CHAPTER XXI

MUD VOLCANOES AND HOT SPRINGS

Mud volcanoes are the more or less conical hillocks from which, under certain conditions, mud is thrown out through the crust of the earth.

Geikie defines mud volcanoes as follows:

"Conical hills formed by the accumulation of fine and usually saline (salty) mud, which, with various gases, is continuously or intermittently given out from the orifice or crater in the centre. They occur in groups, each hillock being sometimes less than a yard in height, but ranging up to elevations of 100 feet or more. Like true volcanoes, they have their periods of repose, when either no discharge takes place at all, or mud oozes out tranquilly from the crater, and their periods of activity, when large volumes of gas, and sometimes columns of flame, rush out with considerable violence and explosion, and throw up mud and stones to a height of several hundred feet."

There are two kinds of mud volcanoes: those in which the mud is thrown out by the action of different kinds of gases, and those in which the mud is thrown out by the action of steam.

Mud volcanoes may or not be volcanic phenomena. Those which occur in the neighborhood of volcanoes whether active, dormant, or extinct, are probably of volcanic origin. There are others, however, which occur in regions far removed from volcanoes. These are probably due not to volcanoes, but to chemical action and the eruptions are caused by the action of gases.

The gases producing these eruptions are either carbonic acid gas (the gas that is given off from soda water); carburetted hydrogen (the gas that is sometimes seen escaping from the bottom of marshy ground); sulphuretted hydrogen (a gas that is given off from rotten or decomposing eggs, and possessing the characteristic odor of decayed eggs) and nitrogen gas derived from the atmosphere. In mud volcanoes of the gaseous type the mud is generally cold, and the water salty. In this latter case the mud volcanoes are also called _salses_. Daubeny has pointed out that the mud volcanoes of this class that occur in the neighborhood of Sicily are due to the slow burning or oxidation of beds of sulphur.

Mud volcanoes which eject hot mud by the force of eruption of steam, which occur in volcanic districts, are of volcanic origin. They are caused by the passage of hot water and steam through beds of volcanic rock such as tufa, or hardened volcanic mud and other volcanic products. The hot water or steam raises the temperature of the mud through which it passes to the boiling point. As Dana remarks, the mud varies in consistency from very liquid muddy water to a thick mass like boiling soap, or in some cases like masses of mud or paint, and, in still other cases, to material like soft mortar, the consistency of the mud varying with the dryness of the season.

There are three regions where mud volcanoes are especially common. One of the best known is in the Yellowstone National Park, four miles north of Yellowstone Lake, and six miles from Crater Hill. Some of these mud volcanoes have circular craters about ten feet in depth around which they have built mounds, the rims of which are several feet above the general level.

There are well-known regions of mud volcanoes in different parts of Iceland. Here, according to Lyell, they occur in many of the valleys where sulphur vapor and steam bursts from fissures in the ground with a loud hissing noise. In these regions there are pools of boiling water filled with a bluish black clay-like paste, that is kept violently boiling. Huge bubbles, fifteen feet or more in diameter, rise from the surface of the boiling mass. The volcanoes pile up the mud around the sides of their craters or basins.

Another part of the world where mud volcanoes are especially numerous is on the western shores of the Caspian Sea at a place called Baku. These are of the gaseous type and are attended by flames that blaze up to great heights often for several hours. These flames are due to the presence of natural gas and petroleum vapor that pass out through the water. Large quantities of mud are thrown out from the craters of these mud volcanoes.

There are also many mud volcanoes in a district in India about 120 miles northwest of Cutch near the mouth of the Indus. In this region the cone built up around the crater is sometimes as high as 400 feet.

The following description of mud volcanoes on Java is quoted from Daubeny's book on volcanoes.

"It would appear likewise from Dr. Horsfield's description, that Java exhibits phenomena of a similar kind to those noticed in Sicily and at the foot of the Apennines, and there known under the name of 'Salses.' In the calcareous district (which I suspect to belong to the same class of formations as the blue clay and tertiary limestone of Sicily) occur a number of hot springs, containing in solution a large quantity of calcareous earth, which incrusts the surface of the ground near it. Of these, some are much mixed with petroleum, and others highly saline.

