Ruapehu is largely composed of andesite and began erupting at least 250,000 years ago. In recorded history, major eruptions have been about 50 years apart, in 1895, 1945 and 1995�1996. Minor eruptions are frequent, with at least 60 since 1945. Some of the minor eruptions in the 1970s generated small ash falls and lahars (mudflows) that damaged skifields.
Between major eruptions, a warm acidic crater lake forms, fed by melting snow. Major eruptions may completely expel the lake water. Where a major eruption has deposited a tephra dam across the lake's outlet, the dam may collapse after the lake has refilled and risen above the level of its normal outlet, the outrush of water causing a large lahar. In 2000, the ERLAWS system was installed on the mountain to detect such a collapse and alert the relevant authorities.
1945 eruption and aftermath
The 1945 eruption emptied the crater lake and dammed the outlet with tephra. The crater slowly refilled with water, until on 24 December 1953 the tephra dam collapsed causing a lahar in the Whangaehu River. The lahar caused 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. It was already known that the river had partially undermined one of the bridge piers and 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 and six of the carriages fell into the river.
1995-96 eruptions
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 nine months. Several lahar were observed, both in the Whangaehu River and other areas of the mountain, between 18 September and 25
