Mount Taranaki (formerly Mount Egmont) is an active stratovolcano in the Taranaki region on the W coast of New Zealand’s North Island. The primary peak is 2,518 m high with a 1,966 m satellite cone Fanthams Peak (Panitahi) on its south flank.
The peak was named by Captain Cook as Mount Egmont in 1770. It was named Pic Mascarin by Marc-Joseph Marion du Fresne in 1772. Mount Egmont remained on maps until 1986 when the NZ Ministry of Lands ruled that Mount Taranaki would be an alternative and equal official name. The surrounding national park is still named Egmont though geologists still refer to is as Egmont. The name will change again in 2020 as part of a treaty settlement with indigenous Maori to Taranaki Maunga.
Taranaki is the fourth volcano in a sequence that has been working its way south for nearly 20 Ma. The volcanoes are surrounded with ring plains of volcanic debris. The region gets 8 m of rain annually which is drained by over 365 turbulent, fast-flowing rivers across the ring plain. There are multiple lakes in the area.
Humans have changed the local ecology as native forests were cleared and native plant and animal species greatly reduced in numbers. Forests were replaced with pastures for dairy farming, one of the prime economic drivers for Taranaki.
Maori venerate the peak. The name of the mountain and the region comes from two Maori words: Tara (mountain peak) and ngaki (glistening), as the mountain typically has snow on its upper reaches.
The history of New Zealand and the indigenous Maori was relatively bloody. Taranaki and the surrounding region had its share of that conflict. The Maori originally arrived 1250 – 1300. Their lands in turn were confiscated in 1865 under the rationale that lands belonging to Maori in rebellion would be transferred to military settlers. It was returned to the Maori in 1978 and immediately gifted back to the NZ government as a gift to the nation. As with all things tribal, this ended up as a mess with a tribal tribunal claiming not all tribal members were aware of the transfer in 1978 and did not support it. The dispute was most recently settled in 2019 with yet another agreement, this one to rename the mountain once again. Both the name and surrounding national park were to be renamed, though the NZ Geographic Board has not yet ratified that action.
A 10 km radius circular area from the summit was protected as a forest reserve in 1881. Areas including neighboring older volcanic remains were added and turned into the Egmont National Park in 1900, the second national park in NZ. The national park has old growth forest and is surrounded by dairy pasture grazing.
Egmont National Park is open to hikers all seasons. The volcano is also climbed in all seasons, though the climb is more difficult in the winter. Plant species range from tropical near the seashore and on the ring plain surrounding the volcano to alpine species at the snow line. There are eight government hiker’s huts and two hiker’s lodges in the park. There are also two privately owned lodges inside the park. Three nearby towns also have affordable accommodations with luxury lodgings available in New Plymouth. Hiking and climbing are the primary tourist activities. A climb to the summit takes 6 – 8 hours. Only experienced snow and ice climbers should attempt the climb during the winter. There is a ski area on the eastern slopes open from June – Oct. The mountain is considered a moderately difficult climb generally because weather on the mountain can change unexpectedly. As of June 2017, 84 have died on the mountain, making it the second most dangerous mountain in NZ.
It is monitored with a webcam and 9 seismographs.
Major eruptive centers in the region have progressively moved S over the past 20 Ma. Initial activity 15 Ma were offshore volcanoes off the coast of northern Taranaki and Waikato. These are deeply eroded and buried with newer sediment.
Paritutu Rock at New Plymouth and Sugar Loaf islands off the coast of New Plymouth date around 1.75 Ma. Paritutu is thought to be the plug of lava from the first onshore volcano. The next oldest volcano is Kaitake (Kaitake Range), 15 km S of New Plymouth, last active around 500,000 years ago (500 ka). It may have been as tall as the current Taranaki. Today it is a range of hills, 680 m high. The nearest volcano to Taranaki is Pouakai (Pouakai Range), that may have been active at the same time as Kaitake but remained active until 240 ka. It may have been as tall as 2,000 m. Most of its extensive ring plain of ash and lahar deposits have been obliterated by more recent activity from Taranaki. Deposits to its north are deeply eroded.
Taranaki is geologically young, with activity starting around 135 ka. It is surrounded by a ring plain of lahar, avalanche, and flank collapse debris stretching as far as the coast, some 40 km away, and in places well into the ocean. Today, Taranaki has two eruptive centers: Egmont and Fanthams Peak. The surrounding ring plain of material is over 150 km3 of volcanic debris. Significant current geologic hazards from the volcano include lahars, debris avalanches and floods.
The current edifice above 1,400 m was mostly constructed in the last 10,000 years. This portion of the volcano is 12 km3 in volume. Lava flows on the upper cone are 2,000 – 400 years old. The remnants of the Sisters dome built in 1800 make the current summit. Fanthams Peak is a basaltic satellite cone that was formed no earlier than 3 ka and was active until 1.2 ka.
