Pacific Ring of Fire
The three andesitic volcanoes at the heart of the park, the mountains Tongariro, Ngauruhoe and Ruapehu, form the southern limits of the Taupo Volcanic Zone, part of the Pacific Ring of Fire.
A plate boundary lies just east of the North Island of New Zealand, where the Pacific Plate slides under the Indian-Australian Plate. This area of 'subduction' has created a line of volcanoes which stretches from the pacific island of Tonga to Ruapehu.
It is also the end point of a sequence of volcanoes which extends through the Solomon Islands, crosses the Philippines, bisects Japan, skirts the eastern sea border of Russia, forms the Aleutian Islands in the North Pacific Ocean and extends the length of the west coasts of both North and South America. This circlet of volcanoes is aptly named the 'Pacific Ring of Fire'.
The Pacific and Indian-Australian plate boundary is almost totally responsible for the existence of the New Zealand islands and the volcanoes of Tongariro provide a graphic illustration of the power generated by the movement of these plates. The region of volcanic activity that occurs from Mt Ruapehu to White Island in the Bay of Plenty is known collectively as the Taupo Volcanic Zone.
Volcanic activity in the zone started about 2 million years ago and continues today.
Tongariro National Park volcanoes
Ngauruhoe and Pukekaikiore
In geological terms, the landforms of Tongariro National Park are comparatively young. Although we identify the park as having three andesitic volcanoes, Ruapehu, Tongariro and Ngauruhoe, both Maori and geologists consider Ngauruhoe to be part of the Tongariro system.
These volcanoes are all less than 500,000 years old and still active. The first activity of the Tongariro complex is thought to have occurred about 260,000 years ago. Eruptions from the twelve or more composite cones making up the complex have continued intermittently since.
The earliest eruptions of Mt Ngauruhoe are thought to have occurred 2500 years ago. Traditionally Ngauruhoe has erupted at least every nine years, although at the time of writing the last eruption was in 1975.
Mt Tongariro's active Red Crater last emitted ash in 1926, and there were eruptions from the Te Maari craters on Mt Tongariro's northern slopes on 6 August 2012 and 21 November 2012.
Ruapehu volcano and its eruptions
Mt Ruapehu is the highest point in the North Island and, like Tongariro, its history began before the last ice age. The eight named glaciers on Mt Ruapehu are the only glaciers in the North Island.
Ruapehu volcanism began at least 250,000 years ago. Since then, the volcano has built up in four main phases. Volcanic activity continues, with periodic eruptions beneath Crater Lake.
Mount Ruapehu has near its summit a deep crater containing a hot acidic lake. The temperature, appearance and depth of Crater Lake provide a delicate indicator to the activity of the volcano.
The lake is approximately 100 m deep and 600 m across, and changes colour from deep green to pale blue to dark grey. The colour changes are usually associated with changes in temperature, from as low as 9°C when green or blue, to as high as 60°C when grey.
More than 60 eruptions from Ruapehu's crater have been recorded since 1945. This includes 1995 when the Crater Lake disappeared completely. Volcanic debris called tephra was deposited over the pre 1995 lake outlet, to form a barrier which became a dam since the lake rose higher than the old outlet.
Lahar - fast volcanic debris mud flow
A sophisticated lahar warning system was established on Ruapehu, because the volcano has the potential to empty its Crater Lake down the slopes at any time. There are alarms stationed around the Whakapapa ski field and in Whakapapa village. The alarms are spoken recorded messages telling people to move out of valleys and to higher ground.
Although the unoccupied Whangaehu Valley on the eastern side is the most common route for lahars flowing from the Crater Lake, lahars have also headed down the western side in the Whakapapanui Valley, the Mangaturuturu Valley, the Whakapapaiti Valley and through the Whakapapa ski field.
In 1969,1971, 1975, and 1977 eruptions of Ruapehu generated ash and lahars that damaged the skifields.
The 1945 eruption and Tangiwai train disaster
During 1945, water in Crater Lake was expelled by a rising dome of lava. This was then destroyed by violent explosions, generating ash that fell as far away as Wellington (250 km). Within a few months water refilled the crater.
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.
Types of volcanoes
Volcanoes are the landforms produced when magma (molten rock) escapes to the earth's surface. Magma can either be erupted passively, as lava flows or domes, or explosively. These occur either when gas originally dissolved in the magma is explosively released or by interaction between the magma and water, producing violent steam explosions.
