New Zealands volcanoes: White Island

VOLCANIC HAZARDS AT WHITE ISLAND
by J.W. Cole+ , I.A. Nairn* & B.F. Houghton*

INTRODUCTION

 White Island is currently New Zealand’s most frequently active volcano. Situated 48 km offshore in the Bay of Plenty (Figure 1), its activity is often clearly visible to inhabitants of the surrounding region. The island was the scene of one of the three major volcanic disasters in New Zealand when 11 sulphur miners were killed there in 1914 (the other disasters were the 1886 Tarawera eruption, and the 1953 Tangiwai rail disaster). Since 1976, White Island has been more active than at any other time in the last few hundred years, and has aroused considerable public interest in the level of volcanic hazard it presents to people in the Bay of Plenty region.

Frequent explosive eruptions present an obvious threat to the increasing number of visitors to White Island, and to boats in close proximity offshore. The threat diminishes rapidly with distance, and no eruptions during the 160 years of White Island’s written history have been large enough to produce significant effects on the Bay of Plenty coast. However, recent scientific investigations have shown that White Island has a potential to produce large eruptions, although these are apparently unprecedented in the long life of the volcano.

The volcanic hazard arising from White Island has therefore to be assessed under two headings: (a) the obvious but local risk arising from typical White Island eruptions affecting the relatively few people on or close to the island at any one time, and (b), the possible but much less likely, chance of an eruption big enough to affect the large number of people and investments around the Bay of Plenty coast.

This booklet is intended to make information on the volcanic history and hazards at White Island available to residents of the Bay of Plenty region.

WHITE ISLAND ERUPTION HISTORY

 White Island (Figure 1) is the summit of a large (16 by 18 km) submarine volcano which has grown up from the sea floor at 300m to 400m depths. Only half the height and a very small proportion of the volume of this volcano are above sea level. The main crater (Figure 1) was formed in prehistoric times, apparently by the collapse of three overlapping roughly circular subcraters (Figure 2). The eastern subcrater was formed first, and now contains only minor hot spring activity. The central subcrater contains the Donald Mount fumaroles (vents emitting steam and hot gases), and the Noisy Nellie and Donald Duck craters and fumaroles. The western subcrater contains most of the eruption sites that have formed since 1960 and has been the main focus of activity during the island’s recorded history. Deep pits mark the sites of recently active vents. Most of the present main crater floor lies less than 30m above sea level, and has an irregular surface covered by mounts of avalanche debris from the 1914 disaster.

Figure 1 (See above) Map of White Island showing positions of features on the island in 1990. The main crater is the near-flat area which extends between 1978/1990 Crater and the coast at Shark, Wilson and Crater bays.

 
Figure 2

(A) Map showing positions of the three subcraters making up the main crater floor at White Island (E = eastern, C = central, W = western), with locations of vents within the western subcrater in 1977. (B) and (C) show the changes which have occurred since 1977, to 1982, and 1990. TV1 Crater formed in October 1990, and is typical of the six short-lived new vents which have formed in 1978/1990 Crater since 1983.

The end result of the 1976-1982 activity was the partial re-excavation of a sediment-infilled prehistoric crater, and the deposition of about 10 million cubic metres of volcanic ash on the island and offshore. All the individual eruptions during the 1976-1982 period rate as "small" (less than 1 million cubic metres) on the world scale of eruption sizes. A significant hazard existed on and immediately adjacent to the island on only a few days of the entire active period. At no time was the Bay of Plenty coast significantly affected, apart from a few days of ash contamination of roof collected tank water supplies in the eastern Bay of Plenty.

Small eruptions in late 1983 and early 1984 were the only activity prior to the next large eruption sequence which began from a new vent in the wall of 1978 Crater in February 1986. This eruption sequence has continued to the time of writing and has included a range of eruptions and collapse events typical of White Island activity. Ash eruptions in late 1986 were followed in January 1987 by an explosion which threw blocks over the main crater floor. Ash eruptions, accompanied by considerable enlargement of the new vent, continued unaffected before and after the 2 March 1987 Edgecumbe Earthquake. Incandescent ash and gas was erupted from late March to May 1987, when activity reached greatest intensity, with glowing rocks erupted as well as ash, accompanied by air shock waves.

