The descriptions which have been given of the chief geyser regions of the world lead to the question. What is the source and character of the geyser waters? It has been plainly indicated that in the fields described the vents are always situated along lines of drainage, on the shores of lakes, or under conditions where ordinary springs of meteoric water would naturally occur.
That the geyser waters are surface waters which have percolated through the porous lavas and have been heated by encountering great quantities of steam and gases rising from the hot rocks below there is no reasonable doubt. The proximity of ordinary cold springs and those of boiling hot water lends support to this view.
These hot waters, traversing the rocks in irregular fissures, readily dissolve out the more soluble constituents of the rocks, the amount and the character of the salts present varying somewhat with the nature and amount of gases held in the waters. Chemical analyses of geyser waters from the three regions described show no greater variation than those from different vents in any one of these regions.
Source of heat.That the source of steam is the still hot lavas below, and is in some way connected with volcanic action, is so evident from the facts that no other conclusion is possible. A very common belief concerning the source of the heat of boiling springs and geysers, but one which no longer has the support of scientific men, is that the heat results from chemical action, as it is vaguely termed. Were not the evidence so directly opposed to this idea, it would merit consideration, but so far as the heat of geyser waters is concerned all observation shows it to be untenable. To this class of theories belongs the popular idea that the geyser basins are underlain by great beds of (quick?) lime, which supply the heat and steam of the geysers.
The smothered combustion of beds of lignite, coal, or pyrites is another form of the same theory that has been received with considerable favor and still commands a few followers. That hot springs may have such an origin is not denied, but the geological conditions and environment clearly show that none of the great geyser regions of the world derive their heat from such action.
Where the source of supply is deep-seated, spring waters always have an elevated temperature, generally proportionate to the depth, but the very high temperatures of the geysers and the local source of the waters exclude this theory. The folding and faulting of rocks is another source of heat made manifest by hot springs.
It has been shown by Dr. Peale, however, that boiling waters are only found in the regions of volcanic rocks, and it was pointed out by L'Apparent that geysers only occur in acid volcanic lavas. In Iceland the volcanic forces are still active, and melted lavas may exist at no great depth. In New Zealand the recent eruption of the eroded mountain Tarawera showed that heated rocks exist and in that case rose up near enough to the surface to cause the explosion which so transformed the country.
In the Yellowstone there are no active volcanoes, and none of even geologically recent activity. The lavas that fill the ancient mountain-encircled basin of the park are scored by glaciers and deeply cut by running water, and the old volcanoes from which the lavas were, in part at least, outpoured show no signs of having been active since Tertiary times. Yet in this region the expenditure of heat by the hot springs, geysers, and steam vents would undoubtedly keep a moderate-sized volcano in a very active state were it concentrated. There is no doubt that this heat is connected with the past volcanic energies of the region and derived principally from the still hot lavas, three-quarters of the entire area of the park (3,500 square miles) being covered by rhyolitic rocks.
The significance alluded to above, of the association of geysers and acid lavas (rhyolites), is possibly to be found in the fact that these rocks are more easily dissolved by the hot waters forming the tubes and reservoirs for geysers. The situation of hot springs and geysers along water-courses has already been mentioned. It is a well-known fact that the presence of water in the pores of a rock increases its capacity to conduct heat, so that we may surmise a rise in the local isogeotherm in such situations.
Geyser eruptions.Geysers have often been compared to volcanoes, presenting in miniature, with water instead of molten rock, all the phenomena of a volcanic eruption. The diversity of form and varying conditions of activity of the hot springs found associated with geysers makes it impossible to determine in every case whether a spring is or is not a geyser. Geyser vents may be mere rifts in the naked rocks or bowls of clear and tranquil water, quiet until disturbed by the first throes of an eruption, and surrounded by white sinter deposits in nowise distinguishable from those about hot springs. In other cases the vents are surrounded by a cone or mound of pearly beaded "geyserite," a certain and distinctive feature of a geyser.
The displays of the great "Geyser" of Iceland have already been briefly described; they may be taken as the type of eruptions from geysers having bowl-like expansions at the top of the tube, the so-called "basin" of the geyser. Where the vent is surrounded by a cone of sinter, as is so often the case among the fountains of New Zealand and the Yellowstone, the first part of the geyser eruption is somewhat different. Perhaps the most familiar geyser of this type is Old Faithful, the one geyser in the Yellowstone that is sure not to disappoint the visitor. Though surpassed by many of its neighbors in the height and magnitude of its eruptions, it holds a front rank for beauty and gracefulness. Previously heralded by loud rumblings, with spasmodic outbursts of 10 to 20 feet in height that mark abortive attempts to send up its steaming pillar, the white column is finally thrown upward with a loud roar, and mounts at once to a height that seems hundreds of feet as we gaze upon it. For one or even two minutes the column maintains a height which measurements show to vary from 90 feet up to 150 feet, with occasional steeple-shaped jets rising still higher, the jets ever varying and giving off great rolling clouds of steam; then the jets gradually decrease in altitude, and in five minutes the eruption is over, the tube apparently empty, and emitting occasional puffs of steam for a few minutes longer.