"The latter are dispersed through a district of country consisting of limestone, several miles in circumference. They are of considerable number, and force themselves upwards through apertures in the rocks with some violence and ebullition. The waters are strongly impregnated with muriate of soda, and yield upon evaporation very good salt for culinary purposes (not less than 200 tons in the year).

"About the centre of this limestone district is found an extraordinary volcanic phenomenon. On approaching the spot from a distance, it is first discovered by a large volume of smoke rising and disappearing at intervals of a few seconds, resembling the vapors arising from a violent surf, whilst a dull noise is heard like that of distant thunder. Having advanced so near that the vision was no longer impeded by the smoke, a large hemispherical mass was observed, consisting of black earth mixed with water, about sixteen feet in diameter, rising to the height of twenty or thirty feet in a perfectly regular manner, and, as it were, pushed up by a force beneath, which suddenly exploded with a dull noise, and scattered about a volume of black mud in every direction. After an interval of two or three, or sometimes four or five seconds, the hemispherical body of mud or earth rose and exploded again.

"In the same manner this volcanic ebullition goes on without interruption, throwing up a globular mass of mud, and dispersing it with violence through the neighboring places. The spot where the ebullition occurs is nearly circular and perfectly level; it is covered with only the earthy particles impregnated with salt water, which are thrown up from below; its circumference may be estimated at about half an English mile. In order to conduct the salt water to the circumference, small passages or gutters are made in the loose muddy earth, which lead it to the borders, where it is collected in holes dug in the ground for the purpose of evaporation.

"A strong, pungent, sulphurous smell, somewhat resembling that of earth-oil (naphtha), is perceived on standing near the site of the explosion, and the mud recently thrown up possesses a degree of heat greater than that of the surrounding atmosphere. During the rainy season these explosions are more violent, the mud is thrown up much higher, and the noise is heard at a greater distance.

"This volcanic phenomenon is situated near the centre of the large plain, which interrupts the great series of volcanoes, and owes its origin to the same general cause as that of the numerous eruptions met with in this island."

There are, in many parts of the world, springs, whose waters issue from their reservoirs at temperatures either at or near the boiling point of water. These are called _hot_ or _thermal springs_. Hot springs are found both in volcanic regions, as well as in regions where there are no volcanoes, but where there are lines of deep fissures or faults. According to Dana, in both of these classes, the cause is to be traced to heat of volcanic or deep subterranean origin. Hot springs are also found in regions where there are no volcanoes. In these cases the heat is due to the gradual oxidation of various sulphide ores, or to some other chemical action.

The waters of hot or thermal springs almost always contain various mineral substances in solution. All spring water contains some little dissolved mineral matter, but in hot springs the quantity of this matter is greater than in cold springs, because hot water can dissolve mineral substances much better than can cold water.

It might surprise you to hear that one of the commonest substances that is found in solution in the waters of many hot springs is silica; for silica is practically sand, and sand does not easily dissolve in water as does sugar. The very hot water, however, which comes from the hot spring, whose temperature below the earth's surface is very much higher than it is when it comes out of the spring, possesses the power of readily dissolving silica from the rocks over which it flows. When the waters of such springs reach the surface the silica is deposited in a solid condition around the outlets of the springs. In this way there are built up craters or mounds, or, more correctly, crater-shaped basins.

Sometimes the hot water contains calcareous substances dissolved in it, the solution being caused not only by reason of the hot water, but also by means of the carbonic gas it contains. When this water flows from the springs, it builds up the same crater-shaped mounds, only in this case the mounds are of lime instead of silica.

There are peculiar kinds of hot springs called _geysers_, that possess the power of throwing huge streams of water up into the air at more or less regular intervals. The word geyser is an Icelandic word meaning to rage, or snort, or gush, the name being given by reason of the manner in which the waters rush violently out during an eruption.

As Dana points out, when the water in a basin of a hot spring merely boils, whether this boiling is nearly continuous, or the water is alternately boiling and quiet, the spring is called a hot or thermal spring, but where the water is thrown violently out at more or less regular intervals, it is called a geyser.

