Popular Science Monthly/Volume 12/February 1878/Geysers and How They are Explained
|GEYSERS AND HOW THEY ARE EXPLAINED.|||
A GEYSER may be defined as a periodically eruptive spring. They are found only in Iceland, in the Yellowstone Park, United States, and in New Zealand. The so-called geysers of California are rather fumaroles. Those of Iceland have been long studied; we will, therefore, describe these first.
Iceland is an elevated plateau about two thousand feet high, with a narrow marginal habitable region sloping gently to the sea. The elevated plateau is the seat of every species of volcanic action, viz., lava-eruptions, solfataras, mud-volcanoes, hot springs, and geysers.
These last exist in great numbers; more than one hundred are found in a circle of two miles diameter. One of these, the Great Geyser, has long attracted attention.
The Great Geyser is a basin or pool fifty-six feet in diameter, on the top of a mound thirty feet high. From the bottom of the basin descends a funnel-shaped pipe eighteen feet in diameter at top, and seventy-eight feet deep. Both the basin and the tube are lined with silica, evidently deposited from the water. The natural inference is, that the mound is built up by deposit from the water, in somewhat the same manner as a volcanic cone is built up by its own ejections. In the intervals between the eruptions the basin is filled to the brim with perfectly transparent water, having a temperature of about 170° to 180°.
1. Immediately preceding the eruption sounds like cannonading are heard beneath, and bubbles rise and break on the surface of the water. 2. A bulging of the surface is then seen, and the water overflows the basin. 3. Immediately thereafter the whole of the water in the tube and basin is shot upward one hundred feet high, forming 3 fountain of dazzling splendor. 4. The eruption of water is immediately followed by the escape of steam with a roaring noise. These last two phenomena are repeated several times, so that the fountain continues to play for several minutes, until the water is sufficiently cooled, and then all is again quiet until another eruption. The eruptions occur tolerably regularly every ninety minutes, and last six or seven minutes. Throwing large stones into the tube has the effect of bringing on the eruption more quickly.
In magnificence of geyser displays, however, Iceland is far surpassed by the Yellowstone geysers in the basin of Firehole River. This wonderful geyser region is situated in the northwest corner of Wyoming, on an elevated volcanic plateau near the head-waters of the Madison River, a tributary
of the Missouri, and of the Snake River, a tributary of the Columbia. The basin is only about three miles wide. About it are abundant evidences of prodigious volcanic activity in former times, and, although primary volcanic activity has ceased, secondary volcanic phenomena are developed on a stupendous scale and of every kind, viz.: hot springs, carbonated springs, fumaroles, mud-volcanoes, and geysers. In this vicinity there are more than 10,000 vents of all kinds. In some places, as on Gardiner's River, the hot springs are mostly lime-depositing; in others, as on Firehole River, they are geysers depositing silica.
In the upper geyser basin the valley is covered with a snowy
deposit from the hot geyser-waters. The surface of the mound-like, chimney-like, and hive-like elevations, immediately surrounding the vents, is, in some cases, ornamented in the most exquisite manner by deposits of the same, in the form of scalloped embroidery set with pearly tubercles; in others, the siliceous deposits take the most fantastic
forms (Figs. 1, 2, 3). In some places the silica is deposited in large quantities, three or four inches deep, in a gelatinous condition like starch-paste. Trunks and branches of trees immersed in these waters are speedily petrified.
We can only mention a few of the grandest of these geysers:
1. The " Grand Geyser," according to Hayden, throws up a column of water six feet in diameter to the height of 200 feet, while the steam ascends 1,000 feet or more. The eruption is repeated every thirty-two hours, and lasts twenty minutes. In a state of quiescence the temperature of the water at the surface is about 150°.
2. The "Giantess" throws up a large column twenty feet in diameter to a height of sixty feet, and through this great mass it shoots up five or six lesser jets to a height of 250 feet. It erupts about once in every eleven hours, and plays twenty minutes.
3. The "Giant" (Fig. 4) throws a column five feet in diameter 140 feet high, and plays continuously for three hours.
4. The "Beehive" (Fig. 5), so called from the shape of its mound, shoots up a splendid column two or three feet in diameter to the height by measurement of 219 feet, and plays fifteen minutes.
5. "Old Faithful," so called from the frequency and regularity of its eruptions, throws up a column six feet in diameter to the height of 100 to 150 feet regularly every hour, and plays each time fifteen minutes.
