Ruapehu Volcano

Ruapehu volcano (2797m) is a massive andesitic stratovolcano located at the southern end of the Taupo Volcanic Zone, just south of the Tongariro volcanic complex on New Zealands north island. Ruapehu has a complex structure formed by numerous large eruptions (and inumerous smaller ones) over the last 120000 years. In the last 50000 years, layers of block lavas, breccias and pyroclastic materials have been erupted from at least 6 vents with a N-S orientation. In the last 10000 years, records show a reduced volume and magnitude of eruptions compared to earlier phases. Historic activity has been restricted to a summit crater known as the West Crater. This usually hosts a warm crater lake, the temperature of which tends to fluctuate in the 10-40'C range but can be higher. Sporadic steam and ash eruptions can occur through the lake without affecting its level significantly. Larger eruptions can partially or completely remove the lake, resulting in the generation of substantial lahars. Although the majority of lahars flow SE into the Whangaehu catchment, the 1995 eruption recently demonstrated that substantial amounts of material can be ejected laterally causing lahars in other drainages and threatening the ski fields located on the flanks of the volcano.

Chateau Tongariro with Ruapehu W flank behind.	Ruapehu viewed from NNW.

Lower Tama Lake with Ruapehu Volcano behind.	Ruapehu Volcano viewed from SSE.

The crater lake volume is estimated at 10 million cubic meters. A small fraction of this is sufficient to cause massive lahars. This was demonstrated in 1953. In the years after the 1945 eruption, the crater lake had gradually refilled, reaching 8m above the pre-eruption level by late 1953. The SE rim of the lake consisted of an unstable agglomerate of volcanic materials and ice. On christmas eve, this structure was no longer able to hold back the crater lake and, without warning, 1.6 million cubic meters of water were released into the Whangaehu drainage. The lahar travelled rapidly downstream and swept away part of the Tangiwai railway bridge 38km downstream. The passenger express from Wellington to Auckland could not be stopped before the bridge. The locomotive and first carriage reached the opposite bank, whilst the 4 following carriages fell into the river and were swept away. The next carriage was partially submerged and three fortunately remained on the track. Although 134 passengers could be saved, at least 151 lost their lives. Fatalities as a result of sudden crater lake drainage have also been documented at other volcanoes, including Kawah Ijen in Indonesia.

Ruapehu summit viewed from NE.	Ruapehu summit viewed from N.

Ruapehu West Crater with warm lake (Nov.2007)	West Crater with warm lake and Whangaehu drainage.	Ruapehu West Crater with warm lake (Nov.2007)

Large eruptions at Ruapehu have occurred in 1861, 1895, 1903, 1945, 1969, 1971, 1979, 1988, 1995 and 1996. Most of these have involved explosive phreatomagmatic eruptions. Minor short-lived phreatic events such as recently in 2006 and 2007 regularly occurred inbetween. The well-documented1995 eruption was initially phreatomagmatic and eventually resulted in removal of the crater lake. Ash-rich magmatic episodes followed and after a several-month pause these resumed in summer 1996. Over a 6 week period ballistics were distributed up to 1.5km from the crater and ashfall was recorded 200km from the volcano. Even short-lived phreatic eruptions can have a major local impact. The September 2007 event was without warning and lasted for 7 minutes. Impact craters were found 1.5km from the crater, an ash column reached an altitude of 5km and several small lahars were reported, including a small one in the Whakapapa ski area. The Dome Shelter Mountain Hut 700m from the crater was coated in ash and mud and was severely damaged by ballistics. A skier inside was critically injured by a large rock which penetrated the roof of the shelter.

West Crater with warm lake and Whangaehu drainage.	Ruapehu West Crater with warm lake (Nov.2007)

Eastern Flank of Ruapehu with West Crater lake at top right. More of the Whangaehu drainage is visible on this picture, extending to bottom left.	Ruapehu W Flank with skiing area. Grey areas are deposits from Sept.2007 eruption which have been partially exposed after melting of subsequent snow .

The summit of Ruapehu can be reached relatively easily on foot in the summer, yet touristic overflights are possible at any time for those who wish to see the crater lake. Further information on New Zealands volcanoes can be found in sections on Tongariro, Tarawera, White Island and Wai-O-Tapu geothermal area.

Further Photos

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