Ash emission was almost continuous into 1988, with occasional larger explosions ejecting blocks and sending ash columns to heights of more than 3 km. Fine ash from White Island was carried inland as far as Rotorua during northerly winds in December 1988. A new crater, named "Donald Duck" (Figure 2C) was formed on the main crater floor to the east of 1978 Crater, and was enlarged by explosions which threw blocks to 450m over the walking tracks on the main crater floor. Two vents were in eruption, with both Donald Duck and 1978 Crater active. 1978 Crater was considerably enlarged by collapse during a period of heavy rainfall in August 1990. The crater rim migrated outwards by about 40m; the enlarged crater was renamed "1978/1990 Crater" (Figure 1). 

The volcanic activity at White Island is caused by the presence of a large body of hot magma deep beneath the island. Gases dissolved in this magma body continually escape and rise towards the ground surface, heating groundwater at shallow depths beneath the crater floor. Steam from this heated groundwater mixes with the volcanic gases from the magma to produce the white steam/gas cloud which is usually present above White Island. The size of this cloud is controlled by the total amount of gas and heat flowing out of the volcano, but is also affected by other factors such as the amount of recent rainfall, and wind strength and atmospheric humidity at the time, so that variations in cloud size and height are not always directly related to volcanic activity. Light winds and high humidity can produce a towering column of white cloud above the volcano, particularly after a period of heavy rainfall. However, actual steam/gas eruptions do occur, often when a blockage beneath the crater is overcome, and an outburst of increased steam and gas discharge results.

A variety of explosive eruptions occur at White Island, where the release of built up steam/gas pressures often blows out solid material blocking the volcano conduits that lead up from the magma body to the vents on the crater floor. Blocks and ash are ejected. The larger ejecta fall back close to the vent, producing rock-strewn areas often covering the usual walking tracks on the main crater floor (see Figure 4). Anybody hit by falling blocks would be killed or severely injured. The ash is carried up in a convecting eruption column, coloured black or grey by the solid material it contains. The column expands into an eruption cloud which drifts downwind, with ash-sized tephra falling out of the cloud as it cools. This ash falls on the island and into the surrounding sea. Only rarely are eruptions large enough, and winds strong enough and in the right direction, for ash to reach the mainland. Thin dustings of fine ash have fallen on coastal areas of the eastern Bay of Plenty a few times in the last 20 years.

Explosions at White Island can occur during otherwise quiet periods, as well as during periods of increased activity such as 1976-1982 and 1986-1989. These intervals of increased activity occur when small volumes of magma rise to shallow depths (less than 1 km beneath the crater floor) from the main body of magma sited at depths of 5km or greater. Gases given off by the shallow magma, and steam derived from heated groundwater, ream out a conduit to the crater floor. The top of the magma body is exposed to the atmosphere, and incandescent lava bombs, block and magmatic ash are erupted. It is at these times, such as in April 1977, that glowing ash and gas columns may be seen at night from the coast.

Particularly large and violent steam explosions occur when groundwater-saturated crater floor material collapses into the eruption conduit, and on to the top of the hot magma column. The resulting steam explosions have produced the highest eruption columns recently observed at White Island (3-5 km in 1977). Such eruptions are particularly likely to produce pyroclastic flows and surges, when part of the high eruption column becomes too dense to remain buoyant in the atmosphere, and collapses on to the ground surface. The high downward velocity attained during the column fall is translated into horizontal speed as a swirling mass of hot ash and gas rapidly flows over the crater floor (Figure 6). Pyroclastic flows and surges would kill anybody caught in their path.

A large eruption column is a threat to aircraft, particularly if ash is sucked into jet engines, where it damages turbines and causes loss of power. No major airline routes cross White Island, but an ash cloud drifting to the northwest or southwest could affect internal and international air routes.

Because the crater floor is underlain by wet volcanic sediments, the eruption of lava flows at White Island is much less likely than is the continuation of explosive ash and block eruptions. Rise of magma into the wet sediments produces explosive steam eruptions which fragment the cooled magma. The eruption of lava flows would require the rise of large volumes of magma to dry out the crater fill, so that liquid magma could reach the surface. With the present active magmatic vents sited deep beneath the main crater floor it is unlikely that lava flows would be erupt4d onto accessible crater floor areas.

Parts of the nearly vertical walls of the main crater at White Island are unstable, particularly at the western end, and it is likely that future collapse will occur, similar to that of 1914. The 1914 avalanche came from the southwest wall of the crater (Figures 2 and 3) where a series of fissures, caused by previous collapse of the main crater, had left a large part of the crater wall very unstable. This collapsed onto the crater floor, where the resultant slurry, containing blocks as large as houses, flowed rapidly eastward, killing 11 sulphur miners living on the island. Newspaper reports at the time described attempts to dig out the bodies as being frustrated by the very hot water seeping from the avalanche deposits.

