Tuesday 29 November 2022

Part 1 of Ōpōtiki, the shifting and fiery land around us. A thrilling story of volcanism and tectonics..

Exploring the geology around my home in Ōpōtiki and the Eastern Bay of Plenty I have found there is so much to talk about that this post is going to be split into two parts. No two forces have had a greater effect on this part of the North Island in recent geological history than tectonics and volcanism. Today’s post tells the story of the volcanism that has and will continue to shape this area of the North Island. Part two of this post will explore plate tectonics and subduction as the structural force behind the volcanism and landscape of this region. Ōpōtiki is in the Eastern Bay of Plenty, with the district stretching eastwards onto the northern Raukumara Peninsula. The town is situated on the floodplain where the Waioeka River and the Otara River merge before flowing out to the Pacific Ocean.

Aerial view of Ōpōtiki showing the Otara river joining the Waioeka before it flows out to sea. Source: LINZ (2017)

Map of Ōpōtiki district. Source: Opotiki District Council (2017).

But to get a real sense of where this township sits in the land we can look under our feet and further afield at the geological forces that have shaped the Bay of Plenty, the coastal lowlands, and the Central Plateau. There is no shortage of signs of these processes occurring beneath our feet and before our very eyes. The gently rolling hills surrounding the town are formed by rivers and streams eroding into volcanic deposits either directly deposited from the air or washed down by rivers and floods from higher areas. When we think of active volcanoes we may think of physically obvious volcanoes such as Ruapehu, Ngauruhoe, or Tongariro. These volcanoes are part of a belt of volcanism running from Ohakune in the south and offshore to Wharaaki/White Island, commonly referred to as the Taupō Volcanic Zone (TVZ), pictured below. The other two significant volcanic zones in the North Island are the Auckland Volcanic Field (AVF) and Coromandel Volcanic Zone (CVZ), both of which I look forward to exploring in future posts.

Simple map of Taupō Volcanic Zone, showing volcanic peaks and volcanic centres (Calderas). Source: University of Waikato (2013).

The most voluminous and, violent eruptions in the TVZ have formed a series of calderas (collapsed magma chambers), forming many lakes such as Taupō and Rotorua, and which may not be obvious as volcanoes. It is the eruptive deposits of these massive volcanic events that have formed most of the hills and soils of the Eastern Bay of Plenty lowlands. To give you a sense of the volume of material erupted from the two largest eruptions from Taupō, the diagram below shows that the Ōruanui eruption 26,500 years ago was 10 x the volume of the 1815 Tambora  eruption in Indonesia, the largest eruption in recorded human history.  

Comparative volumes of eruptive material from historic eruptions. Source: Richard Smith, David J. Lowe and Ian Wright, 'Volcanoes - The Taupō volcano', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/diagram/8716/taupo-eruptions (accessed 13 October 2017)

From the same diagram we can see that the most recent eruption from Taupō, approximately 1800 years ago was 100 times less the volume of Ōruanui but still a very significant eruption (this eruptive episode is referred to as the Taupō eruption). In fact, volcanoes that have erupted these volumes of material are referred to as “supervolcanoes”. It is no wonder that wherever the cover is peeled away in this region one can see signs of ash-falls and massive amounts of volcanic material moved by dynamic natural forces.

Above and below, layers of pumice and ash exposed in roadside cutting, Kutarere. Source: Author (2017).

Coarse alluvial (laid down by flowing water, usually a river) deposits consisting predominantly of reworked volcanic materials and gravels, with layer of fine ash towards top of outcrop. Coastal cliffs, Waiotahi Beach. Source: Author (2017).

Signs of a high energy alluvial (deposited by a river) environment. Cross bedding, and wavy contacts between beds. Coastal cliffs, Waiotahi Beach. Source: Author (2017).

However, one does not need to look far for reminders why the TVZ is considered one of the most currently active volcanic regions in the world. From Ōpōtiki it is hard to miss White Island, approximately 48km from the coast and steam rising from the crater visible on a clear day. 

Te Puia o Whakaari/White Island, view from eastern Bay of Plenty looking into main crater with steam fumeroles visible just above sea-level. Source: Author (2017).

The Māori name Te Puia o Whakaari means “The Dramatic Volcano” or “that which can be made visible” and is explained in two Māori legends. One tells of Maui fishing the North Island out of the ocean, accidentally stepping on the land and some of the fire burning on it. Where he shook this off his foot into the sea this became Whakaari. Another legend tells of the arrival of Chief Ngatoro-i-rangi bringing fire from Hawaiiki. Leaving his sisters at Whakaari, he voyaged to Maketu and onwards to Tongariro. Finding it so cold at Tongariro he called on his sisters to send fire. Volcanic and thermal activity between Whakaari and Tongariro marks the route of the underground journey taken by spirits bearing fire. The volcano, most of which is underwater (the main crater is only 30m above sea-level), which is illustrated in the diagram below, has been continually active since human settlement of New Zealand, with the most active period between 1975-2001.  Presently the volcano is at an alert level of 1 meaning minor volcanic unrest.

