Mount Ruapehu
Mount Ruapehu is an active stratovolcano at the southern end of the Taupō Volcanic Zone and North Island volcanic plateau in New Zealand. It is northeast of Ohakune and southwest of the southern shore of Lake Taupō, within Tongariro National Park. The North Island's major ski resorts and only glaciers are on its slopes.
Ruapehu, the largest active volcano in New Zealand, has the highest point in the North Island and has three major peaks: Tahurangi, Te Heuheu and Paretetaitonga. The deep, active crater is between the peaks and fills with water between major eruptions, being known as Crater Lake. The name Ruapehu means "pit of noise" or "exploding pit" in Māori.
Geography
Ruapehu is located in the centre of the North Island of New Zealand, northeast of Ohakune, New Zealand, and southwest of the southern shore of Lake Taupō, within Tongariro National Park. Ruapehu is the largest and southernmost volcano in the national park, with an estimated volume of 110 km3. The volcano is surrounded by a ring plain of volcanic material, made from lahar deposits, ash fall, and landslide debris.There are three access routes to Ruapehu, and each access route leads to one of the three skifields that are found on its slopes. State Highway 48 leads to Whakapapa Village at the base of the mountain, and from there an access road leads up the mountain to Iwikau Village at the base of the Whakapapa skifield on the northwestern bumpy hilly slopes. An access road from Ohakune leads to Turoa skifield on the southwestern slopes, and a four-wheel drive track leads from the Desert Road to the Tukino skifield on the eastern slopes.
Ruapehu's active crater, dubbed Crater Lake, is situated at the southern end of the Summit Plateau, and as the name suggests, is filled with a warm, acidic lake. The lake's outlet is at the head of the Whangaehu Valley, where the Whangaehu River arises. The Whangaehu River is notorious for destructive lahars caused by Ruapehu's eruptions. In historic times, eruptions have built tephra dams across the outlet on several occasions, most recently in 1945 and 1996. These dams failed in 1953 and 2007 respectively, causing an outburst of Crater Lake each time, which sent destructive lahars down the river. The 1953 lahar was the cause of the Tangiwai disaster, a railway accident in which 151 people died. Even larger lahars occurred in 1862 and 1895.
Glaciers
A total of 18 glaciers have been recognised on Ruapehu, of which six are named. Two glaciers are found in the active crater: one on the north side of the crater under Paretetaitonga Peak and another one to the south, and these are New Zealand's only crater glaciers. Most of the ice on Ruapehu is contained in only three of its glaciers: the Whangaehu, Summit Plateau, and Mangatoetoenui glaciers. The Summit Plateau glacier is not a glacier in the true sense, but rather an ice field that fills an extinct volcanic crater, and the ice there reaches more than 130 m thick. The Whangaehu glacier feeds the Whangaehu River, and the Mangatoetoenui glacier is one of the principal sources of the Waikato River, which arises as a series of streams on Ruapehu's eastern slopes. On the western side of the mountain, many of the streams that arise there, such as the Whakapapa and Manganui o te Ao rivers, feed the Whanganui River.Ruapehu's glaciers are situated at the northern limit for the formation of permanent ice in New Zealand, and thus they are extremely sensitive to changes in climate. Surveys of the glaciers undertaken since 1955 have found that the glaciers have all been thinning and retreating, with the exception of the northern crater glacier, which thickened and lengthened after the 1953 outburst of Crater Lake lowered the lake water level.
Climate
Ruapehu has a polar tundra climate on the upper slopes, with average temperatures ranging from in summer and in winter, depending on elevation and cloudiness. On the lower slopes, Ruapehu has a subpolar oceanic climate.The prevalent wind direction in the region is westerly or northwesterly, and gale force conditions are common on the mountain. Rainfall is higher on the western flanks of Ruapehu than the eastern flanks due to the rain shadow effect. Whakapapa Village receives an average of 2,200 mm of rain per year, whereas the Rangipo Desert to the east of Ruapehu receives slightly more than 1,500 mm of rain annually. Snow falls on average as low as 1,500 m elevation.
Severe weather incidents
Weather conditions can be changeable over the day, and mountain visitors are advised to be prepared and carry basic survival equipment. Severe weather has claimed several lives over the years, including a party of five NZ Army soldiers and one RNZN naval rating, caught in a week-long storm while undergoing winter survival training in 1990. The same storm also trapped an experienced Japanese mountaineer when the weather unexpectedly closed in on him, but he built a snow cave and sheltered in it until he was rescued days later.Extreme weather conditions have caused visitors to be trapped on the mountain in the past. In 2003, about 350 visitors to Whakapapa skifield and 70 staff had to stay overnight in various lodges at Iwikau village after a snow storm made the road too dangerous to descend. In 2008 extreme weather resulted in about 2000 visitors being evacuated from Whakapapa skifield, with cars being led down the mountain in groups of five. About 100 cars were left at the skifield overnight.