Eruptive history over that period has been sub-Plinian events and more frequent effusive events. The effusive events built lava domes that collapsed and deposited extensive block and ash flows filling valleys in most sectors of the edifice.
Activity cycle of Taranaki has been cycles of cone growth to a critical volume / height via explosive and effusive volcanism followed by flank collapses and massive debris avalanches. The most recent of these took place 7.500 ka. There are at least 19 identified debris avalanche deposits below the volcano. For the most part, flank collapses do not appear to be hot events. We will take a look at flank collapses / debris avalanches in the next section.
There are more than 20 tephra layers deposited on the surrounding ring plain over the last 30,000 years. Plinian eruptions ejected plumes as much as 14 – 29 km high with VEI 4 – 5. Pyroclastic flows associated with these eruptions reached as far as 18 km from the source.
There is a cylindrical body around 5 km in diameter beneath the volcano that may extend down to 10 – 15 km. Crustal thickness beneath the volcano is 25 – 23 km.
Minimum runout for debris avalanches is marked by the coastline of the Taranaki peninsula and exceeds 25 km to the W, 32 km to the S, and 39 km to the N. The northern deposits can be traced another 6 km offshore and have been discovered by oil and natural gas exploration. These avalanches were generally able to flow unconstrained except to the N, forming a broad fan around the volcano.
The largest known flank collapse at Taranaki took place some 20 ka producing the Pungarehu Formation with an onshore volume of 7.5 km3.
Debris avalanches have been going on in this part of New Zealand since volcanic activity began half a million years ago. The Old Formation dates back to 520,000 years. The Eltham Formation, Ararata Marine Terrace and Rangitawa Tephra sequence date 360 – over 400 ka and all appear to come from the Pouakai volcano. The Maitahi Formation was a catastrophic collapse from the Pouakai volcano dating 240,000 years.
The oldest confirmed Taranaki debris avalanche deposit is also the oldest known unit from the new volcano. The Motunui Formation is exposed along the N coast of the Taranaki peninsula and is dated 210 – 127 ka.
The Okawa Formation was emplaced around 105 ka. It covers over 255 km2 with a volume of 3.62 km3.
There is some difficulty doing detailed mapping to the S and SE of the volcano. The Stratford Formation is to the E and dates 80 – 50 ka. It contains 3 debris avalanche deposits. The Opunake Formation is lahar dominated and dates 38 – 30 ka.
The Ngaere Formation covers 320 – 500 km2 of the eastern ring plain with nearly 6 km3 of debris. Its age is 23.4 – 22.6 ka. This one was directly related to eruptive activity.
The Pungarehu Formation closely overlies the Kawakawa Tephra and dates 22.1 ka. It is the second most voluminous debris avalanche from Taranaki at over 7.5 km3 and covers 200 – 250 km2 of the western ring plain sector. It extends at least 8 km offshore. Avalanche mounds on the seafloor are visible to 60 m below the waves. Hummocks near the coast are 5 m high.
The Warea Formation was originally identified as volcanoclastic. Dating has varied a bit with initial thoughts that It was emplaced 16 – 12 ka. Later analysis has identified multiple lobes that date 22.5 – 14.8 ka.
The Kahui Formation is a series of block and ash flow and debris flow deposits dating 12.5 – 7 ka that extend up to 20 km from the summit. This formation also has multiple flow deposits on the NW and W flanks mainly in channels of major tributaries. On Taranaki, block and ash flows typically are produced by dome collapse.
The Opua Formation covers large parts of the W and SW ring plain. It is around 0.35 km3 and dates 6.6 ka. There is a large amphitheater between Bob’s Ridge and Fanthams Peak that is considered to be the source scarp. That scarp has been partly filled by younger lava flows.
The most debris avalanche recent formations are the Ngatoro (3.6 – 3.5 ka), Te Popo (3.1 – 2.9 ka). The Hangatahua (400 years old) Formation is a lahar deposit.
The last 600 years of activity is called the Maero eruptive period. A significant eruption took place around 1500 which destroyed much of the forest on the NW slopes. A pumice fall destroyed a Maori campsite around 1350. A single sub-Plinian eruption in 1655 (Burrell lapilli) dusted tephras across North Island. The last known eruption is considered to be 1755 which deposited the Tahurangi Ash. This eruption produced a pyroclastic flow. There were dome collapse events in 1800 and 1854.
Taranaki is interesting in that several researchers have identified what they believe to be a periodicity in major eruptions. As of today, there is no explanation for that periodicity or even unanimous agreement that it exists. The Holocene record of explosive pumice-producing eruptions from Taranaki appears to be cyclic with a 1,500 – 2,000 period. The largest eruptions occur regularly at the trough of the supposed cycles and erupt the most evolved (silicic) andesites. Semi-regular periods of activity separated by intervals of relative quiet have been suggested for Vesuvius, Merapi and Fuji.