Diagram showing the majority of the White Island volcanic cone under water, and magma chamber and conduit during one of several types of eruptive phase typical of White Island. Source: Adapted from Cole et al. (2000). Magma origin and evolution of White Island (Whakaari) Volcano, Bay of Plenty, New Zealand. Journal of Petrology, 41(6), 867-895.

The island passed into the hands of European officers in the mid-19th century, allegedly for two hogsheads of rum. From 1885 onwards the island was the site of commercial sulfur mining, which continued till 1933 despite several miners being killed by a landslide from the crater wall in 1914. Understandably such a unique, dynamic, and relatively accessible volcanic feature has long been of interest, both for scientific interest and tourism , as can be seen in this photo of an early 20th century field trip to White Island.

The island remains in private ownership and has been classified as a scenic and scientific reserve since the 1950’s. Access to the island is limited to licensed operators, with a very popular option a boat trip from Whakatane which not only gives an opportunity to explore the island but also the prolific marine life of the Bay of Plenty.

The most famous and deadliest eruption in recent New Zealand history is the 1886 eruption of Tarawera, Rotomahana, and Waimangu. Tarawera is part of the Okataina Volcanic Centre, which can be seen in the map of the TVZ. The main eruption occurred along the three mountains forming the Tarawera complex, tearing a rift open and forming a series of new craters. At the height of the eruption volcanic activity was occurring along a series of craters running for 17 km, showering the Bay of Plenty with volcanic material and sending ash as far as ships 220km off the coast. The eruption at Lake Rotomahana showered “Rotomahana Mud” over a wide area of the Bay of Plenty, destroyed the world-famous Pink and White terraces and increased the size of the lake three-fold. Explosions were heard as far away as Auckland and the South Island, two Māori villages at Te Ariki and Moura were overwhelmed by volcanic surges and buildings at Te Wairoa collapsed under the weight of falling mud. Approximately 120 people died and the following description by I. Nairn gives one a sense of the wider effects of the eruption as it happened

“The 1886 Tarawera eruption was accompanied by spectacular lightning activity, with fireballs reported to have set fire to a house at Te Wairoa, and to the forest on the north shore of Lake Tarawera. Fissures opened on the numerous faults of the Taupō Fault Belt to the southwest of Tarawera, making travel across country difficult. Strong winds during the eruption damaged the forest at Tikitapu, felling many trees, and suffocating gases were experienced at Te Wairoa, making breathing difficult. The northward passage of the eruption cloud caused darkness during daylight hours at Rotorua, Te Puke, Tauranga, Whakatane, Opotiki, and East Cape”  Source: Nairn (2002). Geology of the Okataina Volcanic Centre, scale 1:50000. Institute of Geological and Nuclear Sciences geological map 25. 1 sheet + 156p. Lower Hutt, New Zealand: Institute of Geological & Nuclear Sciences Limited.

Eruption of Mt. Tarawera, 1886. Source: http://mp.natlib.govt.nz/detail/?id=26464

Rift torn by the 1886 eruption thorough the three domes of the Tarawera complex. Source: http://mp.natlib.govt.nz/detail/?id=21432

At the southern end of the Taupō Volcanic Zone recent activity has centered on the three volcanoes Ruapehu, Ngauruhoe, and Tongariro. Unlike calderas, the distinctive volcanic cones, most typified by the symmetry and steep slopes of Ngaurahoe, are referred to as stratovolcanos. In fact White Island is a stratovolcano, however, almost the whole cone structure is under water. In the case of Ruapehu, much of the original cone has been destroyed by earlier eruptions or lost through massive avalanches when part of the cone collapsed. Tongariro is actually a collection of  vents from one eruptive centre, forming a volcanic complex. The most recent significant eruptions at Ruapehu were over the period 1995-1996. In 2012 a hydrothermal eruption at Te Maari craters in Tongariro caused closure of the Tongariro Crossing track and damaged a DOC hut that fortunately at the time was empty,  and Ngauruhoe last erupted significantly over 1974 and 1975.

Mt Ruapehu Massif at the break of dawn during a restful period, from Tāwhiri, Waiouru. In terms of frequency and eruptive energy the geological record tells us that Ruapehu has been in a "quiet" phase since human settlement of New Zealand. Source: Ross McComish (2017).

