White Island is the northernmost active volcano
in the Taupo Volcanic Zone - a 250km-long zone of intense volcanism
that marks the boundary of the Australian and Pacific tectonic plates.
Scientists from the Institute of Geological & Nuclear Sciences
Limited are regular visitors to the privately-owned island, which
attracts an ever-increasing number of tourists.
Sitting 48km offshore, the island has been built up by continuous
volcanic activity over the past 150,000 years. About 70 percent of
the volcano is under the sea, making this massive volcanic structure
the largest in New Zealand.
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The island has a history of long periods of continuous hydrothermal
activity and steam release, punctuated by small-to-medium eruptions.
Between 1976 and 1993 White Island was more active than at any time
in the past few hundred years, and ash from its 1998 eruptions was
recorded as far inland as Rotorua. The volcano's activity is often
visible to people in Bay of Plenty with gas and ash plumes rising
as high as 10km on clear, still days.
Craters and fumaroles on the island continually emit gases at rates
of several hundred to several thousand tonnes per day. The gases
are mostly steam, carbon dioxide and sulphur dioxide, with small
quantities of chlorine and fluorine. Acid gases combine with water
in the steam to form acid droplets that can sting the eyes and skin,
and can affect breathing. The acid can also damage cameras, electronic
equipment and clothes. In spite of its hostile environment, the
island is host to a number of bird species including a gannet colony.
The upper slopes of the island, which rises to a height of 321m,
are steep and deeply eroded. A sulphur mining venture on the island
stopped abruptly in 1914 when part of the crater wall collapsed
and a landslide destroyed the sulphur mine and miners' village.
Twelve lives were lost. Mining resumed again in the 1920s and the
remains of buildings from that era are a tourist attraction. The
sulphur was used to make fertiliser, for export as sulphur ore,
and for the manufacture of sulphuric acid.
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White Island as an earth science laboratory
Recent scientific investigations by the Institute and international
collaborators have focused on the island's eruption history, volcanic
processes, and its geothermal system. The transition zone of heat
from volcanoes to geothermal systems is commonly 3km to 4km beneath
the surface. At White Island it is within a few hundred metres of
the surface. Consequently New Zealand, Japanese and United States
scientists have proposed a research drilling project to explore
the island's geothermal system.
The project, which will involve drilling down several hundred metres
below the crater floor, will enable scientists to explore the boundary
between the molten magma and the geothermal system. Samples of subterranean
fluids and gases will provide valuable insights that will enhance
the understanding of volcanoes and geothermal systems.
Just as important, the project will tell scientists more about the
type of technology needed to cope with very hot and corrosive fluids
in geothermal fields worldwide. However, it is unlikely that White
Island will become a geothermal energy source in its own right.
Geological investigations have established that the island is capable
of producing eruptions that are larger than any it has produced
in the past 150 years. Scientists have also established that no
eruption deposits from the island are preserved on the mainland.
This indicates that although there have been large eruptions in
the past, they have not had a significant impact on the Bay of Plenty
region.
Geochemical studies have shown that the geothermal system under
the island is similar to those which host precious metals. Scientists
have used the island to study metal corrosion in acid environments,
and it has been used to establish technologies such as power generation
from high-temperature, acid geothermal systems.
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Volcano monitoring at White Island
Investigations of the White Island volcano began in the 1940s, and
more systematic surveillance started in the mid-1960s. Early investigations
involved occasional visits to the island. When semi-continuous, larger-scale
eruptions started in 1975, it quickly became apparent that irregular
visits were not effective for real-time hazard assessment and surveillance
needs.
The Institute of Geological and Nuclear Sciences (GNS) runs the White
Island volcano surveillance programme. GNS scientists make four comprehensive
visits a year, and up to six second-order visits. A solar assisted
seismometer on the island sends back seismic data that are continually
available to our scientists.
Volcano surveillance is based on the philosophy of undertaking ''
an effective minimum level of investigation''. This takes into account
factors such as available resources and the level of hazard. The surveillance
programme is designed to identify the early onset of a volcano crisis,
but not necessarily to provide real-time information, say, to visitors
on the island.
White Island and tourism
More than 10,000 people visit White Island every year. The number
of tourists has grown steadily over the past 10 years. Four helicopter
operators take visitors to the island -- three from Rotorua and
one from Whakatane. A number of launch operators, mostly based in
Whakatane, run charter trips to the island.
Volcanic hazards at White Island
Volcanic hazards in and around the main crater vary considerably
with the level of activity. Hazards exist even during quiet periods.
As well as the risk of isolated eruptions during quiet periods,
there is also the small possibility of a crater wall collapse.
A collapse and resultant avalanche could be triggered by a shallow
earthquake or through increased instability after heavy rain. Given
the history of the last 25 years at the island, the most likely
hazard-producing event is the resumption of enhanced activity.
Hazards during enhanced activity include ashfall, steam eruptions
and occasional explosions throwing ballistic blocks as far as the
sea. Explosive eruptions pose a risk to visitors on the island,
boats anchored in the bays, and to aircraft passing close to the
island.
One of the outcomes of the Institute's surveillance programme is
the realisation that at present there are no reliable short-term
precursors to large, discrete explosive eruptions. One of the difficulties
in forecasting eruptions at White Island is that many are triggered
by the collapse of part of the volcano wall. When this happens,
the avalanche temporarily blocks the plumbing, and an explosion
is almost inevitable. White Island is an active volcano and any
activity on the main crater floor involves a degree of risk.
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