There were two distinct periods with no eruptive events – 7 – 6.5 and 8.7 – 8.2 ka. For the last 6.,25 ka, there may be a 1,500 – 2,000-year cycle in the magma system indicating either a recurrence of regional tectonic events or large magma batches reaching sufficient buoyancy to rise and erupt. Over the last 3,000 years, there have been eruption of more than one magma batch during a relatively short period, which may be a common process at andesitic volcanoes. Analysis of lake sediments indicate that the frequency of identified eruptions has increased significantly over the last 500 years. There are at least 138 separate events visible over the last 10,150 years and that may still underrepresent the true eruption frequency of the volcano.
Eruptions from the summit are generally andesitic to dacitic. Eruptions from Fanthams Peak are generally basaltic andesites.
Concern over a possible eruption from Mt Taranaki triggered a round of moderately hair-raising articles 2018 – 2019 discussing what would happen to the surrounding habitations. These took turns being more breathless than the one before. The NZ Herald article May 2018 referred to an April 2018 paper by Torres-Orozco, et al entitled Volcanic hazard scenarios for multiphase andesitic Plinian eruptions from lithostratigraphy: Insights into pyroclastic density current diversity at Mount Taranaki, New Zealand.
The NZ Herald noted that over 85,000 live within 30 km of the mountain, 40,000 of them in high priority evacuation areas. The region also has dairy and petrochemical industries. The study described flank deposits over the last 5,000 years of eruption. This was the first eruption modeling done on Taranaki. Three scenarios were suggested: A major eruption from Fantham’s Peak; gradual growth and destruction of a dome at the summit and subsequent pyroclastic flows; and finally sputtering dome growth and small explosive events. In every case, pyroclastic flows were expected to reach urban locations 15 – 18 km away. Ashfall and tephras extended 20 – 30 km.
Needless to say, this triggered follow-on media coverage, for who can resist a dangerous monster in your own backyard? Stuff published a follow-up piece by Michael Daily in June 2018, describing Taranaki as “…the NZ volcano most at risk of a large eruption in the next 50 years.” Not being content with that headline, Stuff published another piece by Mike Watson in Apr 2019 entitled “Scientists find evidence that Mt Taranaki eruption could turn region into the next Pompei.” This one took a closer look at evacuation plans necessary following the original Torres-Orozco 2018 paper.
The interesting part of this analysis is the concentration on eruptions and not mentioning debris avalanches, flank collapses or lahars, all of which would seem to me to be a more significant and far less predictable threat than a hot eruption. Why? Because the volcano is well monitored and lahars and debris avalanches do not necessarily show early indications before they release downhill.
The Hikurangi Trough is E of the southern part of North Island. Relative plate motion along the trench decreases from 98 mm/yr in the Tonga Trench to the N to 42 – 50 mm/yr in the Hikurangi Trough. The Hikurangi Plateau to the E of the southern portion of North Island causes resistance to subduction. This forces a clockwise rotation between the trough and axial ranges of North Island. The rotation of these microplates contributes to extension at the southern end of the active Tonga – Kermadec volcanic arc which terminates in the Taupo Volcanic Zone (TVZ). Continental crust within the TVZ is relatively thin at 12 – 15 km and has exceptionally high heat flow, creating one of the most productive magmatic systems on the planet. Mount Ruapehu is located at the southern end of the TVZ.
The subducting plate is 100 km beneath Ruapehu, which is located 280 km from the trench. In contrast, the subducting slab beneath the Taranaki region is 250 km, about 400 km W from the trench. The process causing the rise of magma powering Taranaki is poorly understood, still being debated, and may be due to plate delamination allowing hot asthenosphere material to make it closer to the surface.
Taranaki is a geologically young region. The basin is large sinking block formed by the collision between the Australian and Pacific Plates. Over the last 80 – 100 Ma, erosion has filled the basin with mud and sandstones up to 7 km thick. Sediments are still being deposited. A large block of crust dropped between the Taranaki Fault to the E and the offshore Cape Egmont Fault to the W, forming the Taranaki graben. Most of the region’s gas and oil fields are found along the main fault lines.
Taranaki is a large, active andesitic volcano. While situated in close proximity to neighboring towns, the local population at least does not number in the millions like we have seen in other parts of the world. While it is in a time of relative quiet, it has a large low velocity body imaged below it with what appears to be a healthy plumbing system. Most concerning, it has an extensive ring plain of volcanic debris, most of it from flank collapses, debris avalanches and lahars. Its current active cones are quite steep, unstable and the region gets a lot of precipitation and the occasional earthquake. This is a volcano that must be treated with a great deal of respect and deference.
Volcano hazard scenarios for multiphase andesitic Plinian eruptions from lithostratigraphy: Insights into pyroclastic density current diversity at Mount Taranaki, New Zealand, Torres-Orozco, et al, April 2018
Stratigraphy, age, and correlation of voluminous debris-avalanche events from an ancestral Egmont volcano: Implications for coastal plain construction and regional hazard assessment, Alloway, et al, Mar 2010