Mount Ruapehu, or, according to the Maori, Ruapehu, is an active stratovolcano mountain, 2797 metres (9175 feet) high, situated at the southern end of the Taupo volcanic zone. Ruapehu is frequently active, and is one of the largest active volcanoes in New Zealand.
Between eruptions, a crater lake forms from melting snow. When the lake fills it sometimes overflows, causing a mud flow known as a lahar.
A lahar on 24 December 1953 caused the Tangiwai disaster, with the loss of 151 lives, when the Tangiwai railway bridge, across the Whangaehu river, collapsed just before an express train crossed the bridge while the lahar was in full flood. It was already known that the river had partially undermined one of the bridge piers and it is believed that the lahar finished the job, causing the bridge to collapse. Although warned of the collapsed bridge, the train driver was unable to stop the train in time.
Spectacular eruptions occurred during 1995 and 1996. Ruapehu had been showing signs of increased activity since late November 1994, with elevated Crater Lake temperatures and a series of eruptions that increased in intensity over about 9 months. Several lahar were observed, both in the Whangaehu river and other areas of the mountain between 18 September and 25 September 1995, indicating the Crater lake was being emptied by the eruptions. The Department of Conservation immediately issued hazard warnings and advised all people to keep off the mountain. This eruption also ended the ski season on the mountain. The eruption cloud also disrupted air travel, occasionally closing airports and the central north island airspace. Episodic eruptions continued until the end of November 1995.
Within hours of a major eruption during the night being reported on 25 September 1995, news media were trying to get live video of the eruption and amateur photographers had published eruption images on the World Wide Web. A webcamera, dubbed the world's first "VolcanoCam", was also set up. Since then Ruapehu has been monitored by at least one and sometimes several volcanocams.
After the 1996 eruption, it was recognised that a catastrophic lahar could again occur when the crater lake filled up and flowed over the volcanic ash dam blocking the lake outlet. This is the same mechanism that caused the 1953 lahar. It is estimated that such a lahar might occur by 2005 and the local authorities are currently planning for managing this event safely when it occurs.
Ruapehu has two commercial skifields, Whakapapa on the northern side and Turoa on the southern slope. The season is generally from July to October but depends on snow and weather conditions. Both skifields are accessible by car and chairlifts, with beginners to advanced skiing slopes. Limited accommodation and refreshments are available at Top O'the Bruce, at the entry to Whakapapa, as well as elswhere on the mountain. Alpine huts are also provided for trampers and climbers.
Weather conditions can be changable over the day, and mountain visitors are advised to be prepared and carry basic survival equipment. Although severe weather is unusual and generally forecast, it has claimed several lives over the years, including a party of soldiers undergoing winter survival training. During the same storm a japanese tourist was trapped in a snow cave for several days after he made the shelter when the weather unexpectedly closed in on him. On 5 July 2003 about 350 skiers and 70 skifield staff were trapped on the mountain overnight at Top O'the Bruce when a sudden snow storm blew up and made the access road too dangerous to decend within only a few minutes. They spent the night in relative comfort and all safely decended the next morning. Such rapidly changing conditions are typical of the weather on New Zealand mountains.