The most common type of rock throughout the volcanic zone, and forming much of the volcanic plateau is called ignimbrite (this internationally accepted term for these deposits was coined by a New Zealand volcanologist Patrick Marshall in the 1930’s).  The term ignimbrite is derived from the latin ignis for fiery, and imber for spray, as these deposits are formed from a ground-hugging cloud of ash and gasses and hot rock fragments, rather than lava flowing on the ground-surface. There is a great variety of types of ignimbrite, from loose unconsolidated deposits of ash and pumice fragments, to very hard rock-like welded ignimbrites.

Welded ignimbrite forms a hard rock due to welding processes taking place as ignimbrite cools from a high temperature. Also visible are darker fragments flattened in the heat from pressure of overlying material. Matata, Bay of Plenty. Source: Author (2017).
Detail of above showing matrix of welded ash, fragments of volcanic glass, quartz crystals, and rock. Matata, Bay of Plenty. Source: Author (2017).
Poorly consolidated ignimbrite of coarse pumice in matrix of fine pumice and ash. Height of outcrop approximately 1 metre. Pikowai, Bay of Plenty. Source: Author (2017). 

Some types of welded ignimbrite are popular as a building material. These blocks clearly show large pieces of pumice and other rock in a welded matrix. The largest lighter pieces are older ignimbrites ripped up and reworked as pyroclastic flows move over older deposits. Ōpōtiki. Source: Author (2017).

One of my favourite parts of the drive from Ōpōtiki towards the western Bay of Plenty are the striking white cliffs north of Matata that look out towards the ocean. These cliffs are formed by the products of two separate eruptive episodes forming the Haroharo caldera in the Okataina volcanic centre, the northern part of the TVZ. The geological map below shows the caldera, part of which is infilled by Lake Tarawera, and shows the ignimbrites extending to the coast.
Geological map showing Matahina ignimbrite (fine dark cross-hatching) and Rotoiti ignimbrites (to the left) extending from the Haroharo caldera to the Bay of Plenty coast. Source: Adapted from Bailey & Carr (1994). Physical geology and eruptive history of the Matahina ignimbrite, Taupo Volcanic Zone, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 37(3), 319-344.

The fast-moving and extremely hot (up to 1000 deg. C and moving up to 700kmh) mixtures of fragmented rock, volcanic ash and gasses, and superheated air are called pyroclastic flows, one of the most common and lethal volcanic hazards. They occur when eruptive energy supporting the eruption column into the atmosphere can no longer support the weight of the material, as immense horizontal flows from caldera producing eruptions, or as in the case of Mt. St. Helens when part of a volcano collapses, triggering a lateral eruption.  When one considers the volume of material forming the cliffs on the Bay of Plenty coast, some 30-40km from their source you get a sense of the immense volcanic forces that have shaped this land. 

Massive ignimbrite deposit, approximate height of outcrop 20metres, coastal cliffs Pikowai, Bay of Plenty. Source: Author (2017).

Outcrop of larger pumice fragments in matrix of ash and sand-sized pumice particles. Pikowai, Bay of Plenty. Source: Author (2017).
Detail of above, showing large pumice and rock fragments in fine matrix exposed as the face of the outcrop is eroded. Height approximately 50 cm. Pikowai, Bay of Plenty. Source: Author (2017).

Because of the lethal and unpredictable nature of pyroclastic flows, it is near impossible to study them in real time, we can only infer the physical forces operating from the deposits left behind. One way around this problem has been ingeniously solved by a team at Massey University who have engineered an “eruption simulator", explained in the short video below. In essence, this has allowed a window into the interior fluid dynamics of pyroclastic flows in relation to eruptive forces and materials, as they propagate away from their source.

By now you may be asking if there is a  reason behind this great concentration of volcanic activity in this very distinct zone, and in fact there is.  However, that will have to wait for part two of this story, which will tell the of the relentless tectonic forces taking place offshore of the Bay of Plenty and the Raukumara Peninsula, shaking, faulting, and uplifting the land around us, and driving the volcanism of the Taupō Volcanic Zone.

Click here for part 2 of Ōpōtiki, the shifting and fiery land around us. A thrilling story of volcanism and tectonics..

If you are interested in a more in depth exploration of the geology of the Bay of Plenty, the GNS Qmap of Rotorua is highly recommended. Comes with a fold-out geological map of the Bay of Plenty and a very informative and well illustrated booklet. Maps and publications are available on line from GNS at  https://www.gns.cri.nz/Home/Products/Publications

For a great base from which to explore the volcanic plateau and it's stunning sights, check out Tāwhiri, Waiouru for comfortable and friendly acommodation.


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