The beginnings of volcanic activity at Ruapehu are shrouded in the mists of time but early evidence comes from andesite pebbles preserved on an uplifted marine bench in the Wanganui district. These pebbles, which are about 300,000 years old, are the remnants of an andesite lava flow, which was emplaced following eruptions from an early volcano in the vicinity of Ruapehu. The pebbles were then transported by a proto-Whanganui River to their present position. The oldest lava sequences preserved in site are exposed on the northern flank of Ruapehu and seem to date from this time. This material comprises massive and brecciate lava flows of basic andesite, which formed a steep cone up to about 250,000 years ago.
Subsequently, the range of lava compositions widened as a new phase of cone building began about 130,000 years ago; these lavas appear to have emanated from one principal vent to the northwest of Mitre Peak. During this period, voluminous lahars were also generated. These inundated the nearby hill country of uplifted marine siltstones, sandstones and limestones, creating an extensive ring plain, especially to the north and to the west of the Whakapapa catchments. The resultant deposits total more than 12 km3 volumes.
A second cone-building phase occurred between 60,000 – 15,000 years ago during which time further lavas were extruded from several vents established between Tahurangi and the northern Summit Plateau. Over time, lava composition became more variable, from olivine basalt to dacite. Lahars were also generated during this time, leading to further deposition on the western and southwestern ring plain, especially around Raetihi. Smaller lahars also inundated the eastern ring plain at this time, and considerable thickness of volcanic ash also accumulated downwind of the volcano.
At about 30,000 years ago, basaltic magma erupted scoria and surge deposits from a satellite vent (Rochfort crater) on the northern outskirts of Ohakune. South of Ohakune, the magma also encountered groundwater, which flashed to steam resulting in two additional explosion craters that form Ohakune Lakes.
Around 25,000 years ago basaltic andesite erupted from the southwest flank of Ruapehu, spreading lavas across 4km2. Later similar flows were generated from the southern flank to form the Rangataua lava flows around 18,000 years ago. During this period the climate was colder than at present and most river beds were choked with sediment (aggraded) as further lahars spread laterally both to the west and to the east. Their deposits total in excess of 3-km3 volumes.
Sometime between 4,500 and 3,500 years ago, instability of the upper cone led to 34 million m3 debris avalanche of largely hydrothermally altered rock and clay that swept to the east across the Rangipo Desert. It covered 20 km2 and destroyed a widespread beech forest on the southeastern ring plain at the time.
In historical time the earliest report of eruptive activity at Ruapehu was in 1861 when a lahar was sighted in the Whangaehu River. Mr Henry Sergeant described the lahar in the vicinity of Wyley’s Bridge: "In the mid-summer of 1860-61…I was standing on the bank (of the Whangaehu River) …when I suddenly saw coming around a corner in the distance a huge wave of water and tumbling logs. They filled the whole trough of the stream…As it passed us it appeared to be covered with what we first thought to be pumice but the intense cold which soon made us shiver and turn blue caused us to discover that …[this] was no less than frozen snow. Mixed with this was a mass of logs and debris. Very soon a bridge passed us stuck in the roots of a giant tree and a few minutes later about a dozen canoes came down". Campion et al., (1988).
A major eruption was recorded in 1895 when further lahars were generated in the rivers draining the eastern flank. Smaller eruptions occurred intermittently until March 1945 when a volcanic dome was slowly emplaced within the crater. Crater Lake waters were gradually displaced into the Whangaehu River (no lahars were recorded) until explosive eruptions began tearing the new dome apart. Volcanic ash was spread from Wellington to the Bay of Plenty, with ash eruptions peaking in August to September. By December 1945 the activity subsided leading to a deep, vertically walled crater, which slowly refilled with water. By 1953 the lake had filled to a level 8m above the pre-1945 level. Suddenly, on the evening of 24 December 1953 an ice and volcanic debris barrier built across the outlet collapsed into an ice cave above the Whangaehu River. About 1,650,000 m3 of water were rapidly released entraining boulders and sand across the Whangaehu fan to form a substantial lahar. About 42km downstream the lahar was reaching its peak discharge at Tangiwai (810m3 per second) when the Wellington-Auckland passenger train was crossing the rail bridge. The bridge’s piers were unable to sustain the force of the debris-laden water and the weight of the train, causing the engine to plunge into the far bank and taking 6 of the passenger carriages into the raging torrent. In what was to become New Zealand’s worst rail disaster, 151 lives were lost (Fig 1.). As a result a lahar detection gauge was built 15km upstream to warn of any future lahar events that had the potential to damage the bridge.