Volcanoes have a wide range of forms.
Taranaki, on the west coast of the North Island, and Ngauruhoe, are known as composite or strato-volcanoes because they are composed of layered lava flows and volcanic ash and debris.
Some volcanoes, such as Mt. Tarawera (near Rotorua), are dome-shaped composed largely of viscous lava which does not flow readily.
Another extreme of volcano shapes is the collapsed volcanic centre known as a caldera. These are often filled by caldera lakes such as Taupo and Rotorua.
Different types of volcanoes produce a variety of products which dictate their shapes. The most important factor in determining the shape of the volcano is the composition of the magma. There is a range of magma types from fluid, hot, dense, dark-coloured basaltic magma to viscous, cooler, less dense, paler coloured rhyolitic magma. All usually contain gas which is given off when it reaches the surface.
The less viscous the lava (that is, the more fluid), the more easily the gases are given off. With more viscous lava, gas is usually released with great explosive force, throwing ash and pumice over great areas.
The lava of the central North Island volcanoes is mainly intermediate in composition and viscosity, and alternate between explosive and passive extrusive eruptions. They are called andesites, after the volcanoes of the Andes Mountains in South America.
Volcanic cones in the central North Island
Several volcanic cones may be seen in the central North Island. These are composite cones, and typical layers of lava and pyroclastic material can be seen on the eroded flank of Tongariro.
The cones occur singly, as in Taranaki, or in multiples, as in Ruapehu and Tongariro. Ngauruhoe is regarded as a vent of the Tongariro complex.
Hauhangatahi and Pouakai Range are satellite vents of Ruapehu and Taranaki respectively. Both are old and eroded relative to their more spectacular partners.
Craters on Tongariro
There are several craters on the Tongariro complex. These are steep-sided depressions, the result of explosive volcanism.
The floor of North Crater is the remains of a lava lake that solidified when nearly at the same height as the crater rim. The lava lake has subsequently been damaged by eruptions forming a smaller explosion crater. This small crater is known to have been inactive for about 1800 years.
Lava flows pour down the volcano flanks when they overflow from a crater rim. The long flow below Te Maari Craters is thought to have occurred about 450 years ago. The other shorter flow, from the Upper Te Maari Crater, has partly filled the older Lower Te Maari Crater.
Ketetahi Springs, on the northern side of Tongariro, is a collection of hot mineral springs and steam vents (fumeroles). The steam and hot water are generated by a cooling volcanic rock source beneath the surface.
There are several visible steam vents at the Te Maari craters from the eruptions in August and November 2012.
You can see fumerolic activity in the steam plumes around Emerald Lakes and Red Crater, and sometimes visible on the outer rim of Ngauruhoe.
All these volcanoes are surrounded by ring plains of debris eroded from the cones. Because volcanic cones are usually high with relatively steep slopes, they are rapidly eroded.
Loose ash and lava boulders are washed down lower slopes of volcanoes as bouldery lahars. Deposits of the lahars form the dominant component of the ring plains.
Volcanoes and faults
Volcanoes and faults are often closely associated. The faults on the side of Tongariro traverse the western flank. A further small fault actually crosses the floor of North Crater.
Movement on these faults is known to have taken place after ash accumulated from the eruption of the Taupo Volcanic Centre about 1800 years ago.
Volcanoes are not usually the result of a single eruption. Each volcano in the central North Island has an active life that spans many thousands of years.
The oldest known ash deposits from Taranaki are dated at about 120 000 years ago, making it the youngest volcano. A basaltic scoria was erupted at Fanthams Peak about 3300 years ago.
Its most recent eruption is thought to have occurred in about 1755 AD. The dissected volcanic remnant that forms the Pouakai Ranges is older than Taranaki and probably dates from about 250 000 years ago.
Study and monitoring
The study of these volcanoes is important in the world scene for the understanding of volcanic mechanisms.
By studying past eruptions, geologists derive information allowing assessment of the nature of future eruptions and the hazards they present.
Detailed monitoring of volcanic activity may reduce the the risks to people and property from major eruptions.
Find out more about the study and monitoring of New Zealand's volcanoes on the GNS Science website.