging torrent of mud, ice and boulders. One hundred and fifty-one people died. Fittingly Tangiwai means 'weeping water'. The next lahar is a matter of when rather than if, and scientists maintain a network of sensors and warning systems to avoid a repeat of the Tangiwai disaster.

Maori tradition tells that Ngatoro-i-rangi caused fire to come to the mountains. He was the tohunga or high priest of the Te Arawa canoe, which had travelled from the ancestral Maori homeland of Hawaiki. While he was exploring the area with his slave Ngauruhoe, he stocked Lake Taupo with fish through magic incantation and then climbed Mt Tongariro to claim the region for his tribe. A blizzard blew up and he called to his sisters in Hawaiki to send him fire. The sacred flame they sent burst out firstly in the sea, creating White Island, and then out of the land, forming the thermal areas around Rotorua and Taupo, before finally bringing the cold mountains to roaring life. The eruptions saved Ngatoro-i-rangi from the cold, but it was too late for Ngauruhoe, after whom the mountain adjoining Tongariro is named.

Mt Ruapehu is part of Tongariro National Park, the country's first, dating from 1894. In the late 1800s, Te Heuheu Tukino IV, paramount chief of the local Ngati Tuwharetoa tribe, rightly saw the encroaching European colonists and the desires of other tribes as a threat to the mountains, which were sacred (tapu) to Ngati Tuwharetoa. Through the passion of his oratory he convinced the other tribes not to claim the mountains, but feared that the Europeans would get hold of them and cut them up into sheep paddocks. In 1887, in an act of great vision, he gifted Mounts Tongariro, Ngauruhoe and Ruapehu to the nation to become a national park and thus preserved the sacred mountains of his people forever. In 1894 an act of parliament was passed that created Tongariro National Park - the first in New Zealand and the fourth such park in the world. Today it covers over 80,000 hectares with a diversity of ecosystems, a network of tracks and huts and three skifields. It is a deservedly popular area. The crossing of Mt Tongariro is undertaken as a daywalk by hundreds of people a day in summer, while in winter thousands of people flock to the skifields of Ruapehu.

The current vegetation patterns in the park have been shaped over millenia by successive eruptions. One of the most dramatic was an eruption of the Taupo volcano (Lake Taupo is its crater) in 186 AD. This blasted the central North Island with over 100 cubic kilometres of incandescent rock, ash and pumice. Much of the vegetation was scoured from the park and a lot of the rest was blanketed in metres of pumice. Forest on the south side of Ruapehu survived and has re-colonised the west side of the mountain, but the harsh climate and strong winds have kept much of the rest as scrublands or tussock. There is even a large area of rocky desert on the eastern slopes. This is partly because the prevailing winds bring rain from the west or south-west (about 2,500 mm a year on those sides), whereas the eastern side is mostly in the mountain's rain shadow and so receives very little. Being an alpine area, the park is prone to strong winds and sudden weather changes. I recall reading about a backpacker who set off on a sunny day wearing jeans. A southerly change with rain and icy winds caught him on an exposed ridgeline. He became hypothermic and his body was found the next day. Several years ago most members of an Army training squad died when they were caught out by bad conditions on the summit of Ruapehu. Thousands of people tramp in the park every year though, especially on the 'Great Northern Circuit' over Tongariro, but the route around Mt Ruapehu is less popular.