THERE ARE, on average, about 60 volcanic eruptions each year. They do not occur just anywhere on the earth's surface - although at some time in the past, everywhere on the earth's surface has experienced them. Nowadays more than two-thirds of them take place around the rim of the Pacific Ocean, with such a frequency that vulcanologists have dubbed this zone "the ring of fire". The most recent such event, the eruption of Mount Ruapehu on the North Island of New Zealand, has been the most spectacular for quite a while, and featured in a splendid satellite photograph in The Irish Times last Wednesday.
The centre of the earth, as we know, is very hot indeed, and even relatively close to the surface there can be found vast reservoirs of molten rock - called magma. When rock melts, it expands and needs more space: if the pressure in a magma chamber becomes greater than the strength of the roof above, the molten rock bursts forth at the weakest point, causing what we know as an eruption. The most vulnerable spots are often located along the joints between the vast tectonic plates which make up the earth's crust, which explains the existence of the ring of fire.
The violence of a volcanic eruption often depends on the viscosity of the magma deep below. If the molten rock is thin and runny, like basalt, the gases formed within it bubble freely, and escape easily upwards without explosive consequence - which seems to have happened at Mount Ruapehu. But if the melted rock is highly viscous, almost like toffee, as is she case with molten granite, then the bubbles of gas have great difficulty in making their escape. Then very high pressures build up, and culminate in a violent, earth-shattering explosion.
When an eruption occurs, lava and heavy air-borne material transform the local landscape in a very obvious way. More subtly, lighter particles and gases may be projected high into the stratosphere to form a veil around the world that may affect the global climate for a year or two - as was the case, for example, with Mount Pinatubo in the Philippines in 1991.
In between these two extremes, however, great clouds of dust and ash are injected into the atmosphere at heights that correspond to the cruising altitudes of aircraft. Such debris, ingested into the air-intake of a jet aeroplane, may cause multiple engine failure with the attendant possibility of catastrophic accident. Nothing much can be done to remove this hazard: nature must take its course and all pilots can do is avoid the affected area.