About 2 or 3 million years ago, the collision between the Pacific and Indo-Australian tectonic plates, meeting along the eastern edge of the North Island, shaped vast areas of the country. A chain of pressure-fold mountains was forced up, forming a chain from East Cape to Wellington, and as the Pacific plate was forced down, its melted crust rose to the surface and exploded out of the central North Island. That titanic collision is still happening today and the result is the chain of volcanoes and thermal areas stretching from Tongariro, 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 Tongariro (elevation 1968 metres) is a complex volcano of several craters that have blown their way out of what was once probably a much larger mountain, built up in eruptions over the last few hundred thousand years. It gave off smallish clouds of ash in the 1800s and late 1920s, but the main activity in recent times has come from the perfect cone which has grown out of an old crater on Tongariro's southern slopes - Mount Ngauruhoe (elevation 2291 metres). It's only 2,500 years old and quite active. In the 1950s it pumped fountains of lava 300 metres into the sky and in 1967 sent glowing clouds of gas and ash down its slopes. Since then it's been relatively quiet, just puffing out the occasional cloud of gas or ash.

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 Auruhoe, 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. His words were carried there by the wind. The name for the area, Tongariro, means carried away by the south wind. 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 Auruhoe, after whom the mountain adjoining Tongariro is named. A different version holds that Ngatoto-i-rangi sacrificed Auruhoe to the gods, throwing her body into a crater.

Tongariro National Park was 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 has re-colonised some areas of the mountains, but the harsh climate and strong winds have kept much of the rest as scrublands or tussock. The eastern slopes are generally drier and barer. 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. From the warmth of Mangatepopo Hut I once watched the rescue helicopter pick up one of a school group who had fallen and broken a limb up on the side of Tongariro. It was a dark and icy-cold night and I bet they were happy to see the helicopter's searchlight. Thousands of people tramp in the park every year, especially on the 'Great Northern Circuit' over Tongariro. This is a DOC Great Walk and you have to book a place in the huts if you want to use them.