Taupo Volcanic Centre Geology
By Paul Froggatt
Victoria Link Ltd & Research School of Earth Sciences, Victoria University of Wellington
This booklet is designed to inform you about the volcanic hazards of the Taupo area. It reviews the past volcanic history of Taupo, and explains the different types of eruptions that have occurred. People who live near active volcanoes, such as Taupo, can benefit greatly from clear scientific information about the area. This information can raise awareness and understanding of our environment and of the natural events that shape the land, such as the earthquakes and volcanic eruptions that have been a dominant influence at Taupo.
Everything we know about Taupo Volcano has come from studying the deposits of past eruptions, but our record of these deposits is incomplete. Most of the deposits are covered by forest, farmland or towns and the area close to the past vents is deep under Lake Taupo. However the exposures that we do have are valuable windows into the nature, size and effects of eruptions from Taupo.
The Taupo area owes much to the volcano. Its hills and valleys, mountains and lakes were all shaped by the volcano. Its soils and pumice, hot springs and geothermal energy are all due to the volcano. We benefit greatly from this, but we must not overlook the source of our pleasure: remember Taupo is a sleeping volcano and will certainly erupt again.
Volcanoes come in many different shapes and sizes and only a small number have the "typical" cone shape of Mt Taranaki or Mt Fujiama in Japan. Taupo Volcano is very large and has many vents, most of which are now under Lake Taupo. Our geological studies of Taupo show that the volcano makes up only the northern half of the lake and a small surrounding area but there have been numerous eruptions from different sites within this large volcano. Taupo is not a large mountain because the eruptions have been so explosive that all material has been deposited far from the vent and subsequent collapse of the ground has formed a caldera (a collapsed volcano).
MAGMA TYPES AT TAUPO
Volcanoes throughout the world erupt a wide variety of magmas in a great diversity of eruption styles. In the central North Island, there is a spectrum of magma types from basalt through andesite and dacite to rhyolite. The chemistry of the magma changes from silica-poor (basalt) to silica-rich (rhyolite), gas content changes from low to high,. Viscosity of the magma goes from low to high. With these changes, eruptions generally become more explosive and destructive. At Taupo, most of the erupted magma is rhyolite but there are also small amounts of dacite and basalt.
Rhyolite accounts for about 98% of all erupted material at Taupo. Most of the rhyolite has been erupted explosively as pumice and finer sized ash (the collective term for all material is tephra) which has been spread widely over the Taupo area.
The most prominent dacite at Taupo is Tauhara. It is a lava dome complex made up of seven individual units of lava. The lava was viscious and with low gas content and probably oozed out very quietly to form the high, steep-sided domes. No record has been found of any major explosive activity with the growth of Tauhara.
Basalt scoria cones and tuff rings
Basalt is rare at Taupo, but it has been erupted from several vents. Most of the eruptions have formed small scoria cones, typically about 500 m across and up to 200 m high. Examples of these are found at the scoria quarries between Taupo township and Acacia Bay. However several basalt vents have been within the lake and in these cases the magma has reacted with water in phreatomagmatic eruptions to form wide crates and tuff rings. Some good examples are seen around the lake shoreline near Acacia Bay.
THE HISTORY OF TAUPO VOLCANO
There have not been any eruptions involving fresh magma from the Taupo area in historic times, so all our knowledge about the volcano comes from studying the deposits left behind by past eruptions. Each eruption adds a series of layers of tephra onto the ground surface and long intervals between eruptions allow new soils to form. A sequence of layers interspersed by soils results. By examining these layers of many places the number of eruptions and their sources can be established. The character of the layers tells us a lot about the type of eruption. Older layers become more deeply buried with time and are thus less well exposed, so that our knowledge is more complete for the younger eruptions.
Dating the eruptions
The age of each eruption is a useful indication of the life of the volcano and the time between eruptions. As all the eruptions are pre-historic the radiocarbon dating method has been applied to the younger deposits. This relies on the radioactivity decay of isotopes of carbonas preserved in wood or other organic material. Many of the layers of tephra have incorporated small amounts of charcoal from trees destroyed by the eruptions and this charcoal gives a reliable age of each event. Many of the tephra layers have also been preserved in swamps in the Bay of Plenty, Waikato and Hawkes Bay, and the enclosing peat can also be dated.
The older eruptions are more difficult to date accurately, but methods using the radioactive decay of potassium and uranium have been used.
