3.0 Volcanic Hazards in New Zealand
3.1 Volcanism in New Zealand
The New Zealand region is characterised by both a high density of active volcanoes (Figure 3.1) and a high frequency of eruptions (Wilson et al. 1995). Although the probability of an eruption affecting a significant portion of the North Island is relatively low in any one year, the probability of one occurring in the future is high. The potential impacts of a large eruption are significant and the risk cannot be ignored. The timing of the next eruption of a volcano cannot yet be determined but its probable effects can reasonably be assessed.
We can subdivide New Zealand's volcanoes either by type or by status.
3.1.1 Status of New Zealand's Volcanoes
The National Civil Defence plan subdivides New Zealand's potentially active volcanoes into two classes: frequently active and re-awakening volcanoes.
The former are characterised by numerous often prolonged eruption episodes over the last 150 years; the latter have been less active in historical times but have had numerous eruptions in the last 15,000 years. Frequently active volcanoes include White Island, Tongariro-Ngauruhoe, and Ruapehu. Re-awaking volcanoes include the Auckland Volcanic field, Mayor Island, Mt Edgecumbe, Okataina, Rotorua, Maroa and Taupo volcanic centres, and Taranaki (Mt Egmont).
3.1.2 Our Types of Volcano
New Zealand's young volcanoes represent a cross-section of most of the
types of volcanoes documented elsewhere in the world, the only type missing being an
example of a modern basaltic shield volcano such as Kilauea or Mauna Loa in Hawai'i. In
two respects, New Zealand's volcanoes are world-beaters: our cone volcanoes at Ruapehu,
Ngauruhoe and White Island are among the most frequently active examples known; and Taupo
and Okataina are the most productive and frequently active rhyolite caldera volcanoes on
Earth (Wilson et al. 1995). The short time span for which the eruptive histories of
New Zealand's volcanoes have been observed is inadequate to show the full extent of
eruption styles and sizes that are recorded in the pyroclastic deposits and lavas
generated by past activity. Thus lessons about the sizes, styles and associated hazards of
future activity have to be learned from the prehistoric record as well as historic
eruptions.
FIGURE 3.1 New Zealand's Volcanoes
Volcanic fields: Volcanic fields such as Auckland, are where small eruptions occur over a wide geographic area, and are spaced over long time intervals (thousands of years). Each eruption builds a single small volcano (e.g. Mount Eden, Rangitoto), which does not erupt again. The next eruption in the field occurs at a different location, and this site cannot be predicted until the eruption is imminent.
Composite cone volcanoes: Composite cone volcanoes such as Egmont and Ruapehu are characterised by a succession of small to moderate eruptions occurring from roughly the same point on the earth's surface. The products of successive eruptions accumulate close to the vents to form a large cone, which is the volcano itself. Over a long period of time several cones may form which overlap and are built up on top of each other. The same route to the surface is used repeatedly by the magma so sites of future eruptions can largely be predicted.
Caldera volcanoes: Caldera volcanoes such as Taupo and Okataina (which includes Tarawera) exhibit a history of moderate to large eruptions. Eruptions at these volcanoes are occasionally so large that the ground surface collapses into the "hole" (caldera) left behind by the emptying of the underground magma chamber. The pyroclastic products are usually spread so widely that no large cone forms, except where lava flows may pile up on top of each other to form a volcanic edifice (for example, Mt Tarawera). In the large caldera-forming eruptions, a lot of the erupted material accumulates within the caldera itself as it collapses, and the old land surface may be buried to several kilometres depth.
3.1.3 Distribution of New Zealand's Volcanoes
Volcanoes in New Zealand are not randomly scattered, but are grouped into areas of more intensive and long-lived activity, whose position (and the compositions of the magmas erupted) can be related to the large-scale movement of plates in the New Zealand region. Volcanic activity in New Zealand occurs in six areas (Fig. 3.1), five in the North Island and one offshore to the northeast in the Kermadec Islands.
Most New Zealand volcanism in the past 1.6 million years has occurred in the Taupo Volcanic Zone (Wilson et al. 1995), an elongate area from White Island to Ruapehu, which has been by far the most frequently active area, both in historic times and over the last 1.6 million years. Taupo Volcanic Zone (TVZ) is extremely active on a world scale; it includes three frequently active cone volcanoes (Ruapehu, Tongariro/Ngauruhoe, White Island) and the two most productive caldera volcanoes in the world (Taupo, Okataina).
