The 1886 eruption of Mount Tarawera was a violent volcanic eruption that 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 2:00a.m. and lasted for approximately six hours, causing a ash column, earthquakes, lightning, and explosions heard as far away as Blenheim in the South Island – more than away. A long 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 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 in length by in width, by a series of deep ravines or gullies carved into the landscape. Estimates of the pre-eruption height of Ruawahia vary between , 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 above sea level, which has since been revised to . 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 in length from north to south and 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 inches 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 and a distance of to cover an area of . The Pink Terraces were the smaller of the two, reaching a height of and an area of , but were known for a distinctive pink hue of the silica.