Taupo Volcano has been in existence for more than 65 000 years. In that time it has shown a random pattern of exceptionally large events interspersed by smaller eruptions. This is a pattern typical of all the major rhyolite volcanoes of the central North Island and together they have produced large eruptions about every 50,000 years. At Taupo the Oruanui and the Taupo eruptions are part of this larger pattern.
Pre 65,000 years ago
All deposits at Taupo including a number of early lava domes clearly post-date the exceptionally large Whakamaru ignimbrite eruption dated at 330,000 years ago. About 150,000 years ago new activity formed a pumice-rich ignimbrite found along the northern shores of the lake, several basalt scoria cones and tuff rings about Acacia Bay and Mt. Tauhara. Our knowledge of this time intervals is very incomplete as few deposits of this age are exposed.
65,000 to 27,000 years ago
Between 65,000 years and 27,000 years ago there was a series of at least five explosive eruptions, from vents now under Lake Taupo. The older four eruptions produced layers of coarse pumice. The youngest produced fine grey ash suggesting the mixing of lake water with erupting magma.
The Oruanui eruption 26,500 years ago
The largest eruption from Taupo occurred 26,500 years ago producing 300 km³ of ignimbrite, 500 km³ of pumice and ash fall and a unknown volume of material inside the caldera. The Oruanui eruption is thought to have formed the caldera now filled by Lake Taupo, but this large eruption also shows the influence of lake water in its fine grain size and abundant evidence for heavy rain during the eruption. This implies the existence of a large lake prior to the eruption. The Oruanui ignimbrite is seen in many road cuttings about Taupo, draped by the layers of younger tephra. Fine ash from this eruption has been found throughout New Zealand and in many offshore core samples.
26,000 to 2,000 years ago
Following the major Oruanui eruption, there was a change in the conditions of the magma beneath Taupo. It is thought a new batch of magma rose high in the crust as the material erupted after Oruanui has a different composition, and was much hotter. The vents for the post-Oruanui eruptions appear to be south of the earlier vents and most are now under Lake Taupo.
For these reasons, only the eruptions after the 26,500 year old Oruanui eruption may be relevant to assessing the hazards of future activity at Taupo, especially over the next 100 years.
Events over this time period have ranged enormously in size (from 0.01 to 17km³) and in style. Estimates of the repose periods between eruptions also vary greatly, from 50 to 5000 years.
The Taupo Eruption 1800 years ago
The most recent eruption of Taupo was about 1800 years ago. Although the precise year of eruption is not know, evidence from trees preserved at Pureora Forest suggest it occurred in late summer. The Taupo eruption was a complex series of events. The first phases of the eruption produced a series of five pumice and ash fall deposits over a wide area of the central North Island, especially east of Taupo and beyond Napier into Hawke Bay. The eruption culminated with a large and very energetic pyroclastic flow that devastated an area of about 20,000 square kilometres and filled all the major river valleys of the central North Island with pumice and ash. These pumice deposits can still be seen today and many of the major rivers in the North Island carry large amounts of this pumice when in flood. Rounded pumice found on the beaches of the North Island have come from this eruption.
The Taupo eruption took place from a line of vents near the eastern side of the modern lake. At the beginning of the eruption, the vent was clear of the lake as there is minimal evidence for water involvement with the erupting magma. However the lake eventually breached the vent and several stages of the eruption were dominated by mixing of the magma and lake water, with fine ash being formed. Fall of this ash was accompanied on occasion by heavy rain.
It is important to realise that this most recent eruption of Taupo was unusually violent and destructive compared to the other eruptions from Taupo over the last 26,000 years. The volcanic hazards of interest to residents today are perhaps more properly represented by the other events. But even a ‘small’ volume fall-only explosive event at Taupo will be very destructive and disruptive to human life and activity throughout the North Island.
Summary of Past Eruptions
The last 26,000 years have seen about 28 major eruptions, separated in time by between 50 and 5000 years. There is no simple pattern to these eruptions that would suggest when or where the next event might occur.
In at least 20 of these 28 eruptions, water has reached with the magma during eruption, causing greater fragmentation of the tephra, and the generation of rain that has produced wet, sticky ash deposits. As all but three of the vents in this period are now under Lake Taupo it is highly likely that future eruptions will be from this area and may involve lake water.
Only three eruptions have produced destructive pyroclastic flows, at 1800, 3550 and 9950 years ago. This suggests the chance of future pyroclastic flows is small.
