Volcanic hazards at
White Island is currently New
Zealandís 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.
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 islandís 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.
(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.
at White Island as far back as 16,000 years ago is recorded by volcanic ash
layers found in ocean floor sediment drill cores, but the island was
undoubtedly active long before this time. The ocean floor core data provide
the only presently available record of the prehistoric activity of White
Island, as no old ash deposits from the volcano have yet been identified on
the North Island. In fact, there is no evidence that White Island has ever
significantly damaged the North Island, at least during the last 50,000
years of good geological record.
The written history of White Island observations begins in 1826; since then a state of continuous low level activity with intermittent small eruptions has been recorded. The crater floor was often flooded by hot lakes, until these were permanently drained in 1913 so that sulphur deposits on the crater floor could be mined. In September 1914 the southwest corner of the high crater wall (see Figure 2) collapsed to produce a hot avalanche which buried 11 sulphur miners and destroyed buildings and equipment at the eastern end of the crater. Mounds left by this avalanche are visible in Figure 3.
Since 1914 new vents have formed in the west and central subcraters, associated with small steam and ash eruptions. "1933 Crater" (Figure 2A) formed in that year during an explosive ash eruption; "Noisy Nellie Crater" formed prior to January 1947; "Big John Crater" grew to 50 m diameter during eruptions between 1962 and 1965. A steam and ash eruption in November 1966 accompanied formation of the 60m diameter, 120m deep new vent "Gilliver Crater". Rudolf vent grew from a fumarole during ash eruptions in 1968, to reach 45m diameter and 120m depth by 1969. Some ash fell on the North Island during these eruptions. Two years later, a single explosive eruption formed "1971 Crater".
No further eruptive activity occurred between 1972 and December 1976, when the largest and longest sequence of eruptions recorded at White Island began and continued until the beginning of 1982. These eruptions were caused by the rise of molten rock (magma) beneath the volcano, to reach shallowest levels in 1977 and 1978, when the large individual eruptions occurred and incandescence (glows) could be seen at night from the Bay of Plenty coast. Lava bombs and blocks (see box) were erupted and voluminous ash clouds rose above the island (Figures 4 and 5). The "1978 Crater Complex" (Figure 2B) was formed by collapse and explosive excavation around the active vents. Fine ash fell on the eastern Bay of Plenty coast on several occasions during the 1976-1982 period, but this ash was quickly removed by wind and rain and is not preserved. During 1979-1980, eruptions occurred less frequently as the magma withdrew to deeper levels, and at times the floor of the vent was more than 200m below sea level.
(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.
Mounds left by the 1914 avalanche cover the main crater floor. Steam clouds rise from the Donald Mound fumaroles, obscuring 1978 Crater behind - Photo by D L Homer.
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.
WHITE ISLAND ERUPTION TYPES AND EFFECTS
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.
Ash eruption at White Island on 4 April 1977. Lava bombs and blocks cover the main crater floor in foreground. Note geologist on crater rim for scale - Photo by S Nathan.
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.
Continuous ash eruptions on 9 January 1979. The whole island was covered by voluminous ash deposits - Photo by B J Scott.
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.
Thick pyroclastic surge deposits (light grey) cover the main crater floor 2 hours after explosive eruptions and column collapse on 25 August 1977. Note the rapid feathering out of the surge deposit in foreground where it overlies the dark grey pre-eruption ground surface. Steam rises from Donald Mount fumaroles. Compare with Figure 3 which shows the same area Ė Photo by I A Nairn.
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.
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 (100oC-800oC), 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.
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.
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
Level survey of the ground deformation network installed on the main crater floor at White Island. Steam rises from the recently formed Donald Duck Crater beyond surveyor - Photo by I A Nairn.
Hazard cones at White Island for eruptions of sizes with a return period of one per year to one per 100 years. A central (dark) zone is at risk from debris avalanches, pyroclastic flows and surges, and bomb and block falls on the main crater floor and for a short distance offshore. An outer zone (cross hatched) extends further offshore, at risk from pyroclastic surges. Block falls could occur up to 4 km from the active vents. The crater shape sends most ballistic blocks to the east. Heavy ash falls will be dispersed mostly downwind. The most common wind directions are shown in Figure 9.
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.
A. On and Around White Island
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).
B. Bay of Plenty Region
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.
Figure 9 Bay of Plenty map showing possible ash dispersal ellipses from White Island for a 100 year return period eruption. Ash can be expected to exceed the thickness shown within each ellipse. The dispersal ellipses can be pivoted around White Island depending on wind direction at the time of eruption. The rose (compass) diagram shows the approximate percentage of time that the wind blows in various direction, i.e. it blows from west to east 41% of the time.
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.
Possible ash dispersal map for a one per 1000 year return period eruption, otherwise see caption for Figure 9. The stippled coastline illustrates areas possibly at risk from volcanic-induced tsunami at White Island. Only a Krakatoa style of eruption could give rise to a major tsunami threat to the whole Bay of Plenty coastline. However, some risk of tsunami damage over restricted sectors of the Bay of Plenty coast from major eruptions at White Island cannot be completely excluded. With the present configuration of the main crater at White Island, the area between Whakatane and Hicks Bay is probably at most risk, if a tsunami were to be generated by the passage of debris avalanches or pyroclastic flows into the sea.
EVENTS LEADING TO AN EXPLOSIVE ERUPTION
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.
Convoluting ash clouds rising above the rim of 1978 Crater on 9 February 1989. Blocks are occasionally ejected during this type of activity - Photo by I A Nairn.
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 100s or 1000s. 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.
WHAT TO DO IN AN EXPLOSIVE ERUPTION
A. For Visitors to 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.
B. For Residents of the Bay of Plenty Region During a Major Eruption
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.
CONTINGENCY CONCEPTS AND PLANNING
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.
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;