Activity is monitored using the technique of Volcanic Surveillance. Multiple areas of science are used in studying the behaviour of a volcano. Data from all disciplines are collected, analysed and cross-referenced, to help give an understanding of behaviour at the volcanoes and an insight to future eruptions.
The easiest way of monitoring a volcano is just by looking at it!
It is not possible for a scientist to visit a volcano every day so we now use remotely operated cameras to supplement those observations. Our remote cameras are 'netcams' - digital cameras that are specifically designed for remote operation. The controlling computer asks them to take a photo at regular intervals and downloads the file to our data centres and made available to the website.
Several of the cameras are some distance from the volcano, based at locations where we have power and communications. Some of the cameras in remote locations such as White Island are solar powered. At White Island the pictures are telemetered 50 km to Whakatane. The images are then transferred to our data centres and onto the website across internet data links. Our newest cameras are infrared-capable, allowing pictures to be taken at night when there is enough ambient light such as from a full moon. During the course of an eruption the download rate from the cameras can be increased to capture the eruption sequence in more detail.
As the molten material (magma) rises to shallow levels, gases are released and they rise to the surface.
Usually gases are discharged through gas vents (fumaroles) and we can measure the temperature and composition of these gases by taking regular samples. As magma rises in the volcano, the temperature of fumaroles may increase, and their chemistry will change. Sometimes the gases emerge under a lake, or interact with groundwater in the volcanic edifice. Also the heat from the rising molten material can heat the ground water to form hot springs and lakes.
Changes in the water chemistry of crater lakes 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 selected springs, lakes and streams at places like White Island; Tarawera (hot water beach); Red Crater and Central Crater on Tongariro; and the Crater Lakes of Ruapehu and White Island.
Monitoring specific water parameters such as temperature, pH, conductivity, and concentrations of dissolved gases can provide insight into the processes expected to accompany unrest or renewed volcanic activity. Changes in groundwater, lake levels, rates of stream flow can also 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 magnesium concentrations increase if new/fresh rock is made available to the crater lake waters.
One of the key techniques used in volcano surveillance is monitoring ground deformation.
Ground deformation is the change in shape that can occur prior to, during or after an eruption. Such ground movement can occur in response to the influx or withdrawal of molten material (magma) and hydrothermal or magmatic fluids in the volcano. Increases in ground deformation may signal the start of a new eruptive episode. There are numerous ways to measure such deformation, like levelling, triangulation and more recently using continuous Global Positioning System (cGPS) measurements. It is also possible to use lakes as large tiltmeters.
Geodetic levelling is used to measure elevation differences between benchmarks and, by repeating surveys, elevation changes (vertical displacements) with time can be recorded. Regular surveys are made of the crater floor at White Island for example.
A theodolite and EDM (electronic distance measurements) can be used to measure the change in shape of an area by making repeat measurements. This type of surveying has been replaced by GPS (see below).
A lake can be used as a large natural tiltmeter by measuring changes in the water-surface elevation relative to nearby stable bench marks. A lake located within an area of active deformation presents a unique monitoring opportunity. Lakes are often present about volcanoes. In New Zealand we use lakes Tarawera and Taupo, and Blue Lake on Raoul Island in the Kermadecs.
In order to make accurate cGPS measurements a stable monument must first be constructed that is well anchored to the bedrock at a depth of 5 - 10m. Attached to the monument is a geodetic-grade antenna and GPS receiver combination. These instruments are capable of metre accuracy in standalone mode. However when the data are processed with those of nearby stations, accuracy of a few millimetres can be achieved. The necessity for an extremely stable monument then becomes obvious.
Most of our volcano cGPS stations record their position every 30 seconds which is averaged to produce hourly and daily positions. Some stations transmit their location once a second which allows them to be used as base stations by surveyors for cadastral surveying. In this mode millimetre accuracy coordinates are available on a continuous real-time basis.
When molten material (magma) moves into a volcano it gives off volcanic gas emissions which are measured regularly at our volcanoes. There are several techniques which include measurement done from an aircraft and on the ground. The four primary techniques, two airborne and two ground-based are outlined below.