THE PRESENT STATE OF TAUPO
There is currently no evidence for unrest at Taupo Volcano. The centre is monitored by the Institute of Geological and Nuclear Sciences using networks of seismometers and lake level records at measure tilt (like a giant spirit level).
Swarms of small earthquakes that have regularly shaken Taupo in historical times appear to be associated with fault lines and the ongoing subsidence and widening of the region rather than the movement of magma.
POSSIBLE FUTURE ERUPTIONS
Of course we cannot be sure what the volcano will do in the future, but we can assume that the past history will be a good guide. The most likely event will be a small – to medium-sized explosive rhyolite eruption, and the growth of a rhyolite dome. Basalt and dacite eruptions are also possible but less likely. A future vent will most probably be within Lake Taupo, between Horomatangi Reefs and Motutaiko Island. Tephra fall is most likely to be to the east or northeast, and may extend to the east coast of the North Island.
Possible warning signs
Volcanoes are unpredictable and are not well understood. Also there have not been many rhyolite eruptions world-wide in historic times to give us clues about what to expect. Nearly all caldera eruptions are preceded by weeks to months of local earthquakes. These can be expected to increase in number and strength as the eruption approaches, and will not die away after a few days such as swarms of earthquakes have done at Taupo in the past. The recognition of these earthquakes as volcanic in origin is essential and will rely on detailed scientific monitoring and analysis. Caldera volcanoes worldwide are subject to seismic swarms-clusters of close-spaced small earthquakes.
There may be noticeable changes in the land surface near to the possible eruption vent. The surface may rise or drop, possibly by several metres, but it will be gradual and may not be noticed initially. Ground cracks may appear or streams may start to flow faster or even flow backwards as the ground rises. On the lake shore, a rise or fall of the land will be more obvious, as the shoreline will move out or in locally, without an overall rise or fall of the lake level. It is possible that movements may raise Horomatangi Reefs and Waitahanui Bank above lake level if these are close to the site of the impending eruption. The changing rate of these movements will be a powerful key to the level of unrest.
Geothermal features such as hot springs may change and become hotter or with more steam discharge and new features may form. They may also stop altogether as the ground under them moves with the rising magma.
Any signs of changes in the land, lake shore, or geothermal features should be reported to Civil Defence or the Institute of Geological and Nuclear Sciences at Wairakei
THE TYPES OF VOLCANIC HAZARDS AT TAUPO
Tephra fall has been the most common volcanic hazard at Taupo and occurs with both basalt and rhyolite magma. Basalt eruptions are small and tephra will probably only fall for a few kilometres from the vent. If the vent is within the lake, the tephra will be finer grained and will fall wet and sticky. More than 0.1 metre of wet sticky tephra will collapse some house roofs.
Most rhyolite eruptions form high stable eruption columns (1-040 km), out of which falls pumice and finer particles (ash). Wind velocity will influence the cloud so that tephra falls over a wide area of countryside down wind from the vent. The material falls relatively slowly and accumulates thickest and courses near to the vent.
Moderate to large rhyolite tephra fall will cover all of the Taupo area with a layer up to 1-2 metres thick. The material will range in size from pumice 50 mm across down to fine ash and will be mostly cold or warm. Beyond 3 km from the vent it will be like a gentle rain. Nearer to the vent the tephra will also contain larger fragments of solid rock broken off the vent walls. An impact from one of these fragments could be fatal.
During tephra fall, the sky will be darkened by the eruption cloud and visibility will be very poor. As the tephra accumulates, travel will become difficult and car engines will become affected by the intake of fine dust in carburettors and sumps, and abrasion of fan belts and other moving parts. Similarly, breathing may be difficult outdoors without a face mask or damp cloth across the mouth. Power and telephone lines will be broken and radios may not work due to the disturbed atmosphere and lightning.
Pyroclastic flow (ignimbrite)
These may occur without warning with any eruption, but large flows have been rare in the past. A pyroclastic flow is particularly destructive and may move outward at high speed (perhaps 100 km/hr) smothering and burying everything in its path. We call the moving cloud a pyroclastic flow and the deposit it leaves behind an ignimbrite. Where deposits of ignimbrite are very thick they may still be hot enough for the particles to fuse together and form a hard rock as seen today forming bluffs around Lake Taupo and the cliffs along the Waikato River at Whakamaru and Maraetai dams.
Magma that has lost most of its gas is quietly extruded to form a dome. As was the case at Mt St Helens, dome growth can be accompanied by smaller explosive eruptions, but these decrease in size with time. Domes may take from a few months to several years to grow. The main hazards of dome growth are tephra fall from explosive eruptions and avalanches of hot lava from the sides of the dome.
