VOLCANIC HAZARDS AT TAUPO VOLCANIC CENTRE
By Paul Froggatt
Victoria Link Ltd & Research School of Earth Sciences, Victoria University of
Wellington
 Satellite photograph of Lake Taupo and the surrounding area. Taupo
volcano, comprising several calderas (collapsed volcanoes) makes up the northern part of
the lake. The surrounding lighter-coloured region is the area most thickly covered by the
deposits of Taupo eruption 1800 years ago. Landstat thematic mapper ™ image taken in
1991 courtesy of Landcare Research New Zealand Ltd.
INTRODUCTION
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).
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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.
 Figure 3: Layers of volcanic pumice and ash (tephra) erupted from
Taupo Volcano. This section near De Bretts Hotel contains a record of the eruptions that
have occurred in the last 27,000 years.
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’s History
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.
Figure 4: A 3-dimensional diagram looking north of the thickness of tephra from
the 26,500 years old Oruanui eruption, showing the greater thickness (5m) near the vent in
Lake Taupo and a flat fan of material extending east across Hawkes Bay.
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.
 Figure 5: The volume of tephra plotted against the repose time (a)
before and (b) after the eruption.
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.
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THE TYPES OF VOLCANIC HAZARDS AT TAUPO
Tephra fall
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.
Figure 6: Map of the Central North Island showing the thickness of tephra erupted
from Taupo volcano in the last 22,000 years. Lines are contours (in metres) of equal
tephra thickness and are a useful guide to the probable distribution of airfall tephra in
another eruption from Taupo. A future medium sized eruption may produce a maximum
thickness of 1 metre at the centre of the contours. While the tephra is dry, most house
roofs will be secure, but they should be swept clean at the first opportunity. As little
as 20cm of dry ash or 5cm of wet ash may collapse some roofs.
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.
Figure 7a: Photograph of soft ignimbrite from the Taupo eruption 1800 years
ago.
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Earthquakes
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.
Hydrothermal explosions
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.
Forest fires
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’
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Figure 1: Map of the North Island of New Zealand
showing the location of active and dormant volcanoes and calderas, such as Taupo Volcano,
and the line of andesite volcanoes that form the Pacific "Ring of Fire".
Figure
7b: The hard welded Whakamaru Ignimbrite erupted about 330,000 years ago beside the
Waikato River at Maraetai.
Figure 8: Photograph of the interior structure
of a rhyolite dome, showing a radial pattern of cracks formed as the lava cooled.
(Mason’s Rock on the north shore of Lake Taupo)