GEYSER, a natural spring or fountain which discharges into the air, at more or less regular intervals of time, a column of heated water and steam; it may consequently be regarded as an intermittent hot spring. The word is the Icelandic geysir, gusher or rager, but in native usage it is the proper name of the Great Geyser—the general term hoer, a hot spring, making the nearest approach to the more general sense of the word. Geysers exist in many volcanic regions, as in the Malay Archipelago, Japan and South America; but the three localities where they attain their highest development are Iceland, New Zealand and the Yellow stone Park, U.S.A. The very name by which we call them indicates the historical priority of the Iceland group.
The Iceland geysers are situated about 3om. N.W. of Hekla, in a broad valley at the foot of a range of hills from 30o to 400f t. in height. Within a circuit of about am., upwards of one hundred hot springs may be counted, varying greatly both in character and dimensions. The Great Geyser in its calm periods appears as a circular pool about 6oft. in diameter and 4ft. in depth, occupy ing a basin on the summit of a mound of siliceous sinter ; and in the centre of the basin is a shaft, about loft. in diameter and loft. in depth, lined with the same siliceous material. The clear sea green water flows over the eastern rim of the basin in little runnels. On the surface it has a temperature of from 76° to 89° C, or from 168° to 188° F. Within the shaft there is a continual shifting both of the average temperature of the column and of the relative temperatures of the several strata. The results of the observations of Bunsen and A. L. O. Descloizeaux in 1847 showed that the temperature of the column diminishes from the bottom upwards; that, leaving out of view small irregularities, the temperature in all parts of the column is found to be steadily on the increase in proportion to the time that has elapsed since the previous eruption ; that even a few minutes before the great eruption the temperature at no point of the water column reached the boiling point corresponding to the atmospheric pressure at that part ; and finally, that the temperature about half-way up the shaft made the nearest approach to the appropriate boiling point, and that this approach was closer in proportion as an eruption was at hand. The Great Geyser has varied very much in the nature and frequency of its eruptions since it began to be observed. In 1809 and 181o, according to Sir W. J. Hooker and Sir George S. Mac kenzie, its columns were 1 oo or 9oft. high, and rose at intervals of 3o hours, while, according to Henderson, in 1815 the intervals were 6 hours and the altitude from 8o to i5oft.
About ioo paces from the Great Geyser is the Strokkr or churn, which was first described by Stanlay in 1789. The shaft in this case is about 44ft. deep, and, instead of being cylindrical, is funnel-shaped, having a width of about 8ft. at the mouth, but contracting to about loin. near the centre. By casting stones or turf into the shaft so as to plug the narrow neck, eruptions can be accelerated, and they often exceed in magnitude those of the Great Geyser itself. During quiescence the column of water fills only the lower part of the shaft, its surface usually lying from 9 to 12ft. below the level of the soil. Unlike that of the Great Geyser, it is always in ebullition, and its temperature is subject to com paratively slight differences.
The great geyser district of New Zealand is situated in the south of the province of Auckland in or near the upper basin of the Waikato river, to the north-east of Lake Taupo. The scene pre sented in various parts of the districts is far more striking and beautiful than anything of the same kind to be found in Iceland, but this is due not so much to the grandeur of the geysers proper as to the bewildering profusion of boiling springs, steam-jets and mud-volcanoes, and to the fantastic effects produced on the rocks by the siliceous deposits and by the action of the boiling water. In about 188o the geysers were no longer active, and this condition prevailed until the Tarawera eruption of 1886, when seven gigantic geysers came into existence ; water, steam, mud and stones were discharged to a height of 600 to 800ft. for a period of about four hours, when quieter conditions set in. Waikite near Lake Rotorua throws a column to a height of 3o or 35ft.
In the Yellowstone National Park, in the north-west corner of Wyoming, the various phenomena of the geysers can be observed on a large scale. The geysers proper are about one hundred in number; the non-eruptive hot springs are much more numerous, there being more than 3000. The dimensions and activity of several of the geysers render those of Iceland and New Zealand almost insignificant in comparison. The principal groups are situ ated along the course of that tributary of the Upper Madison which bears the name of Fire Hole River. Many of the individ ual geysers have very distinctive characteristics in the form and colour of the mound, in the style of the eruption and in the shape of the column. The lifts the main column to a of only 5o or 6oft., but shoots a thin spire to no less than 25oft.
The "Castle" varies in height from io or 15 to 25oft. ; and on the occasion of greatest effort the noise is appalling, and shakes the ground like an earthquake. "Old Faithful" owes its name to the regularity of its action. Its erup tions, which raise the water to a height of ioo or i5oft., last for about five minutes, and recur every hour or thereabouts. The "Beehive" sometimes attains a height of 219ft., and the water, in stead of falling back into the basin, is dissipated in spray.
The eruptive action of geysers can be explained as follows: far down the pipe the water is at a temperature much above its boil ing point in the open air, owing to the pressure of the column of water above it, and its temperature is constantly increasing owing to volcanic heat below. The vapour pressure of the steam also increases, till eventually a point is reached when it exceeds the pressure due to the water above. The water at this point passes suddenly into steam, which by expanding raises the column of water above, causing an overflow at the surface. The consequent diminution of pressure allows more water to flash into steam, which blows out the whole column of water and causes the eruption.