A violent volcanic eruption occurred in the early hours of 10 June 1886 at Mount Tarawera, near Rotorua on New Zealand's North Island. The eruption reached an estimated volcanic explosivity index (VEI) of 5 and killed an estimated 120 people, making it the largest and deadliest in New Zealand during the past 500 years, a period that includes the entirety of European history in New Zealand.
The eruption began at roughly 2:00 a.m. and lasted for approximately six hours, causing a 10-kilometre-high (6 mi) ash column, earthquakes, lightning, and explosions heard as far away as Blenheim in the South Island – more than 500 kilometres (310 mi) away. A 17-kilometre-long (11 mi) rift formed across the mountain and surrounding area during the eruption, starting from the Wahanga peak at the mountain's northern end and extending in a southwesterly direction, through Lake Rotomahana and forming the Waimangu Volcanic Rift Valley. This rift is where the basaltic dyke that fed the eruption reached the surface.
Damage in the local area was extensive, with ashfall blanketing nearby villages, including Te Wairoa. The eruption is responsible for the presumed destruction of the famed Pink and White Terraces, which prior to the eruption were New Zealand's most famous tourist attraction and brought visitors from across the British Empire. Lake Rotomahana, the former site of the terraces, significantly expanded as a result of the eruption as it filled portions of the newly formed rift valley.
Geology
Mount Tarawera is one of several volcanoes in the central North Island of New Zealand, sitting within the Ōkataina Volcanic Centre along with other volcanic features including Putauaki and the Haroharo vents, as well as Lakes Rotomā and Ōkareka. The Ōkataina Volcanic Centre, one of eight caldera systems in the Taupō Volcanic Zone, is believed to have begun forming around 400,000 years ago, with five of six major eruptions contributing to the formation of the caldera over a period of ~350,000 years. Eruptions from the caldera have typically been rhyolitic, although evidence of basaltic and basaltic-andesite eruptions, as well as pyroclastics, have also been found. Over the past 21,000 years, an estimated 80 cubic kilometres (19 cu mi) of magma and volcanic material has collectively been ejected from 40 vents in the caldera in 11 distinct episodes of activity.
Mount Tarawera itself is a rhyolitic dome volcano that began forming roughly 23,500 years ago in the southeast of Ōkataina Caldera. The mountain was built up over a series of four major volcanic eruptions prior to 1886, each generating significant rhyolitic lava and pyroclastic material. The most recent of these, known as the Kaharoa eruption, occurred in the early 1300s and was responsible for the formation of the three main peaks of the volcano: Wahanga at the north, followed by the highest peak of Ruawahia in the centre, and Tarawera as the southernmost peak. These peaks were carved from an otherwise uneven tableland, itself roughly 4 km (2.5 mi) in length by 1.6 km (1.0 mi) in width, by a series of deep ravines or gullies carved into the landscape. Estimates of the pre-eruption height of Ruawahia vary between 1,080 and 1,100 metres (3,540 and 3,610 ft), with the summit itself showing no evidence of a crater.
Several lakes sat in the area around Mount Tarawera prior to the eruption. The largest of these, Lake Tarawera, is northwest of the mountain and had an estimated height at the time of 317 m (1,040 ft) above sea level, which has since been revised to 298 m (978 ft). To the southwest of the mountain were lakes Rotomahana and Rotomakariri, the names of which translate as warm lake and cold lake respectively and reflect the respective temperatures of the lakes. The warm lake of Rotomahana was 1.6 km (1.0 mi) in length from north to south and 480 m (1,570 ft) across, with three small streams providing the primary inflow to the lake. The cooler and smaller Lake Rotomakariri sat to the east of Rotomahana, with both lakes draining into Lake Tarawera via the Kaiwaka Stream. Much of the ground around Lake Rotomahana was warm to a few centimetres below the surface, with hot springs and steam fumaroles surrounding the lake.
The largest and most famous of these springs were those on the top of the Pink and White Terraces, two distinct sinter terrace formations on the western and northeastern shores of the lake respectively. The White Terraces are believed to have been the largest silica-based terraces in the world, descending from a geyser-fed spring over a height of roughly 30 metres (100 ft) and a distance of 244 metres (800 ft) to cover an area of 2.8 hectares (6.9 acres). The Pink Terraces were the smaller of the two, reaching a height of 26 metres (85 ft) and an area of 2 hectares (4.9 acres), but were known for a distinctive pink hue of the silica.
Background
In the years before the eruption, the area around Mount Tarawera was home to several settlements of Māori belonging to Tūhourangi and Ngāti Rangitihi, two iwi (tribes) which formed part of Te Arawa, a confederation of various tribes who trace their whakapapa (genealogy) to the migratory waka (canoe) of the same name. Settlement did not extend to the mountain, which was deemed tapu (sacred) as it had been used as a chiefly burial ground for generations. As the Pink and White Terraces grew in fame, several of the villages in the area – particularly Te Wairoa – began to develop into tourist towns catering to the growing number of international tourists visiting the area.
To reach the area, visitors typically came from Auckland to Tauranga via train or coastal steamer, transferring to horse-drawn coaches for the journey to Ōhinemutu (now part of Rotorua) and on to Te Wairoa. From Te Wairoa, visitors to the Terraces were rowed across Lake Tarawera to the settlements of Te Ariki and Mouri, before walking across a narrow isthmus to Lake Rotomahana and the site of the terraces. Several Pākehā-run hotels were established in Te Wairoa, with local Māori acting as guides or selling local produce to visitors. This trade brought an estimated £1800 per year to the economy of Te Wairoa (equivalent to roughly NZ $425,000 in 2023), primarily from boat trips, entertainment, and souvenir photography or sketches.