Deepening of the 178/1990 Crater at the foot of the main crater walls increases the chance of wall collapse, but will also act to trap avalanche material. The crater walls are saturated with hot groundwater so that avalanched material is likely to be very mobile, lubricated by the groundwater and probably fluidised by some flashing to steam. Crater wall material which collapses and avoid 1978/1990 Crater (or after 1978/1990 Crater is infilled) will flow very rapidly across the main crater floor, as in 1914, and may enter the sea through the eastern bays. Debris avalanches are likely to be triggered by strong earthquakes at White Island or by large volcanic explosions. They could also occur at other times, perhaps as a result of heavy rainfall, or long term crater subsidence. Debris avalanches into the sea could also occur from the steep outer slopes of White Island, and might generate small tsunami (see below).

 Volcanic Gases

Gases are continually emitted from the craters and fumaroles on White Island, at rates of several hundred to several thousand tonnes per day. These gases are mostly steam, carbon dioxide and sulphur dioxide, with small quantities of halogen gases (chlorine and fluorine). The acid gases combine with water in the steam/gas clouds to form liquid acid droplets which sting the eyes and skin, and affect breathing. They also severely damage cameras, electronic equipment and clothing. Volcanic gases at White Island are discharged at high temperatures (100^oC-800^oC), so that anybody falling into a vent would be rapidly cooked, an unwelcome prospect when blundering about in a steam cloud with eyes shut against acid drops! Visitors to White Island should avoid steam/gas clouds, and watch for wind changes that could blow clouds in their direction. Gas masks with acid gas filters are advisable, to be work if gas becomes a problem.

Tsunami

 The prospect that an eruption at White Island could cause a tsunami has been discussed for many years. No evidence of past tsunami from White Island is know on the Bay of Plenty coast, but such evidence could be very difficult to detect. The main reason for considering the possibility at White Island comes from the generation of tsunami at other island volcanoes around the world.

Possible mechanisms which have been considered to the generation of a tsunami at White Island include: 

(a) subsidence of an underwater part of the volcanic cone.

(b) collapse of part of the above-water cone into the sea.

(c) collapse of the present crater floor to below sea level during an eruption, allowing sea water access to the active vents, followed by collapse of the upper slopes of the cone into the flooded area in a two-stage mechanism.

(d) The entry into the sea or large pyroclastic flows from a White Island eruption.

Mathematical modelling of the effects of such events has suggested that damaging waves would reach the Bay of Plenty coast only if more than one cubic kilometre of water was displaced at White Island. This would require a very large event, perhaps approaching the size of the 1883 Krakatoa eruption. However, these modelling results conflict with what has actually happened around the world, where damaging tsunami have resulted from quite small eruptions. Analogy with such historic events suggests that the risk of significant tsunami damage over restricted sectors of the Bay of Plenty coast resulting from relatively small eruptions and/or cone collapse cannot be completely excluded. Because the time from generation of a White Island-sourced tsunami to arrival at the shoreline is only 20-30 minutes, there is no prospect of advanced warning which would leave time for reaction. It is thus probably safest to assume that a tsunami could accompany any major eruption at White Island.

Although there is no record that White Island eruptions have ever significantly damaged the mainland, recent work has suggested the volcano is potentially capable of producing large eruptions. This potential arises from the size of the magma body at depth beneath the island, as indicated by the long term outputs of sulphur dioxide gas (400 tons/day) and heat (400 MW) from the crater. Maintenance of these large outputs over the more than 16,000 year history of White Island activity requires the degassing and cooling of several tens of cubic kilometres of magma. A large proportion of this magma still remains to provide the heat source for the present activity. If a substantial fraction of this magma were to be erupted (e.g. more than 1 cubic kilometre) it would be far larger than any recognised previous eruption from White Island, and large enough to produce a threat to the Bay of Plenty coast. These threats principally arise from fall out of ash and pumice and from tsunami.

Volcanic activity at White Island is continuously monitored by a radio-telemetered seismograph which records at the Rotorua Office of DSIR Geology and Geophysics. The seismograph detects earthquakes, and volcanic tremor (vibrations caused by magma/gas movement), as well as air shock waves from volcanic explosions. It is vulnerable to damage by impacts of blocks and bombs during eruptions (as in 1977), and to ash fall covering the solar panels, cutting off the power supply.