Volcanic gases and acid rain
Gases are the driving force of explosive eruptions. The main gases are water vapour and carbon dioxide, with small amounts of sulphur, hydrogen and other gases. These gases rarely pose a direct threat as they quickly mix with the air and are diluted as they blow downward. Some of the gases, such as sulphur dioxide and hydrogen fluoride mix with water droplets in the eruption plume to form acids which will attack skin, clothing and metals. Rain falling from an eruption plume may be very acidic and this acid rain will attack foliage and crops over a wide area, perhaps even greater than the fall of tephra. The acid rain may also contaminate water supplies. Acidic water is not a major health hazard, but the acid may leach lead from nailheads and flashing on roots and lead poisoning may result. There is the remote possibility that heavy gases (mostly carbon dioxide) emitted before or after an eruption will collect in low-lying areas or confined valleys and cause suffocation. Generally there is enough wind about the lake area to disperse any gas cloud that may form.
All volcanic eruptions are accompanied by earthquakes and some of these may be large enough to damage buildings and other structures such as bridges. Earthquakes often occur before an eruption, and increase in number and size before the event. Many will be large enough to feel, but most will only be detected by sensitive equipment. During an eruption there will be continuous ground tremor and shaking.
The rise of hot magma towards the earth’s surface often causes an increase in geothermal activity at existing sites as well as forming new areas of hot ground. Steam explosions may occur, forming craters up to 100 m across and endangering an area of about 1 km 2 . Major changes to geothermal systems have occurred in New Zealand both before and after an eruption.
Ground cracks and land changes
Rhyolitic eruptions involve the movement of large volumes of magma inside the crust. To accommodate this movement the crust is deformed and cracked. Before an eruption, the ground surface often swells upwards by centimetres or metres causing cracks to appear. This will be most obvious in built-up areas where houses could be damaged and roads cracked.
Seiches (large waves) on Lake Taupo
It is possible that large earthquakes before or during an eruption, or an eruption itself, might generate seiches on the lake. These would be waves up to 5m high that would travel across the lake and flood lowlying land on the lake edge and cause torrents down the Waikato River. It would be prudent to keep away from the lake and the river during an eruption.
Contamination of rivers and water supplies
Contamination of water supplies should not be a widespread or long lived hazard. Immediately after an eruption rivers may contain high levels of volcanic ash but this should be no more toxic than existing sediment. If cloudy, ash-laden water is left to stand, the sediment should settle out. In areas where the domestic water supply relies on rain-water collected from roofs, the collection area will need to be swept and washed clean and tanks emptied of ash.
Contamination of grass and grazing land
Farmland will be heavily affected by an eruption. Apart from grass killed by acid rain (see above) grazing and land will be covered with tephra that will vary from a light dust to a layer up to 2 m thick. The tephra itself should not be toxic, but where it is very fine-grained stock may suffer from ingestion of ash. Erosion of the tephra will be a problem in waterways until it is stabilised by compaction or new grass growth. Where the ash is less than 5 cm thick rain will soon wash the ash into the soil but at greater thicknesses new grass will need to be sown. Stock killed by the eruption or stock suffering from lack of feed will be a further problem.
A large part of the area likely to suffer from an eruption at Taupo is forest. Hot pyroclastic flows and surges will kill trees. Nearer the vent, trees will be blown down and may be buried and charred as was seen at Mt St Helens. Fall tephra will cause less damage to trees but where the tephra is thick, trees are likely to lose their leaves and branches. Few historic eruptions have caused forest fires, except where hot lava flows have set trees alight. However charcoal is a common feature in soils on top of tephra layers in the Taupo area and this may indicate widespread fires in forest damaged by an eruption. Lightning strikes during an eruption may also cause local fires.
This text is taken from one of a series booklets which cover volcanic hazards at each active or potentially active volcanic centre in New Zealand.
The series was produced by the Volcanic Hazards Working Group of the Civil Defence Scientific Advisory Committee, which includes scientists from the Institute of Geological and Nuclear Sciences and the Universities.
- Booklets published in the series so far are:
- Number One ‘Egmont Volcano’.
- Number Two ‘Okataina Volcanic Centre’.
- Number Three ‘White Island’.
- Number Four ‘Kermadec Islands’
- Number Five ‘Auckland Volcanic Field’
- Number Six ‘Mayor Island’
- Number Seven "Taupo Volcanic Centre’