A Correlation Spectrometer (COSPEC) or a FLYSPEC measure the absorption of ultraviolet light by SO2, so the SO2 content of the volcanic gases is determined by flying under the gas plume (at right angles to the wind direction) and looking up through it. The FLYSPEC is a miniaturized, lightweight correlation spectrometer more adaptable to different platforms than the COSPEC.
The aircraft flies through the volcanic gas plume at different altitudes. A pump inside the aircraft sucks in the gases and the LICOR and Interscan instruments measure the concentration of carbon dioxide (CO2), sulphur dioxide (SO2) and hydrogen sulphide (H2S).
Gas emitted by volcanic systems can also seep up through the soil in the area surrounding a volcano. These soil gas emissions are measured with a soil gas flux meter. A round metal accumulation chamber is placed on the ground and the gas emitted from the soil is measured in the back-pack analyser. The soil gases we measure are carbon dioxide (CO2) and hydrogen sulphide (H2S).
Soil gas flux measurements can be used to track changes in the volcano. Regular measurements are made at White Island. Measurements of soil gas flux have also been used at Raoul Island, Ngauruhoe and Tongariro in response to eruptions or signs of volcanic unrest.
Soil gas flux measurements are also used in geothermal areas such as Rotorua to map the geothermal area and determine the risks to public health posed by gases.
Sulphur dioxide (SO2) emissions at White Island are also measured by a permanent MiniDOAS installation (Miniature Differential Optical Absorption Spectrometer). The MiniDOAS instrument uses a UV spectrometer to measure the absorption of ultraviolet light by SO2 gas in the atmosphere. At White Island, two MiniDOAS instruments work together to profile the plume.
When volcanic craters cool down after major eruptions, they often fill with water to form crater lakes. Some are cool, just filled by rain water, while others are warm or hot and remain connected to the volcanic plumbing. The colour of crater lakes varies markedly according to the temperature and chemistry of the water, and the type and concentration of particles suspended within it.
Crater lakes occupy volcanic craters and any reactivation or change in status of an active volcano is often reflected in the lake. This may be a chemical change, a temperature increase or water level variation. Many factors can influence a crater lake.
Crater lakes come in many forms and sizes, ranging from a few tens of metres across like Frying Pan and Inferno Craters at Waimangu, to the larger lakes in craters like at Ruapehu, Raoul Island and Tongariro, to the very large caldera lakes like Rotorua and Taupo.
The water level of Green Lake (at Raoul Island) rose more than six metres before the 1964 eruption started. In 1993 it rose 1.2m following an earthquake swarm, but no eruption followed. However in 2006 it didn’t change before the March eruption.
At Ruapehu we monitor the lake temperature, water level and overflow along with the chemistry. The lake temperature varies from about 10 to 50°C; unfortunately this is not always a measure of activity as eruptions have occurred from both hot and cold lakes. The Mg (magnesium) values increase if new/fresh rock is in contact with the crater lake waters, while the Cl (chloride) changes with steam input.
In February 2003 a large pool of water started to collect on the floor of the 1978/90 Crater Complex, drowning the vent that was active during the July 2000 eruptions. This was the start of a complex sequence of the crater basin being flooded. In the first episode, from 2003 to 2005, the lake filled to within one metre of overtopping, before falling over 25m during 2006 and 2007. In the later stages of 2007 the lake started to refill and by mid 2008 was about 8m below overflow level, being sustained around there until 2011. In 2011 the lake again started to evaporate away.
When full, the lake had an area of 74,850m2, whereas when low it only covers 24,800m2. When the lake reaches to near overflow level the volume of the water in the crater is about 1.8 million m3. The primary source of water filling the lake is condensing steam and gas from fumaroles now beneath the lake and runoff from the surrounding crater walls.
The establishment of a semi-permanent Crater Lake at White Island has changed the range and likely impacts of the hazards to visitors on the island.
Within the Okataina Volcanic Centre, heat flow from the large crater lakes at Waimangu has been monitored. Inferno Crater lake shows large water level variations (10 m) while the overflow of Frying Pan lake varies between 120 and 100 litres/sec.
Seismic monitoring is the most widely used of the 'holy trinity' of volcano monitoring methods – seismic, deformation and geochemistry. Worldwide, almost all monitored volcanoes have some kind of seismic monitoring system and it is usually the first technique applied when scientists begin to monitor a volcano.