 Other monitoring techniques are carried out at regular intervals. Level surveys (Figure 7) are made to determine the precise heights of a network of pegs on the main crater floor at 2-3 month intervals, and more frequently during active periods. Changes in heights of the pegs between surveys reveal deformation of the crater floor, with uplift (inflation) of up to 20cm occurring prior to major eruptive episodes, and subsidence (deflation) accompanying declining activity. Magnetic surveys, made during the same visits to the island, can detect changes in temperatures at depth beneath the crater floor, as rock magnetism decreases during heating, and increases with cooling. Fumarole gas temperatures are measured to provide further evidence of heating or cooling trends. Volcanic gases are collected and analysed to help understand the volcanic processes which control White Island activity.

The volcanic hazard existing at White Island is illustrated in Figure 8 for typical small eruptions which range in size from those that occur with a frequency of about one per year to larger events which occur with a frequency of about one per century. The frequency/size relationship of eruptions is similar to that of heavy rainstorms - the larger the event the less likely it is to occur. Very large eruptions at White Island probably occur at intervals longer than 100,000 years - no such eruptions have yet been recognised.

The main crater floor is at risk from debris avalanches resulting from collapse of the crater walls, and (much less likely) lava flows. A larger zone at risk form pyroclastic flows and surges, and bomb and block falls, includes and extends beyond the crater floor, possibly affecting boats some distance out to sea. Blocks fired out of the vents on ballistic trajectories (like cannonballs) can fall up to several kilometres from the active vents. Heavy ash falls will be dispersed mostly downwind (the inset diagram in Figure 9 illustrates the approximate percentages of time that winds blow in various directions), but the largest recent eruptions have covered the entire island with ash (see Figure 5).

A one per 100 year eruption could be expected to dump 1-50mm of ash on the Bay of Plenty coast if wind directions were suitable (Figure 9). Larger eruptions (e.g. one per 1000 year events) could deposit more ash (Figure 10) on a larger area of the mainland, with dispersal again controlled by wind directions during the eruption.

The coastline between Tauranga harbour and East Cape is at risk from a volcanic-induced tsunami at White Island. It appears that only a Krakatoa-scale eruption could give rise to a major tsunami threat to the whole Bay of Plenty coastline. Eruptions of this catastrophic magnitude are unknown in the history of White Island. A very large eruption from White Island would be an unprecedented event from this volcano, suggesting a very low probability (less than 1 per 100,000 years). However, the generation of some historic tsunami by relatively small eruptions at other volcanoes suggests that some risk of tsunami damage over restricted sections of the Bay of Plenty coast from major White Island eruptions cannot be completely excluded (Figure 10). It is estimated that White Island eruptions would have to be at least 10-100 times larger than any in the last 20 years before have any significant effect on the mainland, and then only if wind directions were suitable.

 In recent years White Island has gone through long periods of continuous gas and ash emission, with ash clouds emitted at low discharge rates (Figure 11). This type of activity poses little risk to visitors to the island, provided that wind changes to not cause them to be enveloped by swirling gas and ash clouds. However, discrete explosions can occur at any time, during both active and quiet periods, to produce block-ejecting eruptions which threaten people on the main crater floor as well as boats offshore. These discrete explosions can occur without any warning to people on the island. A few minutes of precursory seismicity may be recorded in Rotorua, but this is of no use to visitors on the island. These relatively small explosive eruptions typically occur several times a year at White Island, and have to be expected at an active volcano.

No information is available on previous White Island eruptions large enough to affect the Bay of Plenty coast. Analogies with other volcanoes suggest that such an eruption would be preceded by months or years of preliminary activity including major uplift of the whole crater floor area, measured by the level surveys and possibly producing ground deformation visible to the naked eye. The frequency of volcanic earthquakes would be expected to increase by factors of 100’s or 1000’s. Fumarole temperatures and total gas outputs would increase. A major eruption would probably be preceded by steadily increasing gas and ash emission, with smaller eruptions leading up to a climax. Such a sequence of events would probably be considerably larger than that of the 1976-1982 sequence.

Although a prolonged build-up in activity is considered most likely before a major eruption, the slight possibility remains of a sudden and unexpected eruption if a fault displacement accompanying a major earthquake were to intersect the White Island magma body. White Island lies close to the western margin of the Whakatane Graben (rift), which contains many faults both on and offshore, and in which the 1987 Edgecumbe Earthquake and fault movement occurred. No faults are known to pass through the White Island massif, but their existence cannot be entirely ruled out.

If earthquakes or other events at White Island were to suggest the possibility of an impending eruption, monitoring efforts by scientists from government departments and universities would be increased, with the aim of better assessment of the likelihood, size and effects of any eruption. Scientific advice would be passed on to Civil Defence and local, regional and national government agencies, who will advise the public in detail about the potential hazards and the public safety actions to be carried out. These actions may include evacuation of threatened areas, closure of roads and airports, and restrictions on entry into dangerous zones on, around and above White Island.

