Ruapehu Please cite as: Ministry of Civil Defence. Volcanic hazards information series 7. 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.
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 years ago. 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: 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.
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.
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. Dacite 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 m across and up to 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.
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. 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, years. At Taupo the Oruanui and the Taupo eruptions are part of this larger pattern.
Photo of typical fall tephra made up of rough pieces of pumice and ash. Pre 65, years ago All deposits at Taupo including a number of early lava domes clearly post-date the exceptionally large Whakamaru ignimbrite eruption dated at , years ago. About , 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.
Our knowledge of this time intervals is very incomplete as few deposits of this age are exposed. 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 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. 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, years.
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, year old Oruanui eruption may be relevant to assessing the hazards of future activity at Taupo, especially over the next years. Events over this time period have ranged enormously in size from 0.
Estimates of the repose periods between eruptions also vary greatly, from 50 to years. The Taupo Eruption years ago The most recent eruption of Taupo was about 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, 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, years.
The volcanic hazards of interest to residents today are perhaps more properly represented by the other events. A 3-dimensional diagram looking north of the thickness of tephra from the 26, 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.
Summary of Past Eruptions The last 26, years have seen about 28 major eruptions, separated in time by between 50 and 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 , and years ago. This suggests the chance of future pyroclastic flows is small. The volume of tephra plotted against the repose time a before and b after the eruption. 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. 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. 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.