Prelude
The months prior to the eruption saw an increase in geothermal activity in the region, particularly through increased discharge at nearby hot springs and geysers. On 22 and 24 November 1885, unusually large eruptions occurred of the geyser at the top of the White Terrace. The second of these was to a height of more than 46 m (150 ft), higher than in the memories of local Māori.
In the days leading up to the eruption, a series of unusual events were documented by those living in or visiting the area. On 31 May 1886, eleven days prior to the eruption, a group of tourists returning from the Terraces claimed to have seen a waka approaching their boat before disappearing into the mist. Witnesses described two rows of occupants, one of which was "standing wrapped in flax robes, their heads bowed and, according to Māori eyewitnesses, their hair plumed as for death" using huia and kōtuku feathers. Tūhoto Ariki, a tohunga at Te Wairoa, declared the tourists had sighted a waka wairua (spirit canoe) which was to herald the destruction of the region.
Further physical signs of unrest in the area were also noted. In one account from 24 May 1886 by Sophia Hinerangi, a renowned Māori guide in the area, her tour party arrived to their boats only to find the creek they were moored in completely dry, with the boats beached in the mud. A series of seiches then occurred, flooding the creek with water from Lake Tarawera before receding again. Local guides refused to go on the lake following this without persuasion, with one reportedly remarking once they had agreed that "we can die but once, so we will all go down together". Two days before the eruption there was more steam activity locally from some steam holes that had been quiet for years. These phenomena were collectively interpreted by a local Māori chief as heralding an impending war, while Tūhoto saw them as consistent with his interpretation of the waka wairua.
Eruption
The evening before the eruption was a clear night, with nothing unusual until shortly after midnight on the morning of 10 June 1886, when a series of more than 30 increasingly strong earthquakes were felt in the Rotorua area. About 1:20 a.m. there was a flash of light. Explosions began at around 1:30 a.m., with a larger earthquake at around 2:10 a.m. being followed by the formation of an eruption cloud 9.5–10 kilometres (5.9–6.2 mi) high. During this initial phase, an 8-kilometre-long (5.0 mi) fissure vent opened from the Wahanga dome at about 1:30 a.m., based on reports of the location of the initial column. This then extended across Mount Tarawera, with fountains appearing in multiple locations. By 2:30 a.m., all three of Mount Tarawera's peaks had erupted, blasting three distinct columns of smoke and ash thousands of metres into the sky and causing a large amount of volcanic lightning. The fissure continued to expand to the southwest as the eruption progressed, at the same time as eruptive activity was increasing around the present location of Waimangu. One observer at Gisborne estimated the height of the eruption column at about 2:20 a.m. as 10 km (6 mi) high.
At around 3:30 a.m., the largest phase of the eruption commenced when the expanding rift reached Lake Rotomahana, causing the lake water to come into contact with magma and resulting in a large explosion and a volcanic crater two kilometres wide at the site of the lake. This explosion distributed a mix of wet ash and lapilli over an area of 10,000 square kilometres (3,900 sq mi), with heavy accumulations of this mud destroying several villages near to the mountain, causing the majority of deaths associated with the eruption. Ashfall of up to 15 metres (50 ft) was reported in the immediate vicinity of the rift, while a layer of ash 8 to 10 centimetres (3 to 4 in) deep fell on Te Puke and lighter amounts were reported on boats nearly 1,000 kilometres (620 mi) offshore from the Bay of Plenty coast. The ashfall at the top of Putauaki 24 km (15 mi) away was recorded soon after the eruption to be 36 cm (14 in) thick. This stage of the eruption saw a dramatic increase in the height of the eruption column, with estimates placing the height at between 27.8 and 34 kilometres (17.3 and 21.1 mi) in order to produce the pattern of ash observed given weather conditions. Approximately 2 cubic kilometres (0.5 cubic miles) of tephra was erupted during the course of the eruption.
This phase of the eruption continued for less than three hours, with the majority of the eruption having stopped by 6:00 a.m. Ash continued to fall in surrounding areas and as far away as Tauranga, prompting city officials to consider evacuating the city. Material continued to be ejected from craters at Rotomahana and Okaro for up to ten days after the initial eruption, although these declined in frequency and intensity. Occasional phreatic eruptions continued to occur at Rotomahana for months after the initial eruption, before declining volcanic activity allowed the former lake to begin refilling.
The noise from the eruption was reported across the country, including South Island towns such as Kaikōura, where it was thought to be a ship in distress, Blenheim, where the shock-wave rattled windows, and Christchurch, over 800 kilometres (500 mi) away. Māori on the Whanganui River believed the noise to be from an approaching war party, while in Auckland the noise and sight of the eruption was thought by some to be an attack by Russian warships.
Later geological knowledge
Immediate studies prompted by the eruption led to the first suggestion by a geologist that Lake Taupō hid a volcano, rather than the more obvious volcanoes near Mount Tongariro, that was the likely origin of the extensive pumice deposits of the central North Island. Other deeper local ash (tephra) deposits were believed to have been related to previous local eruptions.
The eruption was caused by a basaltic dyke reaching the surface rather than a rhyolytic eruption more typical of the Ōkataina Caldera. The dyke trended at 57° and when it reached the surface it formed the new craters that extended to Waimangu in a rift about 17 km (11 mi) long. The whole eruption has been assigned a VEI of 5. The basalt melt was not mantle-derived, as it had already undergone prior crystallisation. At Waimangu only phreatomagmatic and hydrothermal eruptions occurred, while across the whole area there is no evidence that significant amounts of faulting took place during the eruption.