Most of the recent explosive eruptions at White Island have affected only the western half of the main crater floor where blocks have fallen up to 500m from the active vents. Explosions have occurred from both 1978/1990 Crater and Donald Duck vents. Surges of hot gas and ash have swept across the main crater floor. If an explosive eruption occurs while people are on the island they should immediately run towards the eastern (factory) end of the crater floor. This area has been safe in all except the largest eruptions of the 1976-1990 period. If about to be caught in swirling steam or ash clouds, where visibility is nil, people should take shelter behind large rocks (if nearby) and breathe through clothing, handkerchiefs etc., if no gas mask is available. Move only when visibility is adequate, to avoid falling into areas of hot ground, fumaroles and craters. Boats close to White Island should move away, preferably into an upwind position, so that the fallout of ash and acid rain on to them is minimised. Be aware that some recent eruptions have thrown rocks into the sea around the island.

A large eruption capable of affecting the Bay of Plenty Coast is most unlikely to occur, and then only after a period of increased activity which would provide reasonable warning. There should be time for detailed instructions to be issued during the precursory phase. In the very unlikely event of a large eruption occurring without warning, residents of low lying coastal areas which are subject to a tsunami threat, should move to shelter on higher ground. Occurrence of such an eruption would most probably be signalled by the generation of a very large and high (greater than 20 km) eruption column (if the island is visible), and by loud detonations, probably audible on the mainland. A tsunami, once initiated, is not affected by wind, but ash fall on the coast will be strongly controlled by wind direction. If winds are from the northwest to east sector, ash fall could be expected on the North Island anywhere between the Coromandel Peninsula and East Cape.

Ash falls greater than a few centimetres thickness can be expected to prevent road travel by immobilising vehicles, the fine ash blocking airfilters and causing damage to motors. Electrical power and telephone services will probably be cut by shorting across insulators. Water supplies and stock feed will be contaminated. Breathing may become difficult in heavy ash falls due to the presence of fine dust and volcanic gases. Use filter masks or breathe through layers of wet cloth.

Contingency plans (prepared by the Rotorua DSIR Geology and Geophysics Office) for monitoring large and potentially hazardous White Island eruptions are based on the concept that eruptions would have to be at least 10-100 times larger than any in the past century before a significant hazard arose on the Bay of Plenty coast. A potential volcanic hazard presently exists from White Island, although this hazard is small due to the very low probability that eruptions of the magnitude required to affect the mainland will occur. Such eruptions would appear to be unprecedented in the volcanic history of White Island, but this does not completely rule out their future occurrence.

During the 1976-1982 eruption sequence the highest observed eruption columns from White Island reached elevations of about 5 km, during eruptions which ejected 100,000 cubic metres of ash over periods of a few minutes. For eruptions approaching significant emission rates (i.e. 10 times larger) it is considered that the associated eruption columns would reach above 10 km, and this elevation figure has been chosen as the trigger level for a volcano "alert" if continued for more than an hour. If such an eruption were to occur at night and/or in bad weather it may be unobserved, but would probably be accompanied by loud detonations heard on the mainland, and undoubtedly by intense seismic activity and probable destruction of the present White Island seismograph. An eruption of this size would be accompanied by earthquakes large enough (magnitude greater than 3.5) to be recorded on portable seismographs placed along the coast of Maketu, Whakatane and Te Kaha. These instruments would form an array linked by radios so that operators could identify earthquakes at White Island greater than about magnitude 3-3.5. A 24 hour/day watch would be maintained by the staff monitoring these instruments, who would also be in contact with a control office in Rotorua. Such a seismic array could probably be put in place during the 24 hours after an alert was commenced. Regular overflights would be made from Whakatane and Rotorua during the eruption sequence, plus helicopter landings to sample ejecta when possible. Any trend towards more silica-rich magmatic ejecta (i.e. dacite) would be of considerable significance in assessing any development of the eruption towards larger and more dangerous levels.

This booklet is largely based on information in New Zealand Geological Survey Bulletin 103 "The 1976-1982 eruption sequence at White island (Whakaari), Bay of Plenty, New Zealand", edited by B F Houghton and I A Nairn;

New Zealand Geological Survey Report G121 "An updated assessment of volcanic hazard at White Island in 1987" by I A Nairn.

Institute of Geological and Nuclear Sciences Science report 95/36 "Volcanic hazards, White Island" in 1995 by B J Scott, I A Nairn and C P Wood.

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’