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In New Zealand's recorded history, volcanoes have claimed more
lives than any other form of natural disaster. Coupled with the
economic development of the country, this means that any future
eruptive activity has to be watched for so that the sizes and styles
of any eruptions, and their consequent threats to lives and property,
can be forecasted.
Studies of the deposits laid down by volcanoes are used to assess
the hazards posed by past eruptive activity (both historic and prehistoric),
they act as a guide to what might be expected in the future. Surveillance
of New Zealand's volcanoes is carried out in order to provide warning
of any impending eruptive activity so that appropriate steps can
be taken to reduce the risk to lives and property.
The rationale for doing surveillance work at as many of New Zealand's
volcanoes as possible is the need to know what is the 'background'
levels of seismicity, ground movement and gas flux at the volcano.
In addition, to be of any real use in planning for a future volcanic
eruption, this surveillance has to be done essentially in near 'real-time',
that is, analysed and available for the volcanologists to interpret
as soon as possible.
 Volcanic
Surveillance: an introduction
Volcanic surveillance is based on the assumption that movement
of molten rock or magma beneath a volcano will occur before any
eruption can start. Volcanologists primarily use three kinds of
techniques to detect magma and monitor its movements.
In this cartoon you can see the magma (orange) which moves into
the volcano to generate the volcanic earthquakes and change the
shape of the volcano.
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 Monitoring
of ground deformation.
As the magma approaches the surface of the earth, and moves into
the conduit below the vent of a volcano, the pushing aside of the
surrounding rocks to make way for the magma causes the ground surface
to move and the volcano to swell. This rising or swelling can be
detected by ground deformation surveys using levelling, triangulation
and GPS surveys.
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 Monitoring
of the chemistry of volcanic gases and crater lakes.
Magma deep in the earth contains gases dissolved in it. As the magma
rises to shallow levels, these gases are released and, because they
are so mobile when compared to the sluggish liquid magma, they rise
to the surface and are discharged through gas vents as fumaroles.
The temperatures and absolute amounts of the gases and for the relative
proportions of different gases give information on the state of the
magma and how close to the surface it is. In some cases,notably Ruapehu,
these gases emerge under a lake, and the chemistry of the lake water
is then used to work out how much volcanic gas is being released into
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Volcano Surveillance: some more
detail
Two scales of monitoring are required for active volcanoes. The first
is background monitoring between crises using a limited number of
fixed instruments or sampling points. The key here is to strike a
balance between cost and the need for accurate and reliable data.
The second scale is the monitoring during a crisis. This can require
a much greater deployment of monitoring equipment and personnel, but
only for a finite time.
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Monitoring of volcanic earthquakes
Earthquakes commonly provide the first indication of volcanic unrest.
There are five volcanoseismic networks in operation around New
Zealand's volcanoes. Three are operated by GNS in the Taupo Volcanic
Zone. The volcano-seismic networks at Taranaki and Auckland are
operated by their respective regional councils. Raoul Island, New
Zealand's most northern and one of the most active volcanoes, also
has a seismograph on it.
Volcanoes produce two types of seismic signature, the first are
called ‘volcanic earthquakes’, the other is ‘volcanic
tremor’.
When the rocks are broken by the magma moving through them small
earthquakes occur, ‘volcanic earthquakes’ . If the rocks
are relatively cool and brittle the earthquake will be off a higher
frequency than those that break hot-softer rocks, some times you
will hear volcanologists talking about high or low frequency volcanic
earthquakes. When the hot volcanic gases that come from the magma
pass through cracks on the way to the surface they cause a small
ground vibration, this is called volcanic tremor. This can be an
almost continuous signal. As the gas volume, velocity or pressure
changes so does the strength of the signal. Volcanic tremor can
also be caused by the movement of molten magma.
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High frequency tremor
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 Low
frequency tremor
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Monitoring of ground deformation
Background monitoring
Geodetic measurements are used to monitor changes in the shape of
the ground-surface caused by the movement of magma towards the surface.
Techniques used by GNS include traditional triangulation and trilateration
techniques which measure distances with electric distance measuring
equipment (EDM), while ground tilting is measured by precise levelling
and the use of the volcanic lakes as large scale natural spirit
levels. Lake levelling is conducted in one of the lakes on Raoul
Island, and at lakes Tarawera, and Taupo. At White Island a precise
levelling network was established in 1966 on the floor of Main Crater
to measure changes in crater height.
Detailed horizontal control surveys have been repeated across the
Taupo Fault Belt, both north of Taupo and south of Rotorua, and
about the Okataina Volcanic Centre using both traditional triangulation
and trilateration techniques, and since 1991 global positioning
systems (GPS) technologies.
Monitoring the chemistry of volcanic gases and crater lakes
Changes in volcanic gas chemistry, the rate of gas emissions (for
example, SO2) from craters and the chemistry of crater lake and
thermal spring waters are used to detect changes in the behaviour
of the volcanoes and their associated geothermal systems. Geochemical
surveys include sampling of volcanic gases from selected fumaroles
at places like White Island; Mount Tarawera (Okataina Volcanic Centre);
Red Crater, Central Crater, and Ketetahi on Mount Tongariro; and
the summit crater of Ngauruhoe. During regular visits to Ruapehu's
Crater Lake samples of water are collected.
GNS also has a correlation spectrometer (COSPEC) which enables us
to monitor accurately the amount of SO2 gas emitted from
New Zealand's volcanoes. Changes in groundwater, lake levels, rates
of stream flow and the temperatures of such waters often give evidence
of unrest within a volcano. Crater lakes in particular are valuable
indicators of the status of volcanic systems. For example, at Ruapehu
we often see the Mg values increase if new/fresh rock is made avilable
to the crater lake waters.
Other monitoring includes weekly measurements of the levels and
temperature of Blue and Green lakes at Raoul Island, along with
regular ground temperature measurements and water sampling. The
water level of Green Lake (at Raoul Island) rose more than six metres
before the 1964 eruption started. Within the Okataina Volcanic Centre,
heat flow from the large crater lakes at Waimangu has been continually
monitored since 1970 by recording the temperature, level and rate
of discharge of the lakes.
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Volcanic
hazards in New Zealand