Because volcanoes can devastate enormous areas, including people, good prediction of eruptions and their effects is very important. What are believed to be the most dangerous volcanoes can be continually monitored
by accurate measurements. Many surveillance methods are expensive and require skilled operators with sophisticated instruments in observatories. However, many dangerous volcanoes are situated in developing countries which
simply haven't got enough money for careful observance of their volcanoes.
The most important methods and instruments used to predict an eruption are: seismographic monitoring, tiltmeters and surveillance by satellite. These methods are often used in conjuction with each other. Gas and steam emissions are also known to herald a volcanic eruption. Once a volcano has been studied hazard assessment maps may be produced.
(Fig. 2.50) Seismograph in action at Pinatubo response.
Rising magma causes moderate earth tremors, when the magma is approaching the surface the frequency and intensity of the tremors will increase. These shallow volcanic earthquakes are perhaps the most reliable sign a volcano is about to erupt. They can be detected by automatic telemeter recorders, relayed for immediate interpretation to an observatory. Hawaii for example has 51 such seismometers.
Tiltmeters are very accurate levels composed of three graduated pots arranged in a triangle and
filled with water or mercury.
Rising magma within a volcano often causes ground deformation. The ground deformation is measured by tiltmeters.
Tilting is for instance measured when rising magma makes a bulge. This process takes place slowly as the eruption looms. One of the most spectacular instances of bulging occured at Mount St Helens in the 2 months before the famous 1980 eruption.
Surveillance by Satellite
Surveillance by satellite is costly and still in its infancy. Nevertheless it
offers some of the best future prospects in forecasting volcanic eruptions. The Global Positioning System (GPS) for example is used to monitor ground displacements, including those around volcanoes, which might pinpoint future activity. GPS uses data transmitted by orbiting satellites.
(Fig. 2.51) Global Positioning System (GPS) receiver at Cotopaxi Volcano, Ecuador.
Credit: J.W. Ewert, USGS
The TOMS satellite produced an image of the Pinatubo volcanic cloud (emitted during the eruption in 1991) over a 9 day period, showing the regional dispersal of the Sulphur Dioxide plume.
Gas and Steam Emission
(Fig. 2.52) Protected by a gas-mask this scientist researches the gas emitted from a fumarole.
Credit: Bea Ritchie,USGS
Increased emissions of gas and steam from fumaroles, mudpots and solfataras often show that the magma is rising closer to the surface. Such information
is hard to collect because the emissions damage instruments and are noxious to their observers.
Greater fumarole activity gave warning of the Askja eruption of 1961 in Iceland. Gas emissions in 1902 from Mount Pelée did also indicate the volcano was about to burst into life. However, increased fumarole activity on Mount Baker (Washington State)in 1975 raised fears of an eruption that has not materialized.
Hazard Assessment Maps
On hazard assessment maps the areas of greatest danger and highest risk around a volcano are defined. It is very important the danger zones have been pointed out very precisely and correctly so the people who are living in those areas or danger zones can be evacuated.