Nor are they isolated events. "At any time there are about 15 to 25 active volcanoes somewhere in the world, and on average 50 different volcanoes erupting every year," said Professor Stephen Sparks of the geology department at the University of Bristol.
Volcanoes are caused when pressures in molten magma below the Earth's crust break through weaknesses in the solid surface. Occasionally, the eruption of an undersea volcano can cause a tsunami which can flood coastal areas.
Volcanoes can be created where solid crustal plates, floating on a magma layer in the lower crust and upper mantle, collide and one is forced above the other; the lower plate eventually starts to melt and its material rises up to erupt through the upper plate.
The Volcanic Explosivity Index (VEI), developed in 1982, gives eruptions a value between 0 and 8, but is not wholly factual: it takes into account a general description (non-explosive to very large), cloud column height, qualitative description (gentle to cataclysmic), and eruption type.
The biggest eruption in recorded history occurred in 1815, when the Tambora volcano in Indonesia exploded (at an estimated VEI of 7), producing 40 cubic kilometres of ash. The eruption killed 10,000 people, and another 80,000 died from crop loss and famine Eruptions can also have dramatic effects on world climate. When Mt Krakatoa, also in Indonesia, exploded in 1883, the following years were rainy and cold across Europe. Volcanoes can also effect the levels of ozone in the upper atmosphere.
The Montserrat volcano has been comparatively small, and the gasses and ash emitted are not particularly acidic; thus its effects on world weather will be minimal. Popocatepetl may be a different matter. "It hasn't gone on long enough yet, but it is sulphur-rich, which means it will produce acidic particles," said Professor Spark. "And it has pushed out ash to a fair altitude. But I suspect there's not yet enough expelled to cause global effects."
Despite the disruption and loss of life they cause, volcanoes can have benefits. Undersea ones can create entirely new land; and those upon the land, once their flows have cooled, can offer very rich soils for farming.