Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

In this case it is not necessary to assume any special accession of heat, but merely to account for extrusion. There are two phases of this view, (1) the one postulating a general molten interior, (2) the other limiting the molten matter to local reservoirs.

Hypothesis I. Lava outflows from a molten interior.—In the early days of geology, when the earth was supposed to have a thin crust and a molten interior, it was very naturally assumed that volcanoes were but pipes leading down to the molten mass within. This view has been essentially abandoned. The independence of adjacent vents is in itself almost a fatal objection, when it is recalled that the height of recent volcanic craters ranges from nearly 20,000 feet above the sea, to 10,000 to 20,000 feet below. The view would involve the conception of lava-columns connected with a common reservoir varying possibly 30,000 to 40,000 feet in altitude, and certainly more than half that much, simultaneously. The lower outlets should as certainly be selected for the outflow of the great interior sea of fluid rock, as the lowest sag in the rim of a lake for its outflow, for no great differences in specific gravity are presumable under this hypothesis. An equally grave objection arises from tidal strain. If the earth were liquid within and merely crusted over by a shell of rock of moderate thickness, it would yield appreciably to tidal stresses, and this yielding would change the capacity of the interior so that with every distortion of the spheroid a portion of its fluid interior would be forced to the outside, and with every return to the more spheroidal form there would either be a re-flow to the interior or a shrinking of the crust. In any case a very demonstrative response to tidal influence would tell the story of interior fluidity. No such effects are observed. The tidal strains may perhaps have a slight effect in hastening a given eruption when the forces are approaching a delicate balance and an eruption is imminent, but the very triviality of this influence implies not only the absence of a general liquid interior, but also of extensive reservoirs.

Hypothesis 2. Lavas assigned to molten reservoirs.—A modification of the preceding view has been made to escape the difficulties involved in the hypothesis as stated above. It is supposed that while nearly all the subcrust solidified, numerous liquid spots were left scattered through it. This honeycombed substratum is supposed to connect the continuous outer crust with a central solid body, solidified because of pressure in spite of its high temperature. This hypothesis escapes only a portion of the objections. For instance, the lavas in Mauna Loa and Kilauea in Hawaii differ nearly 10,000 feet in height, and hence cannot well be supposed to connect with the same reservoir, but they are both on the same vast cone, which implies at least an equally large molten reservoir as its source. If there were two distinct reservoirs of the required magnitude, they must be singularly placed to supply vents so near and yet so independent. The difficulty grows greater when the whole Hawaiian chain is considered, for the points of eruption seem to have migrated from the northwesterly islands, where the volcanoes are old, to the southeastern end, where volcanic activity is now in progress.

It would be natural under this view to suppose that these residuary lakelets of liquid rock should be gradually exhausted as time goes on, and that vulcanism should be a declining phenomenon. It is not clear that this is the case. The great number of existing volcanoes in regions where great extrusions took place in earlier ages does not seem to be in harmony with the hypothesis.

II. On the Assumption that the Lavas are Secondary.

The serious difficulties that arise in interpreting volcanic lavas as remnants of an original molten mass, and the strong arguments of recent years for a very solid earth, have turned inquiry chiefly toward the second class of hypotheses, which refer the origin of lavas to the local melting of deep-seated rock. These differ widely among themselves. One group seeks for a cause of the melting in the penetration of surface air and water; another, in the relief of pressure; a third, in crushing and shearing; a fourth, in the depression of sediments into the heated interior zone; and a fifth, in the outward flow of deep-seated heat.

Hypothesis 3. Lavas assigned to the reaction of water and air penetrating to hot rocks.—As steam is one of the great factors in the explosions of volcanoes, and as water reduces the melting-point of rocks, it is a natural and simple view that water penetrating through the fissures and pores of the outer crust and coming into contact with the heated rocks below, is absorbed into them and renders them liquid, and that then, being rendered swollen and lighter by the process, they ascend and discharge quietly or explosively according to the special conditions of the case. Naturally the suggestion arises that the waters would be converted into steam long before they could reach rock hot enough to be melted, and that this steam would be forced back along its own track, as the line of least resistance, rather than force itself into the rock material and rise in the lava-column; but to this it is answered that an experiment of Daubree’s has shown that water will penetrate the capillaries of sandstone against high steam pressure and add itself to the steam within. The fact is also cited that certain substances, when highly heated, absorb gases which they give out when they cool. The absorption of hydrogen by platinum, and of oxygen by molten silver, are illustrations. It is certain that the lavas do contain large quantities of absorbed gases, and that these are partly, and in most cases largely, given out in cooling, when the cooling takes place at the surface. The presumption is that the lavas would take the gases up again on remelting under similar conditions. If the lavas of actual volcanoes had the temperatures of aqueo-igneous fusion (700°–1000° Fahr.) only, it would strengthen this view; but as temperatures of lavas often exceed 2000° Fahr., and probably sometimes reach 2500° Fahr., and perhaps 3000° Fahr., it is not easy to account for such temperatures under this hypothesis, because they would only be reached at levels far below those at which aqueo-igneous fusion might be presumed to take place. Perhaps this could be met by invoking pressure which might prevent even aqueo-igneous fusion from taking place until these temperatures were reached, but pressure brings in a grave difficulty in another line, as we shall presently see.

There is a phase of the water-penetration hypothesis which seeks to account for an accession of heat. It is affirmed that the outer rocks are oxidized, while the inner ones were not originally, or at least not completely oxidized, and that air and water from the surface, reaching the unoxidized zone, enter into combination and generate the necessary heat. This view was pardonable before the development of modern thermo-chemistry, but is now quite untenable, as may be shown by working out the reactions thermally.

All views which assign the penetration of surface air or water as a cause meet with a grave, if not insuperable, difficulty in the condition of the lower part of the earth’s crust (see [p. 218]). The fractured condition of the crust, which permits a ready penetration of water, is a very superficial phenomenon. Below the first few thousand feet the crevices and porosities of the rock are rapidly closed by the pressure of overlying rock, and all appreciable crevices and pores probably disappear at a depth of five or six miles. The effective function of fissures is, therefore, limited to the upper few miles of the crust, and even here to certain portions only. The great pressures in gas- and oil-wells show that in many quite superficial beds, even when arched, there are no fissures or pores capable of letting even gas escape effectively. The depths at which the temperatures of lavas are reached are usually estimated, from the downward increase of temperature, at 20 to 30 miles. This leaves from 14 to 24 miles of the compressed zone between the lowest assignable limit of the fissured zone, and the highest assignable zone for the origin of lavas. This thick zone of dense rock must be reckoned with in all hypotheses that involve the penetration of air and water from without, and, as well, the extrusion of lavas from within. In addition to the difficulties of the penetration of ground-water, the limitations of its heat, at penetrable depths, also bear adversely (see [p. 219]), on the descent of air and water.

Hypothesis 4. Lavas assigned to relief of pressure.—It seems to be demonstrated that pressure raises the melting-point of average rock, and hence at twenty or thirty miles’ depth there may be rocks hot enough to melt at the surface, but still solid because of high pressure. If this pressure were in some way relieved they would become liquid. Pressure may be locally relieved somewhat (1) by denudation, (2) by certain phases of faulting, (3) by anticlinal arches, and (4) by continental deformation.