The cause of the eruption of a geyser was discovered by Professor Bunsen, the celebrated German chemist, after a careful study of the geyser regions in Iceland. The waters of geysers contain large quantities of either silica or lime in solution. Bunsen traced the cause of these curious eruptions to be the manner in which the hot springs pile up cones of silica or limestone around their mouths.

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.

It is a well-known fact that in geyser regions generally, the throwing of stones or other materials into the tube will frequently hasten an eruption. This is probably due to the fact that the throwing in of these things results in the raising of the water in the tube, thus hastening the eruption.

The New Zealand region is in the neighborhood of Lake Rotomahama in the northern island.

The geyser region in the Yellowstone Park is by far the most interesting of all geyser regions. This region is situated principally around Fire-Hole Fork of the Madison, and near Shoshone Lake at the head of Lake Fork of the Snake River. There are many geysers in this region, as well as simple hot springs. The temperature of their waters varies from between 160° and 200° F. to the boiling point of water at this elevation. As you are probably aware, water boils at the temperature of 212° F. only under the condition of the ordinary atmospheric pressure that exists at the level of the sea. At higher elevations, such as on the slopes of mountains, or on high plateaus, water boils at a lower temperature. The height of the country in which the Yellowstone Park is situated is so great that the water boils at temperatures of from 198° to 199° F.

The conical hillock of geyser cones from which the waters flow assume various shapes, two of which are shown in Figs. 36 and 37.

That shown in Fig. 36 represents the shape of the cone of the giant geyser in the upper geyser basin of the Fire-Hole, Yellowstone National Park. This cone is about ten feet in height, and twenty-four feet in diameter. As shown in the figure it is broken on one of its sides. It throws out, at long intervals, a column of water the height of which varies from ninety to 200 feet.

Fig. 38 represents the crater of a cone known as the Bee Hive in eruption.

Besides the above named geyser regions there is another region on the shores of Celebes, and a small region on San Miguel, in the Azores Islands, in the Atlantic Ocean.

Besides hot springs and mud volcanoes there are two other phenomena connected with volcanic action that we will now briefly describe.

When eruptions take place and the lava begins to flow down the side of a mountain, the different vapors and gases with which the lava is charged begin to escape or pass out from the boiling or fused mass. When these substances are of such a character that they produce fumes, or the vapors of various chemical substances, that become solid on cooling, they form what are called _fumaroles_, a word derived from a Latin word meaning "to smoke." For the greater part, fumaroles are found on the edge of craters, but sometimes are found in cavernous places either in the crater or in the lava streams.

There is, still, another class of openings through which only sulphurous vapors escape. These are called _solfataras_, a word derived from the Italian word _solfo_, or sulphur. Solfataras are generally found in regions distant from volcanic action. In the materials that escape from recently ejected lava, or molten lava, the temperature is high enough to volatilize many of the solid ingredients. But where the temperature is low, only sulphur vapors are driven off. It is for this reason that fumaroles are only found around the craters of active volcanoes, or on the lines of cracks or crevices of the lava stream where the temperature is very high.

Besides water vapor and sulphurous vapors there are other substances that escape from the earth in volcanic districts. Sulphurous acid, together with hydrogen and nitrogen escape from nearly all lava. At Vesuvius chlorine gas is given off. This, however, as soon as it passes into the atmosphere becomes changed into hydrochloric acid. Sulphurous acid is frequently changed into sulphuric acid, which, combining with various substances, forms such materials as _gypsum_, or sulphate of lime, the chemical name for plaster of Paris; sulphate of soda or _Glauber's salt_; sodium chloride or _common table salt_; and _sal ammoniac_. You will remember in reading the description of Vulcano, in the Grecian Archipelago, that some of these products were collected at the chemical works that had been established on the volcano.

When a volcanic mountain is for the time being passing from an active to an extinct condition, it is sometimes said to be in the _fumarole stage_, since the presence of the fumaroles are the only indication of its activity. The volcanic heat is still great. When it reaches a still greater decline, the fumaroles disappear, and only solfataras are left. The amount of heat is now only sufficient to produce sulphur vapors and the vapor of water. This is called the _solfatara stage_.

Of course, as we have already pointed out, fumaroles and solfataras may occur in the neighborhood of a volcano at different distances from its crater.