The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

The water of a geyser generally issues from the top of a more or less conical hillock, reaching the surface through a funnel-shaped tube. Both the tube and the basin are covered with a smooth coating of silica or limestone. In the case of the Great Geyser in Iceland, the basin is over fifty feet high and seventy-five feet deep. Both the tube and the basin have been slowly deposited by the hot water of the geyser.

It is only when the tube of a geyser has reached a certain depth that the geyser is able to erupt. Moreover, as soon as this tube passes a certain depth the geyser can no longer erupt and forever afterwards becomes an ordinary hot spring. There are, therefore, to be found in most geyser regions, a number of what might be called young geysers or merely hot springs, that are not yet deep enough to erupt; others that have just commenced eruption, others that have reached their prime, while others that, old and decrepit, have again merely become hot springs.

Let us now try to understand the cause of the eruption of a geyser. Bunsen's explanation, which is now generally accepted, is as follows:

The heat of the volcanic strata through which the tube of the geyser extends, gradually raises the temperature of the water that fills the geyser tube. Since the boiling point of a liquid increases with the pressure to which it is subjected, far down in the tube of a geyser, the pressure arising from the weight of the water above it is sufficiently great to prevent the water from beginning to boil until it reaches a temperature far higher than that at which it would boil in the upper parts of the tube. Suppose now, when the water in the funnel-shaped tube is nearly filled to the top, the water at last grows hot enough to begin boiling at some point near the middle of the tube. The pressure of the steam driven off from this portion of the water raises the column of water above it in the tube and begins to empty it out of the top of the geyser. All the water below this point being thus suddenly relieved of its pressure, and being now much hotter than is necessary to boil the water at that decreased pressure, suddenly flashes into steam, and violently shoots out all the water above it to a height that in some cases may be as great as 100 to 200 feet. The steam causes this eruption, then rushes out with a roar, and the geyser eruption is over.

Professor Tyndall in his charming book entitled "Heat as a Mode of Motion" speaks as follows concerning Professor Bunsen's discovery:

"Previous to an eruption, both the tube and basin are filled with hot water; detonations which shake the ground, are heard at intervals, and each is succeeded by a violent agitation of the water in the basin. The water in the pipe is lifted up so as to form an eminence in the middle of the basin, and an overflow is the consequence. These detonations are evidently due to the production of steam in the ducts which feed the geyser tube, which steam escaping into the cooler water of the tube is there suddenly condensed, and produces the explosions. Professor Bunsen succeeded in determining the temperature of the geyser tube, from top to bottom, a few minutes before a great eruption; and these observations revealed the extraordinary fact that at no part of the tube did the water reach its boiling point. In the sketch [not reproduced] I have given on one side the temperatures actually observed, and on the other side the temperatures at which water would boil, taking into account both the pressure of the atmosphere and the pressure of the superincumbent column of water. The nearest approach to the boiling point is at A, a height of 30 feet from the bottom; but even here the water is 2° C., or more than 3-1/2° F., below the temperature at which it could boil. How then is it possible that an eruption could occur under such circumstances?

"Fix your attention upon the water at the point A, where the temperature is within 2° C. of the boiling point. Call to mind the lifting of the column when the detonations are heard. Let us suppose that by the entrance of steam from the ducts near the bottom of the tube, the geyser column is elevated six feet, a height quite within the limits of actual observation; the water at A is thereby transferred to B. Its boiling point at A is 123.8°, and its actual temperature 121.8°; but at B its boiling point is only 120.8°, hence, when transferred from A to B the heat which it possesses is in excess of that necessary to make it boil. This excess of heat is instantly applied to the generation of steam: the column is thus lifted higher, and the water below is further relieved. More steam is generated; from the middle downwards the mass suddenly bursts into ebullition, the water above, mixed with steam clouds, is projected into the atmosphere, and we have the geyser eruption in all its grandeur.

"By its contact with the air the water is cooled, falls back into the basin, partially refills the tube, in which it gradually rises, and finally fills the basin as before. Detonations are heard at intervals, and risings of the water in the basin. These are so many futile attempts at an eruption, for not until the water in the tube comes sufficiently near its boiling temperature, to make the lifting of the column effective, can we have a true eruption."

The principal geyser regions of the world are in Iceland, in New Zealand, and in the Yellowstone National Park in the United States.

There are several geyser regions in Iceland. The best known lies in the neighborhood of Mt. Hecla. Here is a great geyser that shoots up a column of water to a height of about 100 feet every thirty hours. [Fig. 35] represents the appearance of the crater of the great geyser in Iceland.