Lava domes have been extruded at Ruapehu once and possibly twice in historical times (in 1945 and possibly 1861) but little remains of these today. Lava domes are bulbous masses of viscous lava, which are extruded slowly from a vent (Fig 2.). The radius of a lava dome is typically between a few hundred meters and 1-3 kilometres. Extrusion of a lava dome in the currently active summit vent poses few direct problems, all of which can be managed with use of an "exclusion zone" around the summit. The eruption is likely, however, to lead to more widespread hazards during explosive disruption of the dome e.g. ash fall and potentially pyroclastic flows. In contrast, dome extrusion from a flank vent onto the steep slopes of the volcano would be a more serious hazard, creating an eruption scenario similar to the 1990 eruption of Mt Unzen in Japan when collapse at the front of the lava sent rock, hot gases and ash plummeting down the volcano. The impact would be particularly severe if a northern flank vent were to be active as this could lead to an extended evacuation of Iwikau and possibly Whakapapa villages. Such a dome-forming eruption is likely to be prolonged (months to possibly years) and create an extended management problem for officials.
The 1995-96 eruptions produced significant changes to the crater area. Portions of the southeastern rim of the crater were eroded and deposition of new tephra has raised the elevation of the rim by 6-7m in the former outlet area. Studies suggest that the potential for collapse of the main crater wall is low but that the weak, poorly compacted and permeable new tephra barrier is strongly prone to erosion and/or collapse once the lake refills. The worst possible scenario in these circumstances is sudden collapse of the new tephra dam causing a lahar as large or bigger than the 1953 Tangiwai lahar.
Numerous types of hazards may result from a volcanic eruption, often simultaneously. The types of hazards that will occur depend on which volcano is erupting (Ruapehu, Ngauruhoe, Auckland, Tarawera, Egmont, Raoul Island or White Island) and the nature of the eruption. For example an eruption from Ruapehu through the Crater Lake will be different from one at say Ngauruhoe where there is no lake. Potential hazards include ash falls (pyroclastic falls), ballistic fall (lapilli and lava bombs), pyroclastic flows and surges, lava extrusions (flows and domes), lahars, volcanic gases, volcanic earthquakes and atmospheric effects. Many of these phenomena will only affect an area very close to the volcano. However, volcanic ash fall can be deposited hundreds to thousands of kilometres from its source, making it the product most likely to affect the largest area and the greatest number of people.
When a volcano erupts it will eject a wide variety of material into the air above it. The large fragments of material, 0.1 to 10 metres in diameter, rarely land more than 1-2 km from the vent. However the fine material (millimetre-sized ash) can be carried by currents in the eruption column to high above the volcano and pass into the downwind plume to rain out forming ash (pyroclastic) fall deposits.
Volcanoes are cone-shaped mountains made from magma that has come from deep below the ground. Most volcanoes are found in long narrow belts across the earth’s surface. The earth’s surface is made up of several plates that move about very slowly. Volcanoes and earthquakes are most likely to occur where plates meet e.g. New Zealand. Ruapehu is an active volcano in New Zealand. I have learnt a lot from this essay about tectonic plates and the volcano Ruapehu. I have also learnt about how it affects the people. I have thoroughly enjoyed this essay and hope to do similar essays in the future.