Felix, declared Virgil, qui potuit rerum cognoscere causas: "Happy is he who has been able to understand the causes of things." And, goodness knows, it is not always easy. It has long been known, for example, that major volcanic eruptions, by injecting dust and other forms of pollution into the upper atmosphere, can block out enough sunlight to cool the world by half a degree Celsius or more - and thereby trigger a temporary change in global climate. Now it is being suggested that the reverse may also be the case: that a change in climate may be a trigger for volcanoes. Let us try, as Polonius so succinctly put it, to:
Find out the cause of this effect
Or rather say, the cause of this defect
For this effect defective comes by cause.
The possible link was discovered when studying the layers of polar ice in Greenland, which are vast reservoirs of information about past conditions on our planet.
The snow falling on these frozen wastes contains impurities, and these provide a clear signature of the state of the atmosphere at the time. As the snow is transformed into ice, layer by layer corresponding to successive seasons, an almost permanent record is laid down of the climatic history of the planet.
To access this information, scientists drill a very deep hole into the Arctic ice-sheet, and then extract, in manageable segments, a narrow cylinder of ice a mile or more in length. From this long frozen pencil the climate of the region back to prehistoric times can be deduced by chemical analysis. One such project paid particular attention to substances of volcanic origin, in order to obtain a picture of rates of volcanic activity in past millennia. As expected, the researchers found that during the periods of global cooling that precede an ice age, volcanoes tended to be much more common than at other times. But to their surprise, they found that the number of eruptions during periods of climatic warming was also very high. The tentative explanation being put forward is that the increased volcanic activity is due to reduced pressure on the Earth's surface as the ice retreats.
During a period of glaciation, the Earth's crust is pushed down by about one third the thickness of the ice that has accumulated; when the ice melts, the crust rebounds, reducing the pressure on the magma chambers deep below the surface that provide the fuel for volcanoes. The magma chambers, the researchers say, would then erupt violently "just like an uncorked champagne bottle" - producing for a time a surfeit of volcanoes.