Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

THE WORK OF GROUND-WATER.

Ground-water effects very considerable results in the course of its history. These results are partly chemical and partly mechanical, the former being far more important than the latter.

Chemical Work.

The results of the chemical and chemico-physical action of water may be grouped in several more or less distinct categories.

1. The simplest effect is the subtraction of soluble mineral matter. Pure water is in itself a solvent of certain minerals; but the carbonic-acid gas extracted from the atmosphere, and the products of organic decay extracted from the soil make ground-water a much more efficient solvent. Something of the results which it achieves is shown by its composition. All ground-water, whether issuing as springs or drawn out through wells, contains much more mineral matter than the water which falls as rain, and the excess is acquired in its underground course.

The subtraction of soluble matter from rock renders it porous. The amount of material dissolved from a given place may be trivial or considerable, according to the character of the rock, the readiness with which water has access to it, and the character of the water. Locally, the subtraction of mineral matter may be the chief, or even the only appreciable, effect of the ground-water.

2. It sometimes happens that ground-water with certain mineral substances in solution exchanges them for other substances extracted from the rock. Thus the process of substitution is effected. By this process the lime carbonate of a shell imbedded in rock may be removed, molecule by molecule, and some other substance, such as silica, left in its place. When the process is complete, the substance of the shell has been completely removed, though its form and structure are still preserved in the new material which has taken the place of the old. Buried logs are sometimes converted into stone by the substitution of mineral matter for the vegetable tissue. This is petrification. Petrification is altogether distinct from incrustation, which simply means the coating of an object with mineral matter. A bird’s nest may be incrusted with lime carbonate, but it is not thereby petrified. Solution is a necessary antecedent of substitution.

3. The materials which are subtracted from the rock at one point may be added to other rock elsewhere. Thus a third type of change, addition, is effected. Rock may at one time and place be rendered porous by the subtraction of some of its substance, and the openings thus formed may subsequently become the receptacles of deposits from solution. This is exemplified in the stalactitic deposits of many caves. Not uncommonly cracks and fissures are filled with mineral matter deposited by the waters which pass through them. Thus arise veins which, for the most part, are nothing more than cracks and crevices filled by mineral matter brought to them in solution, and precipitated on their walls. Most veins of metallic ores have originated in this way.

4. A further series of changes is effected by ground-water when it, or the mineral matter it contains, enters into combination with the mineral matter through which it passes. One of the commonest processes of this sort, hydration, has already been referred to (pp. [43] and [428]); but in the development of many of the commoner hydrous minerals changes other than hydration are involved. These changes result in new mineral combinations, the new minerals being developed out of the old, usually with some additions or subtractions. In the long course of time changes of this sort may be very great, so great indeed that large bodies of rock are radically changed, both chemically and physically. Much of the old substance may remain, but it has entered into new and more stable combinations with the materials which the water has brought to it.

Quantitative importance of solution.—In general, solution is probably most effective at a relatively slight distance below the surface. In the outermost zone of mantle rock the materials are usually less soluble than below, for they often represent the residuum after the soluble parts of the formation from which they originated were dissolved out. Below this zone the rock contains more soluble matter, and the water, charged with organic matter in its descent through the soil, is in condition to dissolve it. At greater depths the water has become saturated to some extent, and, so far forth, less active. Here, too, the movement is less free. The increased pressure at considerable depths, on the other hand, facilitates solution, which must be understood to take place under proper circumstances in any zone reached by the water.

Calculations have been made which illustrate the quantitative importance of the solution effected by ground-water. The springs of Leuk (Switzerland) bring to the surface annually more than 2000 tons of calcium sulphate (gypsum) in solution, and in the same time the springs of Bath (England) bring up an amount of mineral matter in solution sufficient to make a column 9 feet in diameter, and 140 feet high.[100]

The amount of mineral matter in solution in streams is also significant, for while stream-water is not all derived from ground-water, much of it had such an origin. In the case of several streams, among them the Thames and the Elbe, careful estimates of the amount of dissolved mineral matter have been made. Though the Thames drains an area only about one-tenth as large as the State of New York, it is estimated to carry about 1500 tons of mineral matter in solution to the sea daily.[101]

From the uppermost 20,000 square miles of its drainage basin the Elbe is estimated to carry yearly about 1,370,000 tons of mineral matter in solution. Estimates of the amounts of material carried to the sea in solution by several rivers are given on pp. [102] and [103]. Much of this matter was brought to the rivers by waters which had been underground before reaching the streams.

From these figures it is clear that we have to reckon here with a very considerable factor in the lowering of land surfaces. From the amount of lime carbonate carried by the Thames it has been estimated that the average amount of this material dissolved from the limestone area drained by this stream is 143[102] tons per square mile per year. It is estimated that, on the average, something like one-third as much matter is carried to the sea in solution as in the form of sediment, and that by this process alone land areas would be lowered something like one foot in 13,000 years.[103]

Deposition of mineral matter from solution.—The deposition of material from solution is effected in several ways. (1) It is sometimes deposited by evaporation. This is well shown where water seeps out on arid lands. The same process is illustrated when water is boiled. (2) Reduction of temperature often occasions deposition. In general, hot water is a better solvent of mineral matter than cold,[104] and if it issues with abundant mineral matter in solution the precipitation of some of it is likely to take place. (3) Plants sometimes cause the precipitation of mineral matter from solution. About some hot springs, even where the temperature of the water is very high small plants of low type (algæ) grow in profusion. In ways which are not perfectly understood these algæ extract the mineral matter from the hot water. They are now thought to be a chief factor in the deposits about the hot springs of the Yellowstone Park.[105] The influence of organisms on precipitation from solution is not confined to the waters of hot springs. (4) A fourth factor involved in the deposition of mineral matter from solution is pressure. Pressure increases the solvent power of water with respect to minerals directly; it produces the same effect indirectly by its effect on the solution of gases. As water charged with gas comes to the surface, the pressure is relieved and some of the gas escapes. Such mineral matter as was held in solution by the help of the gas which escapes is then precipitated. (5) Precipitation is also sometimes effected by the mingling of waters containing different mineral substances in solution. Such mingling of solutions would be most common along lines of ready subterranean flow, and while each portion of the water entering a crevice or porous bed may be able to keep its own mineral matter in solution, their mingling may involve chemical changes resulting in the formation of insoluble compounds, and therefore in deposition. This principle has probably been involved in the filling of many fissures and crevices, converting them into veins. (6) The escape of gases from water, whether from increase of temperature or by the disturbance of water, sometimes causes the deposition of mineral matter held in solution.

The deposition of material held in solution is most notable at two zones, one below that of most active solution, and the other at the surface, where evaporation is active. Under proper conditions, however, deposition may take place at any level reached by water.

Mechanical Work.

The mechanical work of ground-water is relatively unimportant. Wherever it is organized into definite streams, the channels through which it flows are likely to be increased by mechanical erosion as well as by solution. Either beneath the surface, or after the streams issue, the mechanical sediment carried will be deposited.

Fig. 202.—Diagram to illustrate the general form and relations of caves developed by solution. The black portions represent the cavern spaces. Some limestone sinks are represented on the surface above.