During the eruption the water falls in heavy masses about the vent, filling the basins that adorn the mound, and flowing off in yellow and orange-colored waterways, while the finer spray drifts off with the breeze and falls upon the neighboring sinter slopes. It is impossible to measure the amount of water thrown out, since it runs off in a number of directions in shallow rills that lead either to the sandy terrace near by or to the river. If, however, we assume that the column of steam and water is one-third water, a fair assumption, the estimated discharge is 200,000 gallons at each eruption.
Comparing Old Faithful with its Iceland prototype we find considerable difference in the behavior of the two vents during the interval between eruptions. The former, like Strokr, has no bowl or basin, and the geyser throat or tube is partly filled with water, which is in constant and energetic ebullition while the geyser is inactive. The tube and bowl of "Geyser" are, on the contrary, filled with comparatively cool water. In each case, however, the eruption is preceded by an overflow from the geyser tube, in the case of Strokr and Old Faithful as jets of 10 feet to 25 feet in height; in "Geyser" by a filling of the bowl and successive overflows, accompanied by the noise of condensing steam bubbles, a simmering of the water in the tube. Such preliminary actions are significant when we consider the theory of geyser action.
It is unnecessary to describe the numerous other theories of geyser action; they all suppose caverns or systems of chambers and tubes, of definite arrangement, a supposition most unlikely to occur in many cases, and made unnecessary by Bunsen's theory. Local expansions and irregularities of the tube do exist, and to them we owe many of the individual peculiarities of geysers, but such chambers do not form a vital, essential part of the geyser mechanism.
In an excellent résumé of the various theories of geyser action, Dr. A. C. Peale states that he believes no one theory is adequate to explain all the phenomena of geyser action, though Bunsen's theory comes nearest to it.1
Where the tube is surrounded at the top by a basin no actual overflow need occur. Indeed there is in the Yellowstone a miniature geyser, aptly named the Model, with a tube but 2 inches in diameter, surrounded by a shallow, saucerlike basin, which has eruptions about every 15 minutes of 3 feet to 5 feet in height in which scarcely a drop of water is wasted, but flows back into the tube after the eruption. During the interval between eruptions no water can be seen in the tube, whose basin and upper part are dry and cool. The first signal of the coming display is a quiet welling up of the water in the tube filling the little basin, which being relatively large and shallow relieves the water column of a considerable height. During the eruption which follows, the spray is chilled by the air, falling back into the basin; at the end of the display the water is quickly sucked back into the tube and reheated for the ensuing eruption.
At first thought the constant boiling of the waters in the tube of Strokr, Old Faithful, and many other geysers seems to oppose the theory which we have just given. Observations show however that in many cases the boiling is confined to the surface and deep temperatures do not reach the boiling point corresponding to the depth. It is quite likely also that in some cases a lesser and independent supply of heat may connect with the upper part of a geyser tube; Strokr, we know, has two vents (see figure), one of which is the geyser tube, the funnellike throat of Strokr being really but a nozzle to the geyser.
Theories of geyser action.The intermittent spouting of geysers was long a riddle to scientific men, for although several theories seemed each to offer a satisfactory explanation of the eruptions of "Geyser," they supposed conditions unlikely to occur in many vents. The investigations of Bunsen, and of Descloizeaux, who spent two weeks studying the Iceland fountains, resulted in the announcement of a theory of geyser action which, with slight modifications, has satisfied all requirements and is to-day generally accepted as the true explanation of the action of these natural steam engines. This theory, which bears the name of the illustrious Bunsen, depends upon the well-known fact that the boiling point of water rises with the pressure, and is therefore higher at the bottom of a tube of water than at the surface. The temperature of water heated in any vessel is generally equalized by convective currents, but in a long and narrow or an irregular tube this circulation is impeded, and while the water at the surface boils at 100° C. (at sea level), ebullition in the lower part of the tube is only possible at a much higher temperature, owing to the weight of the water column above it. In the section of Geyser shown in the figure the observed temperatures are given on the left, and the temperatures at which the waters would boil, taking into account the pressure of the water column, are given on the right. In Geyser the nearest approach to the boiling point is at a depth of 45 feet opposite a ledge and fissure discovered subsequent to Bunsen's experiments. At this depth the temperature is 2° C. below the temperature at which the water can boil. If by the continued heating of this layer by steam from the fissure it attains the temperature at which it can boil, steam is formed, whose expansive force lifts the superincumbent column of water, causing a slight overflow at the top, which, shortening the column, brings the layer B to the position C, where its temperature is above the boiling point of C, wherefore steam is formed at this point and a further lifting and relief of pressure ensues, followed by an eruption.
