Mt Ruapehu is the largest active volcano in New Zealand and is located at the southern end of the Taupo Volcanic Zone. , Ruapehu is the highest peak in the North Island (2797m), with several subsidiary peaks and several small glaciers.
Three summit craters have been volcanically active in the last 10,000 years. The active vent is now beneath the crater lake of South Crater. The crater lake contains warm, acidic water that is fed by snow melt.
Ruapehu is a stratovolcano (composite cone volcano) made of successive layers of andesite lava and ash deposits. The mountain is surrounded by a ring plain of volcanic material from lahars, landslides and ash falls. Tephra ranging in size from dust (ash fall) to bombs and blocks are produced in every eruption. Usually the crater lake causes magma to cool and fragment (explode) quickly and violently leading to fine ash eruptions. There are frequent lahars during eruptions or later due to collapse of the crater lake wall.
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- 25 September 2007: An explosive eruption, lasting about 7 minutes. During the eruption explosions spread ash, rocks and water across the summit area, producing lahars in two valleys including one in the Whakapapa ski field. In contrast with the previous eruptions in 1996, there was no high ash plume to produce ash fallout over a wide area.
- 18 March 2007 : the long predicted dam break lahar occured.
- 4 October 2006: Very small eruption leading to wash up of the lake several metres along the shore line in the crater.