Mount Ruapehu, Ruapehu District, Manawatu-Whanganui Region, New Zealandi

Regional Level Types
Mount Ruapehu	Mountain
Ruapehu District	District
Manawatu-Whanganui Region	Region
New Zealand	Country

Mount Ruapehu is an active stratovolcano at the southern end of the Taupo Volcanic Zone in New Zealand. It is 23 kilometres northeast of Ohakune and 40 kilometres southwest of the southern shore of Lake Taupo, within Tongariro National Park. Ruapehu is one of the world's most active volcanoes and the largest active volcano in New Zealand. It is the highest point in the North Island and includes three major peaks: Tahurangi (2,797 m), Te Heuheu (2,755 m) and Paretetaitonga (2,751 m). The deep, active crater is between the peaks and fills with a crater lake between major eruptions.

The volcano is an interesting (and scenic) geological area, rather than a specific specimen site. It is an active multi vent andesite composite volcano, constructed during four separate periods of activity. Space here only allows a brief summary of the rock units of this complex geological area (see Hackett, 1985). The upper near vent geology will be included under the existing Crater Lake sub-heading.

The slopes and outer flanks consist of extensive lava flows associated with autoclastic breccias, laharic tuff breccias, volcaniclastic debris, and sub-ordinate airfall tephra layers. Most of the lavas are medium K acid and basic andesites, with rare basalt, and minor dacite. All the lavas are porphyritic with plagioclase, augite, hypersthene, olivine, with volcanomagmatic micro phenocrysts, and abundant metamorphic and igneous rock inclusions.

The oldest deposit is the Te Herenga Formation, named after a northern ridge on the volcano, showing on the northern and north-west slopes of the volcano, consisting of planeze (sloping triangular facet, protected by lava)surfaces, and aretes (narrow rocky ridge separating valleys). It includes lava flows, tuff breccia, and small igneous intrusive bodies, surrounded by a hydrothermal alteration zone. The formation is up to 300 metres thick.

The unit shows conspicuous glomerophenocrysts of pyroxene in contract to plagioclase showing prominently against the pale matrix. Olivine is absent. It is intruded by microdiorite plugs. The intrusives are highly jointed, and shattered, transected by a complex network of highly altered breccia dykes, surrounded by a brownish aureole of hydrothermal alteration. Plagioclase is commonly replaced or veined with pale olive green clays of montmorillonite, illite and minor pyrophyllite.

Lava flows are lenticular, anastromising, with successive flows over the topography, averaging 6.5 metres thick. The tuff breccia is associated with the lava flows consisting of clasts of spinose clinker breccia formed from viscous basaltitic to low silica andesitic lava flows, causing brittle, fragmentation, and blocky jointing. There is minor sands and gravel associated with the tuff breccia.

The Wahianoa Formation is younger, named after a prominent glacial valley on the south-east slopes, exposed on the south-east flanks of the volcano, consisting of lava flows, tuff breccia, of acid and basic andesite. The unit is up to 220 metres thick. It is intruded by dykes and plugs near the vent region. Some olivine bearing lava flows are present but uncommon. Pervasive hydrothermal alteration shows as yellow-brown discolouration across the surface of the area.

The lava flows average 4.5 metres thick, associated with tuff breccia. Some have conspicuous flow banding, and near the edge of the flows are tightly convolute. The lava flows are irregular, rubbly, with slaggy margins, passing into welded coarse and angular breccia. The breccia is chaotic, containing blocky clasts, oxidised to bright red colours. The welded breccia passes outwards to partially welded and non-welded breccia. There is minor un-welded airfall magmatic deposits from nearby vents. The breccia contains fresh grey and black andesite, red oxidised andesite, and weathered brown andesitic pumice, with some black volcanic bombs. There is also minor sands and gravel.

The Mangawhero Formation, is named after a river on the south-west flank, is exposed on many sides of the volcano except the south-east, consisting of upper lavas and pyroclastics, which form the high peaks and main cone seen today. It is olivine basalt to dacite, with some lower olivine bearing lava andesites being hybrid in origin. The lava flows average 8.4 metres thick. They are associated with breccia, with rubbly margins, blocky and platy jointing, with some glassy flows showing columnar jointing. The flows range from black glassy dacites to medium grey olivine basalt. The tuff breccia is chaotically associated with the lava flows. There is also minor sands, gravel, mantle and shower bedded tuffs and lapilli, dark olivine bearing vertical intrusive dykes.

The Whakapapa Formation is named after a ski field on the volcano slopes, consists of young lava flows, tuff breccia, airfall pyroclastics, and minor pyroclastic flows, consisting of acid to basic andesite. The material has erupted from six vents near the top of the mountain, each given its own rock member unit. More information for this has been added under the Crater Lake sub-heading.

The volcano is surrounded by a ring plain of laharic debris and airfall tephra, except to the north where the volcanic rocks merge with similar geology of other mountains in the volcanic chain.

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References