In illustration of this theory a model geyser is easily constructed of a glass tube an inch or so in diameter and several feet long. When this tube is closed at one end, filled with water and placed upright we have all the mechanism necessary to produce all the phenomena of a geyser. By heating the water at the bottom by the introduction of steam (or with a spirit lamp), we can produce eruptions whose period will depend upon the intensity of the heat. At first the bubbles of steam collapse in the cool waters at the bottom of the tube, but as the temperature rises the bubbles rise part way up the tube and heat the lower part of the column to a high temperature while the water near the surface is still cool. Eventually the water at the bottom reaches the pressure boiling point, when steam is formed, lifting the water above it and causing an overflow at the top. This overflow or its equivalent, the filling of a shallow basin at the top of the tube relieves the pressure and all that part of the column whose temperature was previously below the boiling point but now exceeds it, flies into steam and ejects the water above with great violence. The glass walls of our geyser tube permit us to watch the gradual heating of the water by means of thermometers suspended in the tube, the ascent and collapse of steam bubbles, the overflow and abortive attempts to erupt and the final ejection of the water from the tube.
I believe, however, that Bunsen's theory is a perfect explanation if we but admit that the geyser tube may be neither straight nor regular, but of any shape or size, and probably differing very much for each vent. The shape of the bowl or basin exercises but little influence upon the eruption save to produce the many individual peculiarities of the geyser column.
Origin of geysers.It should be noted that Bunsen's theory of geyser action is quite independent of his theory of geyser formation. The building up of a siliceous tube by the evaporation of the waters at the margin of a hot spring is a process which may be seen in operation in any of the geyser regions of the world; but it is not a necessary prelude to the formation of a geyser, for a simple fissure in the rock answers equally well, as is shown at the Norris geyser basin in the Yellowstone Park.
The life history of a geyser varies, of course, for each one, but observations show that the following sequence of events often takes place. The hot vapors rising from unknown depths penetrate the rocks along planes of fracture and shrinkage cracks, decomposing and softening the rock until the pressure of the steam and water is sufficient to force an opening to the surface. If this opening affords an easier exit for waters issuing at a higher level, the fissure is probably opened with a violent ejection of mud and debris; more often the process is a gradual one, accompanying the slow eating away of the rock walls along the fissure. The flowing waters slowly clear out the fissure, forming a tube that permits the freer escape of hot water and steam, while at the same time the waters change from a thick mud to a more or less clear fluid. The spring, at first a simple boiling mudhole, is now an intermittently boiling spring, which soon develops true geyser action. If the opening of the fissure afforded a new outlet for the waters of some already existing geyser, these changes take place rapidly, and eruptions begin as soon as the pipe is sufficiently cleared to hold enough water. The bare rock about the vent or fissure is soon whitened by silica deposited by the hot waters. This sinter may form a mound about the expanded tube or basin, or, if the vent shall be small and spray is frequently ejected, it builds up the curious geyser cones so prominent in the Yellowstone. In certain cases the building up of these deposits may partially choke the geyser's throat and cause a diminution of the geyser's energy, whose forces seek an easier outlet. In other cases the eating out of new subterranean waterways deprives the geyser of its supply of heat, and the vent becomes either tranquil or wholly extinct, while the pearly geyserite forming its cone disintegrates and crumbles into fine shaly debris, resembling comminuted oyster shells. Thus there is a slow but continual change in progress at the geyser basins, in which old springs become extinct and new ones come into being and activity.
With few exceptions, where the vents are very new, geysers spout from basins or from cones of white siliceous sinter, or geyserite, deposited about the vent by the hot waters. Such deposits are formed very slowly, one-twentieth of an inch a year being an average rate of growth for the deposit formed by evaporation alone. These deposits of sinter are therefore an index to the age of the geyser. In many cases these sinter cones are very odd, fantastic structures of great beauty while wet by the geyser spray, but becoming white, opaque, and chalk-like upon drying. Where the spattered drops fall in a fine spray the deposit is pearly and the surface very finely spicular. If the spray be coarse, the rods are stouter and capped by pearly heads of lustrous brilliancy. Thus the cone is not only a measure of a geyser's age and activity, but it tells, in a way, the nature of the eruption.
Last Updated: 02-Apr-2007