The water of geysers is not volcanic water, but simple spring-water. A geyser is not, therefore, a volcano ejecting water, but a true spring. There has been much speculation concerning the cause of their truly wonderful eruptions.
According to Mackenzie, the eruptions of the Great Geyser may be accounted for by supposing its pipe connected by a narrow conduit with the lower part of a subterranean cave, whose walls are heated by the near vicinity of volcanic fires. Fig. 7 represents a section through the basin, tube, and supposed cave. Now, if meteoric water should run into the cave through fissures more rapidly than it can evaporate, it would accumulate until it rose above, and therefore closed, the opening at a. The steam, now having no outlet, would condense in the chamber b until its pressure raised the water into the pipe, and caused it to overflow the basin. The pressure still continuing, all the water would be driven out of the cave, and partly up the pipe. Now, the pressure which sustained the whole column a d would not only sustain, but eject with violence, the column c d. The steam would escape, the ejected water would cool, and a period of quiescence would follow. If there were but one geyser in Iceland, this would be rightly considered a very ingenious and probable hypothesis, for without doubt we may conceive of a cave and conduit so constructed as to account for the phenomena.
But there are many eruptive springs in Iceland, and it is inconceivable that all of them should have caves and conduits so peculiarly constructed. This theory is therefore entirely untenable.
The investigations of Bunsen and his theory of the eruption and the formation of geysers are among the most beautiful illustrations of scientific induction which we have in geology. We therefore give it, perhaps, more fully than its strict geological importance warrants.
Bunsen examined all the phenomena of hot springs in Iceland. 1. He ascertained that geyser-water is meteoric water, containing the soluble matters of the igneous rocks in the vicinity. He formed identical water by digesting Iceland rocks in hot rain-water. 2. He ascertained that there are two kinds of hot springs in Iceland, viz., acid springs and alkaline-carbonate springs, and that only alkaline-carbonate springs contain any silica in solution. The reason is obvious: alkaline waters, especially if hot, are the natural solvents of silica. 3. He ascertained that only the silicated springs form geysers. Here is one important step taken—one condition of geyser-formation discovered. Deposit of silica is necessary to the existence of geysers. The tube of a geyser is not an accidental conduit, but is built up by its own deposit. 4. Of silicated springs, only those with long tubes erupt—another condition. 5. Contrary to previous opinion, the silica in solution does not deposit on cooling, but only by drying. This would make the building-up of a geyser-tube an inconceivably slow process, and the time proportionally long. This, however, is not true, for the Yellowstone geyser-waters, which deposit abundantly by cooling, evidently because they contain much more silica than those of Iceland. 6. The temperature of the water in the basin was found to be usually 170° to 180°, and that in the tube to increase rapidly, though not regularly, with depth. Moreover, the temperature, both at the surface and at all depths, increased regularly as the time of eruption approached. Just before the eruption it was, at the depth of about forty-five feet, very near the boiling-point for that depth.
1. It is well known that the boiling-point of water rises as the pressure increases. This is shown in the adjoining table. 2. It follows from the above that if water be under strong pressure, and at high temperature, though below its boiling-point for that pressure, and the pressure be diminished sufficiently, it will immediately flash into steam. 3. Water heated beneath, if the circulation be unimpeded, is very nearly the same temperature throughout. That it is never the same temperature precisely is shown by the circulation itself, which is caused by difference of temperature, producing difference in density. The phenomenon of simmering is also a well-known evidence of this difference of temperature, since it is produced by the collapse of steam-bubbles rising into the cooler water above. 4. But if the circulation be impeded, as when the water is contained in long, narrow, irregular tubes, and heated with great rapidity, the temperature may be greater below than above to any extent, and the boiling-point may be reached in the lower part of the tube, while it is far from this point in the upper part.
We will suppose a geyser to have a simple but irregular tube, without a cave, heated below by volcanic fires, or by still hot volcanic ejections. Now, we have already seen that the temperature of the water in the tube increases rapidly with the depth, but is, at every depth to which observation extends, short of the boiling-point for that depth.