Volcanic Fields
Northland and Auckland: Three volcanic fields occur in Northland and Auckland, where small individual eruptions occur at intervals of hundreds to thousands of years. The best known of these is the Auckland field, where around 50 small volcanoes have formed, Rangitoto being the youngest (600 years old) (Allen and Smith 1994). The magma is basaltic in composition, and eruptions tend to be small (typically 0.01 - 0.1 km3), and the areas affected are, at most, a few tens of km2; therefore hazards are very localised (Johnston et al. 1997). However, the growth of New Zealand's biggest commercial centre almost exactly on top of one of these fields has led to much greater awareness of the risks posed by a potential renewal of activity in this area.
Cone Volcanoes
Egmont: The modern cone of Egmont is only the latest in a series of cone volcanoes that stretches back in time for 1.7 million years (Neall and Alloway 1986). The older cones (1.7 - 0.13 million years) have now been eroded down to relics which form the Pouakai and Kaitake Ranges, and the Sugarloaf Rocks at New Plymouth. The main Egmont cone is about 130,000 years old, and has a complex history of multiple cone building episodes followed by cone collapse episodes when much of the cone was destroyed by huge debris avalanches. Most of the actual mountain that we see today is only about 10,000 years old and has rapidly built up since the last major collapse. The latest eruption where magma reached the surface is thought to have occurred in 1755 AD, so the volcano is considered to be dormant (Alloway et al. 1995). Eruptive products of Egmont are andesitic to dacitic in composition. They form domes and lava flows that, together with some pyroclastic material, have built up the modern cone itself. Comparable volumes of pumice, scoria and ash have spread as thin pyroclastic fall and flow deposits beyond the cone.
Tongariro/Ngauruhoe: Tongariro is a large (100 km3) cone volcano of which the youngest cone, Ngauruhoe, is the main active centre (Topping 1973). Tongariro, like Egmont, has been both built up by eruptions producing lava flows and pyroclastic material and has been partially destroyed on occasions in the past. However, the main destructive force at Tongariro does not appear to have been cone collapse, so much as erosion by ice during glacial periods. The oldest lavas from Tongariro are at least 340,000 years old, and occur in places that imply there was a substantial "Mt Tongariro" at that time. New work is showing that the modern cone has grown since 275,000 years ago, with intervals of cone building occupying a few thousand to tens of thousands of years (Ngauruhoe is only 2,500 years old) (Hobden et al. 1996). These cone-building periods are separated by times when either most activity was expressed as widespread pyroclastic deposits (which did not contribute much to cone building) or the volcano was much less active. In most eruptions the magma was andesite, but minor amount of dacite and basalt are also known here. The most prominent vent, Ngauruhoe, has been frequently active in recorded times, but has not erupted since 1975 and is now undergoing its longest break from activity in recorded history.
Ruapehu: Ruapehu is New Zealand's largest cone volcano and, like Tongariro and Egmont, has been built up and partially destroyed on several occasions during its history (Hackett and Houghton 1989). The oldest dated lavas are 230,000 years old, but there has probably been a "Ruapehu volcano" for at least 0.5 million years. Destructive influences at Ruapehu include both cone collapse and glacial erosion, the latter continuing to the present day. Like Tongariro, Ruapehu has erupted mostly andesite, and only minor amounts of basalt and dacite have been found. Ruapehu is unusual among the cone volcanoes in having a crater lake which, in historic times, has greatly modified eruptive behaviour such that even small eruptions are accompanied by potentially dangerous mudflows or lahars. With the exception of the 1945 eruption, the lake has acted as a trap for magmatic heat and volatiles, so making it warm and highly acidic. Ejection of lake water leads to the formation of lahars, one of which in 1953 led to New Zealand's worst volcanic disaster, at Tangiwai. Only in 1945 was the lake displaced, and lava extruded at the surface during the largest volcanic eruption in New Zealand this century (Johnston and Neall 1995).
White Island: White Island is the 320 m high emergent tip of a 17 km wide, 750 m high cone volcano largely submerged beneath the Bay of Plenty (Nairn et al. 1991). It is unusual in being one of the very few privately owned volcanoes in the world. White Island is currently New Zealand's most active volcano with three long cycles of eruption recorded between 1976 and 1994. Our knowledge of the earlier history of the volcano is severely limited by a lack of data on the age of prehistoric eruptions. This early history includes two major episodes of cone growth with both extrusion of lava flows and explosive eruptions. There are no recognisable products of primeval or historic activity preserved on the mainland. Historic activity included a small collapse of the west wall of the main crater in 1914, forming a debris avalanche which killed 11 sulphur miners. All subsequent events have been small explosive eruptions, linked to the formation of collapse craters through the 1914 deposits. Since 1976, White Island has erupted low-silica andesitic magma, whereas most early activity involved higher-silica andesite or dacite.