Geology
Ruapehu is a composite andesitic stratovolcano located at the southern end of the Taupō Volcanic Zone and forming part of the Tongariro Volcanic Centre. Volcanism at Ruapehu is caused by the subduction of the Pacific Plate under the Australian Plate at the Hikurangi Trough to the east of the North Island. Ruapehu has erupted from multiple craters over its lifetime, however, only one crater is presently active, a deep crater at the southern end of the summit plateau which is filled with hot, acidic water, dubbed Crater Lake. The lake water currently covers separate north and central vents.Ruapehu sits on a basement of Mesozoic greywacke overlain by a thin layer of sediments of the Wanganui Basin, composed of sands, silts, shell beds, and limestone. It has not been clearly established when Ruapehu first began erupting, only that eruptions began at least 250,000 years ago and possibly as early as 340,000 years ago. Ruapehu has been built in four distinct stages of relatively intense eruptive activity followed by periods of relative quiet. Each of these four stages of activity has left behind distinct rock formations, named the Te Herenga Formation, the Wahianoa Formation, the Mangawhero Formation, and the Whakapapa Formation. Each of these rock formations is composed of lava flows and tuff breccias, and studies of these formations has revealed how volcanic activity at Ruapehu has developed over time. During the Te Herenga stage of activity, magma rose quickly through the crust during eruptions. However, by 160,000 years ago a complex network of magma dikes and sills had formed in the crust under the volcano, and lava erupted since that time shows signs of extensive mixing between different magma chambers prior to eruptions.
File:MtRuapehu 23oct2002.jpg|thumb|300px|A composite satellite image looking west across Ruapehu, with the older eroded volcano Hauhungatahi visible behind it, and the cone of Ngauruhoe visible to the right.
In more recent times, volcanic activity has been centred on Crater Lake. There are two active vents under the lake, dubbed North Vent and Central Vent. Activity is characterized by cyclic heating and cooling of the lake over periods of 6–12 months. Each heating cycle is marked by increased seismic activity under the crater and is accompanied by increased emission of volcanic gases, indicating that the vents under Crater Lake are open to gas escape. Evidence suggests that an open-vent system such as this has been in place throughout Ruapehu's 250,000 year history. This prevents build-up of pressure and results in relatively small, frequent eruptions at Ruapehu compared to other andesitic volcanoes around the world.
Crater Lake is emptied by major eruptions, such as the ones in 1945 and 1995–1996, but refills after eruptions subside, fed by melting snow and vented steam. In historic times, major eruptions have deposited a tephra dam across the lake's outlet, preventing lake overflow into the Whangaehu valley. The dam collapses after several years causing a large lahar down the valley. The tephra dam created by the 1945 eruptions collapsed on 24 December 1953, sending a lahar down the Whangaehu River and causing the Tangiwai disaster. 151 people died when the lahar swept away the Tangiwai railway bridge just before an express train crossed it. Another dam was deposited by the 1995–1996 eruptions, which collapsed on 18 March 2007. A warning system, the Eastern Ruapehu Lahar Alarm and Warning System system began operation on the mountain in 2002 to detect such a collapse and alert the relevant authorities. The ERLAWS system detected the 2007 lahar, and roads were closed and railway traffic stopped until the lahar had subsided.
Early eruptive history
The earliest known volcanic activity in Tongariro National Park was approximately 933,000 ± 46,000 years ago at Hauhungatahi, northwest of Ruapehu. Subsequently, andesitic clasts found 100 km southwest of Ruapehu, near Whanganui, demonstrate that volcanism was likely present in the Ruapehu area 340,000 years ago. However, the oldest rocks on Ruapehu itself are approximately 250,000 years old. Eruptions during this period are believed to have built a steep volcanic cone around a central crater, which would have been located somewhere near the present-day upper Pinnacle Ridge. Cone-building eruptions ceased about 180,000 years ago, and the cone began to be eroded away by glacial action. Rock formations that date to this period are collectively named the Te Herenga Formation, and today these formations be seen at Pinnacle Ridge, Te Herenga Ridge, and Whakapapanui Valley, all on the northwestern slopes of Ruapehu.Approximately 160,000 years ago, cone-building eruptions began again, this time from a crater that is thought to have lain northwest of present-day Mitre Peak —southeast of the original Te Herenga vent. Eruptions continued until approximately 115,000 years ago, and the lava erupted during this period is known as the Wahianoa Formation. This formation has also been heavily eroded by glacial activity, and it now forms the southeastern flanks of modern Ruapehu. The formation consists of lava flows and tuff breccias.
Beginning approximately 55,000 years ago, a third phase of cone-building eruptions began, creating the Mangawhero Formation. This formation was erupted onto the eroded Wahianoa Formation in two phases: the first occurring 55,000–45,000 years ago and the second 30,000–15,000 years ago. Multiple summit craters were active during this period, all lying between Tahurangi and the northern summit plateau. Possible Parasitic eruptions also occurred at Pukeonake, a scoria cone to the north-west of Ruapehu and at several isolated craters near Ohakune making up the Ohakune volcanic complex and the maar lakes of Rangatauanui and Rangatauaiti. The Mangawhero Formation can be found over most of modern Ruapehu, and it forms most of the mountain's high peaks as well as the Turoa skifield.