Let absciss a d (Fig. 8) represent depth in the tube, and also pressures; and the corresponding temperature be measured on the ordinate a n. If, then, a b, b c, c d, represent equal depths of thirty-three or more feet, which is equal to one atmospheric pressure, the curve e f passing through 210°, 250°, 275°, and 293°, at the horizontal lines, representing one atmosphere, two atmospheres, three atmospheres, etc., would correctly represent the increasing boiling-points as we pass downward. We shall call this line, e f, the curve of boiling-point. The line a g commencing at the surface at 180°, and gradually approaching the boiling-point line, but everywhere within it, would represent the actual temperature in a state of quiescence. Now, Bunsen found that, as the time of eruption approached, the temperature at every depth approached the boiling-point for that depth, i. e., the line a g moved toward the line e f There is no doubt, therefore, that, at the moment of eruption, at some point below the reach of observation, the line a g actually touches the line e f—the boiling-point for that depth is actually reached. As soon as this occurs, a quantity of water in the lower portion of the tube, or perhaps even in the subterranean channels which lead to the tube, would be changed into steam, and the expanding steam would lift the whole column of water in the tube, and cause the water in the basin to bulge and overflow. As soon as the water overflowed, the pressure would be diminished in every part of the tube, and consequently a large quantity of water before very near the boiling-point would flash into steam and instantly eject the whole of the water in the pipe; and the steam itself would rush out immediately afterward. The premonitory cannonading beneath is evidently produced by the collapse of large steam-bubbles rising through the cooler water of the upper part of the tube; in other words, it is simmering on a huge scale. An eruption is more quickly brought on by throwing stones into the throat of the geyser, because the circulation is thus more effectually impeded.
The theory given above is substantially that of Bunsen for the eruption of the Great Geyser, but modified to make it applicable to all geysers. In the Great Geyser, as already stated, Bunsen found a point, forty-five feet deep, where the temperature was nearer the boiling-point than at any within reach of observation, though doubtless beyond the reach of observation the temperature again approached and touched the boiling-point. This point, forty-five feet deep, plays an important part in Bunsen's theory. To illustrate: if e f (Fig. 9) represent again the curve of boiling-point, then the curve of actual temperature in the Great Geyser tube would be the irregular line a g h. At the moment of eruption, this line touched boiling-point at some depth, h, beyond the reach of observation. Then followed the lifting of the column, the overflow of the basin, the relief of pressure by which the point g was brought to the boiling-point, the instantaneous formation of steam at g, and the phenomena of an eruption. But it is extremely unlikely that this condition should exist in all geysers; neither is it at all necessary in order to explain the phenomenon of an eruption.
To prove beyond question the truth of his theory, Bunsen constructed an artificial geyser. The apparatus (Fig. 10) consisted of a tube of tinned sheet-iron about ten feet long, expanded into a dish above for catching the erupted water. It may or may not be expanded below for the convenience of heating. It was heated, also, a little below the middle, by an encircling charcoal chauffer, to represent the point of nearest approach to the boiling-point in the geyser-tube. When this apparatus was heated at the two points, as shown in the figure, the phenomena of geyser-eruption were completely reproduced; first, the violent explosive simmering, then the overflow, then the eruption, and then the state of quiescence. In Bunsen's experiment, the eruptions occurred about every thirty minutes.
According to Bunsen, a geyser does not find a cave, or even a perpendicular tube, ready made, but, like volcanoes, makes its own tube. Fig. 11 is an ideal section of a geyser-mound, showing the manner in which, according to this view, it is formed. The irregular line, b a c, is the original surface, and a the position of a hot spring. If the spring be not alkaline, it will remain an ordinary hot spring; but, if it be alkaline, it will hold silica in solution, and the silica will be deposited about the spring. Thus the mound and tube are gradually built up. For a long time the spring will not be eruptive, for the circulation will maintain a nearly equal temperature in every part of the tube—it may be a boiling,
but not an eruptive spring. But, as the tube becomes longer, and the circulation more and more impeded, the difference of temperature between the upper and lower parts of the tube becomes greater and greater, until, finally, the boiling-point is reached below, while the water above is comparatively cool. Then the eruption commences. Finally, from the gradual failure of the subterranean heat, or from the increasing length of the tube repressing the formation of steam, the eruptions gradually cease. Bunsen found geysers in every stage of development—some playful springs without tubes; some with short tubes, not yet eruptive; some with long tubes, violently eruptive; some becoming old and indisposed to erupt unless angered by throwing stones down the throat.
It is evident, however, that Bunsen's theory of geyser-eruption is independent of his theory of geyser-formation. A tube or fissure of any kind, and formed in any way, if long enough, would give rise to the same phenomena. The Yellowstone geysers have mounds or chimney-like cones, but it is by no means certain that the whole length of their eruptive tubes has been built up by siliceous deposit. Bunsen's theory of eruption none the less, however, applies to these also. The more chimney-like form of the craters in the case of the Yellowstone geysers is probably due to the greater abundance of silica in solution.
- From Le Conte's "Elements of Geology."