Kermadec Islands: Many oceanic volcanoes occur along a line from the North Island, trending north-northeast towards and including Tonga, and the Kermadec Islands which represent points where some of these volcanoes have constructed cones above the surface of the sea (Latter et al. 1992). Although shaped like their mainland counterparts, the three major cone volcanoes in the Kermadecs (Raoul, Macauley and Curtis) differ in two respects. The first respect is that they have erupted substantial amounts of both dacite and basalt, rather than andesite. The second is that the main processes causing destruction of the cones are marine erosion, and caldera collapse, the latter accompanying dacite eruptions. Many details of the volcanic histories of the Kermadec volcanoes are unknown, as the oldest rocks available on the islands are only a few thousand years old, whereas by analogue with similar-sized volcanoes on the mainland, the individual volcanoes would have taken several hundred thousand years to be constructed. Raoul Island has experienced several historic eruptions, the most recent in 1964. The size range of eruptions at the Kermadec volcanoes is higher than that usually considered the norm for cone volcanoes, and pyroclastic deposits (including ignimbrites) are prominent features of the young eruptive records.
Caldera Volcanoes
Below we describe only the youngest, and hence potentially active, calderas within New Zealand.
Taupo: Taupo is a large caldera volcano, whose shape reflects collapse following two large eruptions about 26 500 and 1 800 years ago (Wilson 1993), although the volcano itself first began erupting about 300,000 years ago. The modern Lake Taupo partly infills this caldera structure. Taupo has erupted mostly rhyolite, with only minor amounts of basalt, andesite and dacite, and is the most frequently active and productive rhyolite caldera in the world. The eruptions are notable for varying enormously in size, from <0.01 km3, up to the largest (26 500 years ago) which ejected about 800 km3 of pumice and ash (if expressed as dense rock, this would be similar to the volume of Ruapehu). There have been 28 eruptions at Taupo since 26 500 years ago, of very different sizes and spaced at different intervals. The variability in the sizes and repose periods makes it impossible to predict when the next eruption will occur and how big it will be. The latest major eruption from Taupo caldera volcano about 1,800 years ago was the most violent volcanic eruption in the world for the past 5000 years and has left marks on the landscape and vegetation patterns which are still visible today (Wilson and Walker 1985).
Okataina: Okataina is a large caldera volcano which has been erupting over a similar time span to Taupo, at similar rates of production, and involving the same types and proportions of magma (that is, almost entirely rhyolite) (Nairn 1991). However, the superficial appearance of the volcano and the styles of recent eruptions at Okataina are different. The last caldera collapse occurred about 64,000 years ago, and the many eruptions since then have largely infilled the hole left behind by the collapse. These young eruptions at Okataina have been fewer in number than at Taupo, but more uniform in size, so that the smallest rhyolite eruptions at Okataina were bigger than all but the four or five largest eruptions at Taupo in the same time period. Many eruptions at Okataina have produced large volumes of rhyolite lava; this lava has piled up over the vent areas to produce two large massifs, Haroharo and Tarawera. However, Okataina has also produced some unusual eruptions such as the basaltic eruption of Tarawera in 1886 AD which is not only New Zealand's largest historic eruption, but also the largest basaltic eruption known in the entire 1.6 million year history of the Taupo Volcanic Zone.
Mayor Island: Mayor Island (Tuhua) is the emergent summit, 4 km in diameter and 350 m high, of a caldera volcano which is roughly 15 km across and 750 m high (Houghton et al. 1992; Houghton et al. 1994). Our present understanding of the history of the volcano is therefore limited to what we can see on the island, the oldest portion of which is about 130,000 years old. Although Mayor Island erupts almost entirely rhyolite magma, this rhyolite is unusual in containing higher amounts of sodium and potassium than the more "normal" rhyolites at Okataina or Taupo, reducing the magma viscosity and therefore the degree of explosivity of many eruptions. The volcano has produced many explosive and effusive eruptions during its history above the water surface, punctuated by at least 3 occasions when caldera collapse occurred. The latest of these occurred about 6,300 years ago, following the largest eruption known in the history of the volcano, and later lavas have only partly filled in this caldera. The eruption 6,300 years ago was so large that substantial amounts of fall material fell on the North Island, and large pyroclastic flows entered the sea, building up fans that (temporarily) roughly doubled the area of the island.
