An erupting volcano could be said to be one of the more terrifying manifestations of nature in the raw. Together with the more extreme meteorological phenomena, it was looked upon in Biblical times as an instrument of the Almighty, used as required for disciplinary purposes; the author of Psalms 11, who was privy to such matters, tells us that "Upon the ungodly He shall rain snares, fire and brimstone, storm and tempest, and this shall be the potion whereof they shall drink". Nowadays, however, scientists like to be able to anticipate such vengeful whims. How do they do it for volcanoes?
Volcanic eruptions occur when molten subterranean rock, called (I) magma (EI), surges upwards through weak points on the earth's surface. One of the most obvious signs that a major event may be about to happen is the development of minor volcanic activity where there was none before. But it is often difficult to tell if a volcano is merely "clearing its throat", as it were, or building up for catastrophe. More subtle methods are needed to obtain a clearer picture.
One common technique is to observe closely the physical shape of the volcano: the subterranean movement of the magma often causes a volcano to "swell" slightly, so a subtle change in shape may be the precursor of a major event. Other clues can be deduced from seismic activity in the vicinity: the rising magma results in seismic groanings and creakings deep beneath the earth's surface, and the final warning of a major eruption is often a chorus of rumblings more insistent than any heard before - a seismic drum-roll known as a "volcanic tremor". And a third methodology involves the use of infrared radiometers on satellites to detect threatening hot spots in the vicinity of a volcano.
Finally, another technique developed in recent years involves monitoring the concentration of certain gases in the atmosphere in the vicinity of the feared eruption. Magma far beneath the surface of the earth has dissolved in it certain substances such as carbon dioxide and sulphur dioxide. At great depths, the huge pressure keeps these substances "in solution", but as the molten rock moves upwards, the reducing pressure liberates the gases, allowing them to escape and reach the surface first: the process is similar to the way in which CO 2 is released in the form of bubbles from a fizzy drink with the bottle cap removed. In the weeks before an eruption, therefore, changes in the concentrations of these gases in the local atmosphere may give a warning of the worst to come.