Introducing The Tongariro volcanic complex

[BrickFielder]

Introduction
My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing The Tongariro volcanic complex

Reason for Interest
This complex of Volcanoes are capable of significant eruptions and have been quiet in recent years. Despite being in a remote area there is infrastructure in the form of electricity and water supplies along with transportation routes which can be threatened by volcanic activity. Besides who can resist Mount Doom from the Lord of the Rings Films.

The Setting
The Volcano Complex is found in New Zealand, an island country in the southwestern Pacific Ocean. It consists of two main land masses the North Island (Te Ika-a-Māui) and the South Island (Te Waipounamu) and more than 700 smaller islands. The two main islands are separated by Cook Strait which is 14 miles wide at its narrowest point. The country's varied topography and sharp mountain peaks, including the Southern Alps, owe much to tectonic uplift and volcanic eruptions. The capital city is Wellington and the largest urban area Auckland are both are located on the North Island. 

New Zealand's geographic isolation for 80 million years has influenced evolution of the country's species of animals, fungi and plants. Physical isolation has resulted in a dynamic evolution with examples of distinctive plants and animals. Over 20 percent of New Zealand is covered in national parks, forest areas and reserves with two World Heritage Areas, Tongariro in the Central North Island and Te Wahipounamu in the south-west of the South Island.

New Zealand’s climate varies wildly with the far north having subtropical weather during summer, while inland alpine areas of the South Island can be as cold as -10°C in winter. Most of the population lives close to the coast with mild temperatures all year round. January and February are the warmest months, and July is the coldest month of the year.

Today, the population of New Zealand is made up of people from a range of backgrounds. The Maori indigenous people, of New Zealand arrived more than 1000 years ago from their mythical Polynesian homeland of Hawaiki. Today, one in seven New Zealanders identify as Maori.

The Tectonic environment

New Zealand is located on the boundary beneath the Australian and Pacific Plates. Beneath the islands the Pacific Plate is being subducted beneath the Australian Plate. As the Pacific Plate sinks deeper into the Earth it is partially melted by the friction and the heat of the planet's interior. Some of the melted material then rises through the overlying Australian Plate, fueling the volcanoes of New Zealand. The East Coast of the North Island is rotating clockwise, relative to Northland, Auckland and Taranaki, stretching the Bay of Plenty, and producing the Hauraki Rift and Taupo Volcanic Zone. 

The Taupo Volcanic Zone is a NE trending arc of volcanic activity that has been the focus of volcanism in New Zealand. The central zone has a relatively thin crust with a thickness of about 15 km owing to active faulting and extension. The most recent activity in the zone has been concentrated within the Tongariro Volcanic Centre.

The Tongariro Volcanic Centre is at the southern most extent of the Taupo rifting zone with the volcanoes along an actively extending graben (rift) about 18km wide. Total basement subsidence is estimated at 650m with the the rifting being accommodated by a mixture of Magma filling and fault slipping. Steep-sided volcanic bodies coincide with the Waihi fault and the rift axis suggesting that this fault system may have provided magma pathways to the surface. Study of previous eruptions suggest that at times the fault has unzipped causing eruptions from many vents at the same time. This resulted in very large eruptions from all the volcanoes in the Tongariro Volcanic Centre at the same time.

Tongariro  volcano
Mount Tongariro which means ‘fire carried away’ or ‘seized by the cold south wind’ is part of the Tongariro volcanic center. There are many explosion craters on the volcano complex with water filling some of these to form the Blue Lake and the Emerald Lakes. Red Crater last erupted ash in 1926 and contains active fumaroles. Ketetahi Springs, on the northern side of Tongariro, is a collection of hot mineral springs and steam vents (fumeroles). Three 220 kV electricity transmission lines are within 15km of the volcano along with Hydro-generation facilities and water from the area is used as a source of drinking water.

On 6 August 2012, Mt Tongariro had what was initially believed to be a hydrothermal eruption after a month of increased activity. The eruption occurred at the Te Mari Craters which had been dormant since 1897. The eruption occurred in a new vent below the Upper Te Mari crater, and sent blocks as large as 1 meter  in size up to a mile from the vent. An ash cloud 4 miles high deposited ash into the surrounding area, especially to the east of the volcano. The ash cloud was very large with the smell of sulphur reported as far away as Wellington.

