CHAPTER III.

GEYSERS.

The hot springs of the Yellowstone National Park may be roughly divided into two classes, eruptive and non-eruptive. To the first the term _geyser_ is applied, while the term _hot springs_ is restricted to the second. These two classes pass into each other by insensible gradations and the line of demarcation it is not possible to draw. The following description will pertain only to those examples about which there is no doubt, and which may be taken as types of their class.

A geyser may be defined as a periodically eruptive hot spring. The name, as might be expected, is of Icelandic origin, and comes from the verb _geysa_, _to gush_. The general characteristics of a true geyser, as illustrated by the most perfect example known, Old Faithful in the Yellowstone Park, are the following:

(1.) There is an irregular tube descending from the earth's surface to some interior source of heat.

(2.) The mouth of this tube may be either a self-built mound or cone (as in the example), or simply an open pool.

(3.) Into this tube meteoric water finds its way and is subjected to the action of heat.

(4.) The result is an eruption and expulsion of the water from the tube with more or less violence.

(5.) The eruption is generally preceded by slight preliminary upheavals leading gradually to the final outburst.

(6.) After cessation of the eruption there is usually a considerable escape of steam.

(7.) A quiescent period, generally of indeterminate duration, follows during which the conditions necessary for an eruption are reproduced.

Geyser phenomena have attracted a great deal of scientific attention, and many theories have been advanced to explain them. Passing over for the present the various less important views, attention will first be given to Bunsen's theory, because it is, upon the whole, the most satisfactory explanation yet advanced. This theory was a direct deduction from observations upon the Great Geyser of Iceland, and has been experimentally illustrated by artificial examples.

The fundamental principle upon which it is based is the well known fact that the temperature of the boiling point of water varies with the pressure to which the water is subjected. At the sea level, under the pressure of one atmosphere (fifteen pounds to the square inch), the boiling point is about 212 degrees Fahrenheit. Under a pressure of two atmospheres it is 250 degrees; of three, 275 degrees; of four, 293 degrees, and so on. At an altitude like that of the Park plateau, where the atmospheric pressure is much less than at sea level, the normal boiling point is about 198 degrees, but the law of variation due to pressure conditions applies exactly as in lower altitudes.

If water, subjected to great pressure, be heated to a temperature considerably above that of its normal boiling point, and if then the pressure be suddenly relieved, it will almost instantaneously be converted into steam; a fact which always operates to enhance the danger from the explosion of steam boilers. Applying this principle to the case of an ordinary geyser, it will readily be seen that in the long irregular tube descending to great depths there are present the necessary conditions for subjecting the water to great pressure. At the surface the pressure is that of the weight of the atmosphere corresponding to the altitude; at a certain depth below (33 feet at the sea level, but less at higher altitudes) it is twice as great; at double this depth three times as great, and so on.

Suppose, now, that there is an interior heat at some point along the geyser tube well below the surface. The boiling point of water in the vicinity of the heat supply will be higher than at the surface in definite relation to its distance down. If the tube be of large diameter and the circulation quite free, the water will never reach this point, for it will rise nearer the top, where the boiling point is lower and will pass off in steam. The spring will thus be simply a boiling or quiescent spring. But if the tube be comparatively small and if the circulation be in any way impeded, the temperature at the source of heat will rise until it reaches a boiling point corresponding to its depth. Steam will result, and will rise through the water, gradually increasing the temperature in the upper portions of the tube. After a time the water throughout the entire tube becomes heated nearly to the boiling point and can no longer condense the steam rising from below; which then rapidly accumulates until its expansive power is great enough to lift the column above and project some of the water from the basin or cone. This lessens the weight of the column and relieves the pressure at every point. In places where the water had been just below the boiling point, it is now above, and more steam is rapidly produced. This throws out more water, still further lightens the column, and causes the generation of more steam, until finally the whole contents of the tube are ejected with terrific violence.

From this explanation it is apparent that any thing which impedes the circulation of water in the geyser tube will expedite the eruption. The well-known effect of "soaping geysers" may thus be accounted for. As oil thrown upon waves gives a viscosity to the surface, which greatly moderates their violence, so does the addition of soap or lye make the water of the geyser tube less free to circulate, and thus hasten the conditions precedent to an eruption.

The apparently contrary process of violently agitating the water of the geyser, as by stirring it with a stick, sometimes produces the same effect; but this results from the sudden forcing upward of masses of superheated water, instead of allowing them to rise and gradually cool.

That Bunsen's theory really explains the phenomena of geyser action there can be little doubt. It is true that in no single geyser does one find a perfect example of the theory. But it must be remembered that typical conditions probably never exist. The point of application of heat; the mode of application, whether from the heated surface of rocks or from superheated steam issuing into the tube; the diameter and regularity of the tube; the point of inflow of the cold water; are all matters which influence the eruption and determine its character. In the endless variety of conditions in nature one need not wonder at the varying results. He should rather wonder that in a single instance nature has produced a combination of such perfection as is found in Old Faithful, which, for thousands of years has performed its duty with the regularity of clock work.

