GEYSER

any of a class of hot springs that discharges jets of steam and water intermittently. The term is derived from the Icelandic word geysir, meaning to rush forth. Geysers are generally associated with recent volcanic activity and are produced by the heating of underground waters that have come into contact with, or very close to, magma (molten rock). The water is ultimately of surface origin, having percolated into the ground through cracks and fissures. Geysers often make a most spectacular display as they discharge a roaring column of steam and boiling water high above the surface. Discharges as high as 500 m (1,640 feet) have been recorded, but 50 m (164 feet) is much more common ( e.g., Old Faithful in Yellowstone Park, United States). Water at a high temperature is able to dissolve many of the chemical constituents of the rocks with which it comes into contact. In many cases the geyser waters are saturated with silica, which is precipitated in the immediate vicinity of the geyser discharge, commonly building a small mound around the geyser neck. The activity of geysers varies somewhat with time, and on geological timescales most of them have an extremely ephemeral existence of tens to hundreds of years. Within its life span a geyser can display markedly different kinds of behaviour. Some may discharge continuously, whereas others may erupt violently for a few minutes and then remain quiescent for a period of hours or days. Occasionally, a geyser will adopt an extremely regular and predictable pattern of intermittent activity and discharge for a few minutes every hour or so (e.g., Old Faithful). The physical explanation of geyser activity depends on a knowledge of the boiling behaviour of water. At the Earth's surface close to sea level, fresh water boils at about 100 C (212 F). If the pressure is increased, however, the temperature of boiling rises. It seems that the neck of a geyser can be considered a vertical pipe at least tens and probably several hundreds of metres long. When it is full of water the pressure at the bottom of the column of water is sufficiently high to prevent the water at that depth from boiling, even though its temperature may be higher than about 150 C (302 F). If the upper part of the water column is removed in some way, the effect is immediately felt as a pressure reduction at the base of the column. Boiling occurs the instant pressure is reduced; this process, known as flashing, is violent and explosive. It is believed that violent geyser-steam discharges are generated by the flashing of groundwater at some depth below the surface (100 m [328 feet] or more). The flashing process, however, demands that a release of pressure take place by removal of the upper part of the water column. The plumbing system associated with each geyser is different. In some cases the water in the upper part of the hole is simply warmed by the conduction of heat from below and begins to boil (at about 100 C). As it boils, the water spills out and thus slightly reduces the pressure beneath it, which in turn allows slightly deeper water to boil. In this way the boiling level propagates rapidly down the throat, changing in character from a passive boiling at the surface to a violent flashing in the lower parts. Geysers are nearly everywhere associated with various other, less violent manifestations of the interaction of magmatic heat and groundwatere.g., bubbling hot pools and fumaroles (steam vents). In many areas it has proved possible to tap and control the steam discharged from geysers and to use it in turbines for generating electricity (e.g., the geysers near San Francisco). The main drawback of such operations is that frequently the site on which a power station must be located to exploit this steam is vulnerable to damage either by volcanic activity or by earthquakes. Further, the highly corrosive nature of the impure steam that is discharged can cause severe maintenance problems in the surface plant. There may also be precipitation of dissolved material as the steam passes through surface pipework, which may fur-up the pipes. Nevertheless, in certain parts of the world this method of power generation is economically competitive with and, in some cases, more attractive than conventional fossil-fuel power plants.