Ngauruhoe Volcano

Mount Ngauruhoe (throwing hot stones in Maori) which some may recognize as Mount Doom from Lord of the Rings is one of the most famous volcanoes in New Zealand.  Ngauruhoe isn’t actually a volcano on its own, but rather a cone of the larger Tongariro Volcanic Complex, which resides in the Tongariro National Park. At 2291m high it is the highest point of the Tongariro complex. Ngauruhoe is the youngest (7000 years old) and one of the currently active vents of the Tongariro Volcanic Centre alongside the Te Maari craters, Emerald Lakes, North and Red craters. The Ngauruho eruptions have  been fed from a series of small complex interconnected magma chambers that are part of a larger system underlying the area. The summit of Ngauruhoe is currently composed of two craters, an older outer crater with a diameter 450m and a younger inner crater formed in the NW sector of the outer crater that is approximately 220m in diameter. Fumaroles exist in the inner crater and on the rim of the eastern and northern outer crater. Up until 1975 the volcano erupted roughly every 9 years but has not erupted since then. During both 2006 and 2015 earth quake swarms were detected but no eruption occurred.
 

An eruption  commenced in June 1954 when lava fountaining was observed reaching 300m above the Crater Rim and producing lava flows down the northwestern  sector. Fire fountaining and ash emission continued throughout July with more lava flows descended down the western slopes. Near continuous emission of black ash occurred and incandescent blocks were thrown onto the northern slopes of Ngauruhoe. During September Multiple loud explosions produced visible shock waves and dark ash clouds that rose above the crater to about 1.2km and deposited ash as far away as Taupo township.

The first activity of the 1974-75 eruption was of a black cloud and the ejection of incandescent bombs that lit up the summit flanks. Partially collapsing ash columns resulted in pyroclastic flows and large blocks and bombs reaching 5m diameter were ejected reaching the lower flanks of the volcano. Activity reached a climax on February 1975 with the largest explosion of the eruption episode with an ash cloud reaching 6000m.

Mount Ruapehu

Ruapehu located within  Tongariro National Park is the largest active volcano in New Zealand  and is the highest point in the North Island. It has three major peaks which are Tahurangi (2,797 m), Te Heuheu (2,755 m) and Paretetaitonga (2,751 m). The deep, active crater is between the peaks and fills with water between eruptions with the lake being known as Crater Lake which is filled with acidic water. The lake's outlet is at the head of the Whangaehu Valley which 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. 
 
 A total of 18 glaciers have been recognized on Ruapehu with two glaciers found in the active crater. These are New Zealand's only crater glaciers with one crater on the north side of the crater under Paretetaitonga Peak and the other one to the south. Due to altitude and snow cover Ruapehu is home to the North Island's major ski resorts, Whakapapa on the northern side and Turoa on the southern slope. The season is generally from June to October but depends on snow and weather conditions. 

Ruapehu entered an eruptive phase in March 1945 after several weeks of volcanic tremors. The first indication of an eruption was reported on 8 March, with ashfall seen on the eastern slopes. A lava dome was observed in Crater Lake on 19 March but was destroyed in a series of explosive eruptions over the following week. A second, larger lava dome appeared in May, which continued to grow over the following months and had emptied Crater Lake of water by July. A particularly powerful eruption in the early hours of 21 August was heard in Hawkes Bay and the Tararua District, loud enough to awaken people from sleep and cause alarm. Eruptions began declining in December and had ended by January.

On 24 December 1953, a debris dam near the lake outlet collapsed, sending an ash-laden lahar down the Whangaehu Valley. Soon after the lahar swept away the Tangiwai rail bridge, the Wellington-Auckland express plunged into the swollen river, killing 151 people.

Ruapehu saw a period of heightened activity between 1966 and 1982, with multiple small eruptions occurring in Crater Lake and two larger eruptions in 1969 and 1975, which ejected rocks across the summit region and produced significant lahars. The eruption in 1969  was a moderate phreatic eruption, which blasted rocks up to 1km northwest of the crater and sent lahars down several valleys. 

Earthquake swarms to the west of Ruapehu between November 1994 and September 1995 marked the beginning of renewed heightened activity at the volcano. The first significant eruption took place on 18 September 1995 raining tephra onto the summit region and sending lahars down the mountain. On 23 September, an even larger eruption blasted rocks up to 1.5 km from the crater, sent lahars down three valleys, and generated an eruption column 12 km High with ash fall occurring up to 250 km downwind. During July and August eruption columns reached more than 10 km high and shot rocks 1.4 km from the crater. These eruptions produced more than 7 million tons of ash, which contaminated water supplies, destroyed crops, and lead to the deaths of livestock. Ash in the Tongariro River also damaged the intake turbines at the Rangipo power station, and ash clouds caused airport closures as far away as Auckland and Wellington.

Ruapehu erupted in October 2006 marked by a magnitude 2.9 volcanic earthquake and sent waves 5 meters tall crashing into the wall of the crater.  On 18 March 2007, the tephra dam which had been holding back Crater Lake burst sending a lahar down the mountain. 

Discussion
Emerald lakes, ski slopes, and photogenic scenery may suggest a benign environment but history suggests these are dangerous volcanoes. What piques my interest is that when considering volcano hazards we need to consider  things which are not immediately obvious like infrastructure.