|Last Updated: 11 September 2000||
Ruapehu, the tallest mountain on the North Island, is a massive andesite stratovolcano. The currently active vent is an acidic crater lake near its summit. The volcano has a volume of 110 cubic km and the surrounding ring plain has a similar volume. Photo is looking south across the flank of Ngauruhoe to the September, 27, 1995 eruption of Ruapehu. Photo courtesy of Thor Thordarson.
Simplified geologic map of Ruapehu. The volcano began to form more than 120,000 years ago. The Te Herenga Formation is made of lava flows, breccia, plugs, and dikes. It is exposed on the northwest side of the mountain. The Wahianoa Formation is made of lava flows, breccia, and lahars and is exposed on the southeast side of Ruapehu. The Mangawhero Formation form the resent-day high peaks and main cones of Ruapehu. The formation consists of lava and pyroclastic deposits. The Whakapapa Formation is mostly younger than the most recent glacial period (about 15,000 years ago) and is made of block lavas, breccias, and pyroclastic material that erupted from 6 vents. Crosses show the locations of vents active in the last 50,000 years. From Cole (1990).
Simplified cross-section of the overlapping cones that construct Ruapehu. Cross-section courtesy of Thor Thordarson.
Summit area of Ruapehu. Water from Ruapehu feeds four major rivers. Six glaciers flow down the mountain to elevations as low as 2,000 m. From Nairn and Cole (1975).
Cronin and Neall (1997) summarized the eruptive history of Ruapehu for the last 75,000 years:
- 75,000 to 64,000 years ago: moderately frequent (1 eruption at every 600 years) large volume and large magnitude eruptions and frequent low volume and low magnitude eruptions
- 64,000 to 35,800 years ago: very low frequency (1 eruption at least every 7,000 years) large volume and large magnitude eruptions and frequent low volume and low magnitude eruptions
- 35,800 to 22,500 years ago: moderate-low frequency (1 eruption at every 800 years) large volume and large magnitude eruptions
- 22,500 to 10,000 years ago: moderately frequent (1 eruption at every 500 years) large volume and large magnitude eruptions
- ca. 10,000 years ago: 1 large volume and large magnitude eruption including a pyroclastic flow
- 9,700 to 1,800 years ago: infrequent, low volume and low magnitude eruptions
- 1,800 years ago to present: Frequent (1 every 100 years) low volume and low magnitude eruptions.
Lahar deposit from the September, 1995 Ruapehu eruption. Note the abundance of ice in the lahar. Photo courtesy of Thor Thordarson.
Lahars have been common throughout Ruapehu's history. On the northeast ringplain of the volcano there are 15 distinct lahar deposits ranging in age from greater than 65,000 years to 5,000 years in age. The most active periods of lahar deposition correspond to cool and stormy climatic conditions about 75,000 to 65,000 years ago and 23,000 to 14,000 years ago. These cooler climates allowed the growth of glaciers which, in turn, helped supply more sediment to lahars. Lahars distribution has been influenced by a large lava flow (22,500 years old) and by younger (less than 15,000 years ago) glacial moraines. These barriers continue to direct lahars on the northeast side of Ruaphu into the Whangaehu and Mangatoetoenui catchments (Cronin and Neall, 1997).
Recent historic eruptions at Ruapehu are described in Houghton, and others (1994). They include:
- 1968: lahars associated with lava near the lake floor;
- 1969: lahars associated with phreatomagmatic eruption;
- 1971: lahars and fallout ash associated with phreatomagmatic eruption;
- 1975: lahars associated with a very large phreatomagmatic eruption;
- 1977: pyroclastic surges associated with a phreatomagmatic eruption.
Sources of Information:
Cronin, S.J., and Neall, V.E., A late Quaternary stratigraphic framework for the northeastern Ruapehu and eastern Tongariro ring plains, New Zealand: New Zealand Journal of Geology and Geophysics, v. 40, p. 185-197.
Hackett, W.R., and Houghton, B.F., 1989, A facies model for a Quaternary andesitic composite volcano: Ruapehu, New Zealand: Bulletin of Volcanology, v. 51, p. 51-68.
Houghton, B.F., Latter, J.H., and Hackett, W.R., 1987, Volcanic hazard assessment for Ruapehu composite volcano, Taupo volcanic zone, New Zealand: Bulletin of Volcanology, v. 49, p. 737-751.
Houghton, B.F., McPhie, J., and Simmons, S., 1994, Physical volcanology and modern geothermal systems; Field Guide 1 North Island, New Zealand: Masters of Economic Geology Course Work Manual 8, Centre for Ore Deposit and Exploration Studies (CODES), University of Tasmania.
Malahoff, A., 1968, Origin of magnetic anomalies over the central volcanic region of New Zealand, in, The Crust and Upper Mantle of the Pacific area: eds., Knopoff, L., Drake, C.L., and hart, P.J., Geophysical Monograph 12, p. 218-240.
McClelland, L., Simkin, T., Summers, M., Nielsen, E., and Stein, T.C., 1989, Global Volcanism 1975-1985: Englewood Cliffs, New Jersey, Prentice Hall, 655 p.
Nairn, I.A., Wood, C.P., Hewson, C.A.Y., and Otway, P.M., 1979, Phreatic eruptions of Ruapehu: April 1975: New Zealand Journal of Geology and Geophysics, v. 22, p. 155-173.
O'Shea, B.E., 1954, Ruapehu and the Tangiwai Disaster: New Zealand Journal of Science and Technology, B36 (2), p. 174-189.
The 1995-96 Ruapehu Study Group, 1996, The 1995 Ruapehu eruption, New Zealand: Eos, v. 77, p. W131.