Is volcanic eruption predictable?

Inhabitants of the eastern Congolese town of Goma have suffered three disasters in 8 years – the aftermath of the Rwanda massacres of 1994, the episodic war centred on control of Congo’s immense physical resources since 1995, and now the devastating eruption of the Nyiragongo volcano that threatens half a million people.  The last is a grim reminder of the difficulty in predicting geological disasters, and follows closely on claims that spotting impending volcanic eruptions is now “sorted” (Marshall, T. 2002.  There she blows.  New Scientist 12th January 2002, p. 29-31; Horizon, BBC2 17th January 2002, Volcano Hell).  There are four phenomena that have been investigated as signifying threats of  eruption.  Most obvious are increases in temperature at existing vents that can easily be measured using infrared images from daily orbits of meteorological and environmental satellites.  A remote sensing approach is so cheap that it ought to be applicable world-wide, yet most devastating eruptions emerge with insufficient  time following thermal signs for emergency evacuations to begin.  Fundamentally the clearest evidence that magma beneath a volcano is rising is that the edifice swells.  Interferometric radar can detect millimetre-scale changes in surface topography, and such pre-eruption inflation is detectable (see Interferometric radar and faults of the Mojave Desert in Earth Pages, December 2001).  However, the lengthy periods between overpasses by radar imaging satellites (two images are a minimum for radar interferometry), and the need for immensely powerful computer processing has rendered this approach one of retrospection rather than early warning.  Individual volcanoes’ ground motions, and the minute changes in their gravitational potential that also relate to magma movements can be monitored at permanent ground stations, but apart from a select few on which volcanologists conduct long-term research, some thousands of dangerous volcanoes go unwatched.

The central theme of both the Horizon programme and the New Scientist article was a method based on monitoring low-energy seismicity emanating from magmatic movements.  The observation of low-frequency, long-period seismicity  by US Geological Survey volcanologists while Mount St Helen’s was active in 1980 is probably connected to a natural resonance of each volcano as magma begins to move.  Follow-up work at a small number of volcanoes has fine tuned such signals to the timing of eruptions, with sufficient confidence levels that believable warnings are possible.  Believability is essential, for a mass evacuation followed by no threat to life could deter future responses by endangered people, on the “crying Wolf” principle.  Mexican volcanologists were able to give two day’s warning of the immense eruption of Popocatapetl on 18th December 2001, and evacuation prevented any loss of life.  However, none would have been threatened, as it happened, for the eruption on the vast massif was far from habitations.  Yet so spectacular were the fire fountains, that the exercise served to habituate locals to take such warnings very seriously indeed.

Nyiragongo volcano and its companions in the western African Rift regularly erupt low-viscosity lavas that flow quietly over long distances.  They pose less violent threat to life than explosive volcanoes, such as those around the Pacific rim, but chance may channel such flows through inhabited areas disrupting communications and destroying buildings.  Many of the 45 confirmed deaths in Goma arose when people tried to rescue belongings from their engulfed homes.  The current Goma disaster is not one primarily of volcanic origin, but of poverty, poor communications and fragile provision of basic necessities, such as unpolluted water and emergency food supplies.  After the 1994 humanitarian tragedy, and threats from Nyirangongo to the 800 thousand Rwandan refugees camped around Goma, the US Geological Survey and Japanese volcanologists set up seismometers to monitor the volcano’s internal activity.  Five days before the eruption, only two remained functional, yet transmitted signs of abnormal seismic activity (Clarke, T. 2002.  Seismic rumbling foretold Congo eruption.  Nature. v. 415, p, 353).  Despite that, warning did not get through to Goma in time for local people to flee, or any assistance to arrive. There was nowhere for the victims to go and relief followed only days and weeks after the event, when the damage was done.  The same fate hangs over millions of people living in volcanic areas in poor countries – they favour such risky areas to live because of the richness of soils and the encouragement of rainfall by high mountains..  As things stand, communities in volcanic areas of  North America, New Zealand, Japan and a few of the richer 3rd World countries stand a good chance of escaping magmatic events because of believable warnings and efficient communication.  For the majority, survival is a matter of luck alone.

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