There are various other theories, each with some particular merit, which may be briefly referred to. Sir George Mackenzie, who visited Iceland in 1810-11, thought the geyser tube at some point beneath the surface curved to one side and then upward, communicating with a chamber in the immediate vicinity of the source of heat. The water in this chamber becomes heated above the boiling point, and, expanding, forces the water from the chamber into the tube until the chamber is finally emptied to the level of the mouth of the tube. Any further expulsion of water lessens the weight of the column of water above. Bunsen's theory comes into play, and with the accumulated pressure of the steam in the chamber, produces a violent eruption.

Prof. Comstock, who visited the Park in 1873, thought that there were two chambers, the lower being in contact with the source of heat, and the upper acting as a sort of trap in the geyser tube. After a sufficient force of steam has accumulated in the lower chamber, it ejects the contents of the chamber above.

S. Baring-Gould, who visited Iceland in 1863, observed that if a tube be bent into two arms of unequal length, the shorter of which is closed, and if the tube be filled with water and the shorter arm then heated, all the characteristic phenomena of geyser action result, the water being finally ejected, with explosive violence from the longer tube.

Now, it is probable that in nature each of these theories may find illustration, but it must still be acknowledged that in all cases Bunsen's theory is the partial explanation, and in many the only adequate one.

The most superficial examination of the geysers in the Park will disclose two widely different characteristics as regards their external appearance and mode of eruption. On this basis they may be divided into two classes--the fountain geysers and the cone geysers.

In the fountain geyser there is no cone or mound, but in its place a considerable pool which in intervals of rest bears perfect resemblance to the larger quiescent springs. The eruption generally consists of a succession of prodigious impulses by which vast quantities of water are thrown up one after another. There is ordinarily no continuous jet. To geysers of this class, Mackenzie's and Comstock's theories would seem to find closer application than to any others. Noted examples are the Fountain, the Great Fountain, the Grand and the Giantess Geysers.

The cone geysers, on the other hand, have no pool about the crater, and water is not generally visible in the tube. There is always a self-built cone or mound of greater or less prominence, ranging from a broad gently-sloping mound, like that of Old Faithful, to a huge cone like that of the Castle. The eruptions from these geysers usually take the form of a continuous jet, and are more in accordance with the theory of Bunsen. Prominent examples are the Giant, the Castle, Old Faithful, the Lone Star, and the Union.

[AU] This sketch and a similar one of the Castle Geyser cone and two of the Yellowstone Falls are the _very first_ ever made of these objects. They were made in 1870 by Walter Trumbull, a member of the Washburn Party, and by Private Charles Moore, one of the escort under Lieutenant Doane. Moore was a man of excellent education and considerable culture, and it was a matter of comment among the members of the Expedition that he should be content with the condition of a private soldier. His quaint sketches of the Falls forcibly remind one of the original picture of Niagara made by Father Hennepin in 1697.

An interesting and singular fact pertaining to this region is that in most cases the springs and geysers have no underground connection with each other. Water in contiguous pools stands at different levels, and powerful geysers play with no apparent effect upon others near by.

It is another interesting question to know whence comes the water for these geysers and hot springs. Into the hidden caverns of "Old Faithful" flow nearly a million of gallons per hour. This is a large stream, but it is a mere trifle compared with the entire outflow of hot water throughout the Park. The subterranean passages by which the necessary supply is furnished to all these thousands of springs, certainly constitute the most intricate and extensive system of water-works of which there is any knowledge.

Not least wonderful of the features of the great geysers are the marvelous formations which surround them, more exquisitely beautiful than any production of art. They are really much handsomer than those to be found around the ordinary quiescent springs. The falling or the dashing of the hot water seems to be in some way essential to the finest results. To say that these rocky projections simulate cauliflower, sponge, fleeces of wool, flowers or bead-work, conveys but a feeble hint of their marvelous beauty. It is indeed a most interesting fact that nature here produces in stone, by the almost mechanical process of deposition from cooling water, the identical forms elsewhere produced by the very different processes of animal and vegetable life.

These formations are all silica and are of flinty hardness. Bunsen, and Prof. Le Conte following him, assert it to be a rule that the presence of silica in the water is essential to the development of a geyser. In one sense this is true, and in another it is not. Should the heated waters find a ready-made tube, like a fissure in solid rock, this would serve for a geyser tube as well as any other. The Monarch Geyser, in Norris Geyser Basin, seems to have originated in this way. But in the general case, geyser tubes are built up, not found ready made. In such cases silica is an indispensable ingredient of the water. A calcareous deposit, like that at Mammoth Hot Springs, would lack strength to resist the violent strain of an eruption. So it is found to be a fact that silica is the chief mineral ingredient in the water of all important geysers.