Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Artesian wells.—From the natural fissure spring an artesian well differs in the artificial character of the perforation of the impervious cover to the water layer. The water of artesian wells may flow out at the surface under pressure, or it may require pumping to raise it from some lower level. Ideal conditions are furnished where the geological structure of the district is that of a broad basin or syncline. The water which falls in a neighboring upland is here impounded between two parallel, saucer-like walls and will flow under its head if the upper wall be perforated at some low level ([Fig. 201, 3]).

Fig. 201.—Schematic diagrams to illustrate the different types of artesian wells, (1) A non-flowing well; (2) flowing wells without basin structure caused by clogging of the pervious formation; (3) flowing wells in an artesian basin. The dotted lines are the water levels within the pervious layers (after Chamberlin).

A monoclinal structure may furnish artesian conditions when the generally pervious layer has become clogged at a low level so as to hold back the water ([Fig. 248, 2]). Pumping wells may be used successfully even when such clogging does not exist, for the slow-moving underground water flows readily in the direction of all free outlets ([Fig. 201, 1]).

Hot springs and geysers.—Thermal springs whose temperature approaches the boiling point of water are known as hot springs. A geyser is a hot spring which intermittently ejects a column of water and steam. Both hot springs and geysers are to be found only in volcanic regions, and appear to be connected with uncooled masses of siliceous lava. In two of the three known geyser regions, Iceland and New Zealand, the volcanoes of the neighborhood are still active, and the lavas of the Yellowstone National Park date from the quite recent geological period which immediately preceded the so-called “Ice Age.”

Wherever found, geysers are in the low levels along lines of drainage where the underground water would most naturally reappear at the surface. Their water has penetrated to considerable depths below the surface, but has been chiefly heated by ascending steam or other vapors. The water journey has been chiefly made along fissures, as is shown by the cool springs which often issue near them. Though some hot springs and geysers may disappear from a district, others are found to be forming, and there is no good reason to think that geysers are rapidly dying out, as was at one time supposed.

The action of a geyser was first satisfactorily explained by the great German chemist Bunsen after he had made studies of the Icelandic geysers, and the mechanics of the eruption was later strikingly illustrated in the laboratory by an artificial geyser constructed by the Irish physicist Tyndall. In many respects this action is like that of the Strombolian eruption within a cinder cone, since it is connected with the viscosity of the fluid and the resistance which this opposes to the liberation of the developing vapor. In the case of the geyser, a column of heated water stands within a vertical tube and is heated near the bottom of the column.

Fig. 202.—Cross section of Geysir, Iceland, with simultaneously observed temperatures recorded at the left, and the boiling temperatures for the same levels at the right (after Campbell).

Though the water may at its surface have the normal boiling temperature and be there in quiet ebullition, the boiling point for all lower levels is raised by the weight of the column of superincumbent liquid, and so for a time the formation of steam within the mass is prevented. In [Fig. 202] is shown a cross section of the Icelandic Geysir from which our name for such phenomena has been derived, and to this section have been added the actual observed temperatures of the water at the different levels as well as the temperatures at which boiling can take place at these levels. From this it will be seen that at a depth of 45 feet the water is but 2° Centigrade below its boiling point. A slight increase of temperature at this level, due to the constantly ascending steam, will not only carry this layer above the boiling point, but the expansion of the steam within the mass will elevate the upper layers of the water into zones where the boiling points are lower, and thus bring about a sudden and violent ebullition of all these upper portions. Thus is explained the almost universal observation that just before geysers erupt the hot water rises in the bowls and generally overflows them.