TABLE 3.1: Summary of volcanoes, eruption sizes and frequency of occurrence
| Volcano | Last known eruption | Future eruption size (km3) | Estimated frequency of occurrence | ||
|---|---|---|---|---|---|
| Auckland | ~600 years B.P. | small - medium (0.1-2.0) | 1000-2000 years | ||
| Mayor Island | 6340 years B.P. | small - medium (0.1-1) large (>1.0) |
?1000 years ?10 000 years |
||
| White Island | 1998 AD | small (<0.01) medium (0.01-0.1) large (> 0.1) |
1-5 years ?100 years ?10 000 years |
||
| Tongariro Volcanic Centre | Ruapehu | 1996 AD | small (0.01-0.1) medium (0.1-1.0) large (>1) |
20 years 100-500 years 10 000 years |
|
| Ngauruhoe | 1975 AD | small (< 0.01) medium (0.01-0.1) |
10-20 years 100-200 years |
||
| Tongariro | 1896 AD | small (<0.01) medium (0.01-0.1) large (0.1-1) |
100 years 1000 years 10 000 years |
||
| Egmont | 1755 AD | small (<0.01) medium (0.01-0.1) large (<.1) |
300-500 years 1300-1600 years 10 000 years |
||
| Taupo | 181 AD | small (0.1-0.9) medium (1-10) large (10-100) |
1300-1600 years 2500-5000 years 5000-10 000 years |
||
| Okataina | 1886 AD | medium (1-10) large (10-20) |
1500-2000 years 2000-5000 years |
||
FIGURE 3.2 The frequency of wind directions at various heights above Auckland Airport derived from 1966 to 1979 data.
Wind Frequency
The distribution of ash is highly dependent on wind direction. Winds at higher altitudes are far more uniform than at lower levels. It is these high level winds which have the greatest control on widespread ash distribution. There is little variation in character of the upper-level winds above North Island because of the small difference in latitude between them (see Figure 3.2).
Even though winds from a non-westerly direction occur only for a small proportion of the time, the distribution of ash during such times cannot be ignored. A community on the side of a volcano towards which the wind blows only 1% of the time has only a 1% probability that a single discrete random eruption will drop ash on it. However, if an eruptive episode has 100 discrete ash producing events, there is a 63% probability that one or more of the eruptions will disperse ash in the 1% wind direction, assuming the events are randomly distributed. Therefore, using the wind frequency data to estimate the likelihood of an ash fall occurring, without considering the cumulative frequency probability that results from multiple eruptive events, may lead to a dramatic underestimation of the true likelihood of ash fall at a certain location.
3.1.4 Possible Future Eruption Scenarios
An understanding of past eruptive activity is a key to assessing the location, size and nature of future eruptions. The following section illustrates the range of possible future volcanic hazards by presenting examples of past eruptions and scenarios. However, a future eruption at any volcano will not necessarily be the same as events described here.
Auckland Volcanic Field
Eruption scenarios for the Auckland Volcanic Field have been developed by
Allen (1992), Smith and Allen (1993) and Johnston et al. (1997). Figure 3.3 shows a
phreatomagmatic eruption scenario centred in the Tamaki Estuary (see Johnston et al. (1997)
for a more detailed description). Surges travel out to 3 km and a total of 107m3
of tephra is erupted. Beyond a few kilometres only light ash falls occur.
FIGURE 3.3 A: Vent location for the scenario eruption, extent of tuff cone, and the 1,
2, 3 km zones affected by base-surges and ballistic clasts. B: Tephra fall distribution
pattern (107m3) for scenario eruption. Figure modified from Figures 1.12 and
1.13 of Johnston et al. (1997).
Mayor Island
Eruption scenarios for Mayor Island are described by Houghton et al. (1994). Figure 3.4 illustrates a worst case scenario; a repeat of the 6340 year B.P. eruption. Pyroclastic flows, surges and ballistic blocks would devastated the entire island. Thick ash falls would cover a large area of South Auckland,Waikato and the Bay of Plenty. Since the Mayor Island magma is exceptionally rich in chlorine and fluorine (Houghton et al. 1992) the poisoning of stock in ash-affected areas may result from any eruption, even in areas where only minor amounts have fallen.
FIGURE 3.4 Distribution of fall deposits associated with a repeat of the 6340 year B.P. eruption assuming an easterly wind. The map is from Houghton et al. (1994).
White Island
Figures 3.5 and 3.6 show the distribution of ash from scenarios developed by Nairn et al. (1991). The entire island is subject to debris avalanche, pyroclastic flow and surge and ballistic block hazards. A major eruption (or a very large debris avalanche entering the sea) could generate a tsunami affecting the Bay of Plenty coast, although the risk is thought to be extremely low.
