Icelandic volcanoes are unusual in that they occur on a divergent plate boundary; that is to say a boundary where tectonic plates are moving apart, with new crust material being created. Volcanoes are not unusual on divergent plate boundaries, a series of them run the entire length of the Atlantic, but they are only found in appreciable numbers above the ocean in two places, in Iceland and in East/Central Africa. The Central volcanoes occur where Africa is slowly breaking apart along the Great Rift Valley. Arabia, formerly part of Africa, has already broken away and is separated from the rest of the continent by the Red Sea, a 'baby' ocean with a row of volcanoes along its sea floor. Running south from this volcanoes occur in a broad belt running through Ethiopia, Kenya, Uganda, Rwanda, Burundi, The Congo and Tanzania. The Icelandic volcanoes are unique in that they occur on a divergent margin where two continents (Europe and North America) have long since split apart. Iceland is on a mid-ocean ridge, surrounded by oceanic crust on all sides, but so much volcanic material has been ejected that a major landmass has been created.
Most volcanoes are associated with convergent plate margins, where one tectonic plate is forced beneath another. Where this happens the subducting plate is forced down into the warmer mantle where it melts, and lighter minerals bubble up through the overlying plate. This is the source of volcanoes around the Pacific Rim, in North and South America, the Kamtchatka Peninsula in Russia, Japan, the Philippines, Indonesia, New Guinea and New Zealand; in Southern Italy, where North Africa is being subducted beneath Europe; in the Caribbean and the South Sandwich Islands.
Other volcanoes occur on 'Hot Spots' places where plumes of magma well up from deep within the Earth. These Hot Spots remain more-or-less fixed upon the surface of the Earth for millions of years as the plates roll over them, creating chains of volcanoes. These are the source of volcanoes in the Hawaiian Island volcanoes, the Canaries, the Galapagos Islands and volcanoes in Cameroon in Africa.
In spring 2010 air-traffic across Europe was halted by ash clouds from the Eyafyallajökull eruption. Volcanic ash is extremely hazardous to aircraft in a number of ways. At its most obvious it is opaque, both visually and to radar. Then it is abrasive, ash particles physically scour aircraft, damaging components and frosting windows. However the ash is most dangerous when it is sucked into jet engines, here the high temperatures can melt the tiny silica particles, forming volcanic glass which then clogs engine. When this happens the only hope the aircraft has is to dive sharply, in the hope that cold air passing through the engine during the descent will cause the glass to shatter, allowing the engine to be restarted. Obviously this is a procedure that pilots try to avoid having to perform.
In 2010 a number of aircraft operators objected to the length of time for which airports were closed, citing heavy financial losses and a belief that planes would have been able to fly safely through ash clouds of the density covering much of Europe. Many European governments were receptive to this idea, and were considering relaxing safety regulations. Pilots unions were rather less keen on resuming flights prematurely. In the event the ash clouds were short lived, and lifted before regulations could be changed.
In 2011 the UK new government, which has a track record of describing health and safety legislation as unnecessary red tape, has already stated a willingness to try to keep planes flying during any protracted ash coverage, hoping to track less dense areas in the clouds and steer planes through them. It is possible that other European nations may follow suit.
In addition to the danger to aviation volcanoes can present a number of other hazards. Lava flows, caldera explosions and dense ash-falls are, obviously, rather localized events, and the Icelandic government is experienced in dealing with these. There is no realistic danger of any such event at Grímsvötn affecting any other nation. Volcanoes have also been known to produce considerable amounts of toxic gasses, though again it is unlikely that these could reach beyond Iceland.
More worrying are the potential for toxins in the ash clouds to be distributed widely, and the possibility of climatic disturbance.
In 1970 the Hekla Volcano in southern Iceland deposited toxic hydrogen fluoride rich ash across thousands of acres of farmland, destroying crops and poisoning livestock, though again it is unclear if this could reach beyond Iceland.
More alarming is the potential for climatic disturbance. In 1783 the Laki Fissure eruption, part of the same volcanic complex as Grímsvötn, spewed out in excess of a hundred million tonnes of sulphuric acid in aerosols, causing freak weather conditions across the whole Northern Hemisphere, and widespread crop-failures and famine in Europe, an event some historians have cited as a cause of the French Revolution (though poor behavior on the part of the French Royalty and Aristocracy clearly played a role).
In the modern world the infrastructure is present to prevent crop failures turning into widespread famine in Europe, but it is in theory also there to prevent famines in Africa and India, yet these still occur. Current political thinking allows for speculation on food markets, which can drive up prices on short commodities, making food shortages much worse (something the World Development Movement is currently campaigning against quite vigorously). At the same time European many governments are cutting back on measures to help the less well off in their societies. Potentially this philosophy could turn a crop failure into a major crisis, though it is probable that faced with widespread famine, and the resultant upheaval would lead to a change of policy direction, the process of getting there would be unlikely to be a pleasant one.
See also The Puyehue-Cordón Caulle Volcanic Complex
The Puyehue Eruption, Chile, 2011. and
Volcanos on Sciency Thoughts YouTube