FIGURE 3.5 Bay of Plenty map showing possible ash dispersal ellipses from White Island for a 100 year return period. The dispersal ellipses can be pivoted around White Island depending on the wind direction at the time of the eruption. The rose (compass) diagram shows the approximate percentage of time that wind blows in various directions.
FIGURE 3.6 Possible ash dispersal map for a one per 1000 year return period eruption. See above caption.
Okataina Volcanic Centre
Detailed eruption scenarios have been developed for the Okataina Volcanic Centre by Johnston and Nairn (1993). A future Okataina eruption is expected to resemble those that have occurred in the past (Nairn 1991). Figure 3.7 show the preserved deposits of an event that occurred 9000 year B.P.. A rhyolite eruption will most probably occur from one of two vent lines. Hazard zones for the Okataina Volcanic Centre are shown in Figure 3.8.
FIGURE 3.7 Present day distribution and thickness of fall deposits erupted from the Okataina Volcanic Centre 9000 years ago. Thickness in centimetres. Figure from Nairn 1991.
FIGURE 3.8 Hazard zones defined at Okataina Volcanic Centre based on effects of past eruptions. The hatching defines areas at risk of complete devastation as a result of eruptions at (A) Haroharo; (B) Tarawera; (C) Mt Edgecumbe. The dashed lines enclose areas at risk from hydrothermal eruptions. The dot pattern defines rivers at risk of eruption-induced flooding and lahars. The risk is greater for the Tarawera River than for the Kaituna. Figure from Nairn 1991.
Taupo Volcanic Centre
The range of sizes and types of eruptions likely to occur at Taupo
Volcanic Centre in the future are difficult to forecast because of the chaotic nature of
the volcanic systems (Wilson 1993). There is no simple pattern to past eruptions as shown
in Figure 3.9 but the distribution of tephra from three past eruptions is shown to
illustrate the range of possible future events (Figures 3.10-3.12).
FIGURE 3.9 The volume of tephra erupted in cubic kilometres plotted against the age of the eruptions.
FIGURE 3.10 Distribution of the 1800 year old Taupo fall deposits (in centimetres) and the outer limit of the accompanying Taupo ignimbrite. There was total devastation within the zone of the ignimbrite. (map from Wilson 1993).
FIGURE 3.11 Isopach map of fall deposited erupted from the Motutaiko Island vent ~ 7000 years B.P.. Map from Wilson (1993).
FIGURE 3.12 Isopach map of fall deposits erupted from multiple vents along a fissure with ends marked by filled triangles around 10 000 years B.P.. Map from Wilson (1993).
Tongariro/ Ngauruhoe
The most likely eruption scenario from the Tongariro Volcanic Centre (which includes Ngauruhoe) is a small event similar to historic eruptions (e.g. Te Mari 1896, Ngauruhoe 1954, 1975). A worst case scenario is likely to be an event the size of the c. 10 000 year B.P. eruption that deposited the Te Rato Lapilli (Figure 3.13). The area potentially affected by pyroclastic flows and ballistic clasts from such an eruption would be within the Tongariro National Park.
FIGURE 3.13 Isopach map (in centimetres) showing the distribution of Te Rato Lapilli erupted from a vent since buried under Ngauruhoe ~ 10 000 years B.P..
Ruapehu
A range of eruptions are possible from Ruapehu with a "worst
case" event similar in size to that shown in Figure 3.13. The Crater Lake of Ruapehu
also presents a significant lahar hazard (Neall et al. 1995) and a hazard map has
been produced by Latter (1987). The 1995-1996 eruption illustrated the impacts of even
small eruptions on agriculture in New Zealand. Similar events can be expected every 20 -
50 years. The distribution of ash is shown in Figure 3.14
FIGURE 3.14 Isopach map of the three largest 1995 and 1996 Ruapehu tephra falls.
Egmont
Eruptions from Egmont can be expected every few hundred years, with the last eruption around 250 years ago (Alloway et al. 1995). A range of near-vent hazards pose a significant hazard in Taranaki and are shown on the hazard map of Neall and Alloway (1996). An eruption scenario is shown in Figure 3.15 and the distribution of tephra from an event 3 600 years B.P., the Inglewood tephra is shown in Figure 3.16.
FIGURE 3.15 Tephra fall distribution pattern (108m3)
for scenario eruption.
FIGURE 3.16 Isopach map (in centimetres) for the 3600 B.P. Inglewood Tephra. Map from Alloway et al. 1995.
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