Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Modes of Reaching the Surface.

All of the views that locate the origin of the lavas deep in the earth must face the difficulties of the passage through the dense portion of the sphere below the fracture zone. Near the surface, the lavas usually take advantage of fissures or bedding-planes already existing or made by themselves. There is little evidence that they bore their way by melting, though they round out their ducts into pipes as they use them, much as streamlets on glaciers falling into crevices round out moulins. But this use of fissures and bedding-planes for passage is probably merely a matter of least resistance where the lavas are relatively cool, and their capacity for melting is low or perhaps even gone. Daly has recently urged that lavas work out reservoirs and enlarge passageways for themselves by detaching masses of rock from the roofs and sides of the spaces already occupied by them, these masses either melting and mingling with the lava, or else sinking to lower positions in the column. This process he designates stoping.[287]

In the denser and warmer zone below, the alternatives seem to be (1) melting or fluxing, or (2) mechanical penetration without fracture. As rocks “flow” in this zone by differential pressure without rupture, an included liquid mass may be forced to flow through the zone by sufficient differential pressure. If local differential pressures at the surface be neglected as probably incompetent, there only remain the stress-differences of the interior and the differences of hydrostatic pressure between the lava-column and the surrounding solid columns. The latter would not be great until a column of liquid of much depth was formed, and the former would probably not be concentrated on the liquid in such a way as to force it bodily through the solid rock. Probably fusing or fluxing its way with the aid of stress-differences is the chief resource of the lava in the initial stages. In this it may be supposed to be assisted by its gases, by its selective fusible and fluxing nature, by its very high temperature if it comes from very great depths, as held in the seventh hypothesis, and by the stress-differences which prevail in the deep interior, as shown in the last chapter. In ascending from lower to higher horizons, the lava would be constantly invading regions of lower melting-point because of less pressure. It would thus always have an excess of heat above the local melting temperature until it invaded the external, cool zone, where the regional temperature is below the melting-point of surface pressure. From that point on it must constantly lose portions of its excess of temperature by contact with cooler rocks, and probably in the process of fluxing its way in the compact zone. If this excess is insufficient to enable it to reach the zone of fracture, the ascending column is arrested and becomes merely a plutonic pipe or mass. If it suffices to reach the zone of fracture, advantage may be taken thereafter of fissures and of rupturing, and the problem of further ascent probably becomes chiefly one of hydrostatic pressure, in which the ascent of the lava-column is favored by its high temperature and its included gases. The hydrostatic contest is here between the lava-column, measured to its extreme base, and the adjacent rock-columns, measured to the same extreme depth. The result is, therefore, not necessarily dependent on the flowage of the outer rocks, but may be essentially or wholly dependent on the deep-seated flowage of the rock of the lower horizons. The ascending column may reach hydrostatic equilibrium before it reaches the surface, and may then form underground intrusions of various sorts without superficial eruption, or it may only find equilibrium by coming to the surface and pouring out a portion of its substance and discharging its gases.

Additional Considerations Relative to the Gases.

The question whether the volcanic gases are a contribution to the atmosphere and hydrosphere is so important in its bearings on the whole history of the atmosphere as to merit additional consideration here. As already noted, if the volcanic gases arise from water and absorbed air that have previously passed down through the strata, there is no real contribution to the hydrosphere and atmosphere, but merely circulation. If the gases are chiefly derived from the deep interior, they are an important accession to the atmosphere and hydrosphere.

Most views are more or less intermediate, assigning a part of the gases to the interior and a part to the exterior. No one will question that some part at least of the steam is due to the contact between the ground-waters and the hot lava, and probably no one will question that some gas comes from the interior if the lavas originate there. The vital question is, whence comes the major portion? Are the constant ebullitions of some volcanoes and the terrific explosions of others due mainly to surface-waters, or to interior gases?

It seems to be certain that in most cases the gases are diffused through the substance of the lava, and are not simply in contact with the walls of the column or with its summit. Without doubt steam is generated around the lava-column by external contact, and perhaps some explosions are due to the entrance of the rising lava upon a crevice or cavern filled with water, or to the invasion of a lake gathered in an old crater; but it still remains a question whether the importance of such explosions has not been exaggerated. Such action does not seem competent to produce inflated lavas, but merely shattered ones. Water thus “suddenly flashed into steam” could scarcely diffuse itself intimately through the lava, for the process of diffusion is exceedingly slow. But inflated lavas, pumice, scoriæ, and cinders are the typical products of explosive vulcanism. Not only in the ordinary Vesuvian type, but in the extraordinary Krakatoan type, inflated lavas are the dominant product. Prodigious quantities of this covered the sea about Krakatoa after its tremendous explosion in 1883. Judd estimates that the volume of included steam involved in the inflation of the pumice examined by him, was from three to five times that of the rock, and that the amount involved in exploding the lava into the fine dust that floated in the upper atmosphere for months, was presumably much greater.

If the sudden flashing into steam of bodies of water in external contact with hot lava be rejected as only an incidental source of explosion, it remains to be considered whether waters permeating the rock and becoming converted into steam may not be absorbed into the rising lava, become diffused through it, and ascending with it, explode at the surface. So far as access through fissures and cavities large enough to be entered by lava are concerned, it may safely be concluded that since the hydrostatic pressure of the lava must be greater than that of the water in the fissures, or else it could not rise, the lava will enter them, forcing back the water or the steam generated from it, and, having penetrated as far as accessible, will solidify as a dike, and plug up the avenue of contact between the ground-water and the portion of the lava still remaining molten. The numerous dikes that attend volcanic necks testify to the prevalence of this action. The capillary pores of the wall-rock, which cannot be thus bodily occupied by the lava, must doubtless become filled with steam, and this, following the principles of Daubree’s experiment, will force itself into contact with the lava and be absorbed by it, but whether this will be in sufficient quantity, and will become sufficiently diffused through the body of the lava-column to produce the observed effects, is an open question. The increasing testimony of deep mining is that relatively little water flows through the deeper horizons. It is urged that the water remaining in solidified lavas is very unequal in distribution, as though due to unequal access and partial diffusion. The argument seems strong, but to make it thoroughly good, it must be shown that this inequality is not due to irregularity of discharge of the gases during and after eruption, rather than to irregularity of original accession. There is, perhaps, as much ground for assigning differences in the degree of parting with the included gases, as in acquiring them. Doubtless those lavas that boiled and seethed for a long period in the caldron were more fully deprived of their gases than those that were more promptly disgorged and cooled with less convection and surface exposure.

Thermal considerations.—Probably the most important consideration relates to the heat effects. If underground-waters enter the lava-column and come forth as steam, great quantities of heat are consumed in the process. Has the lava a sufficient excess of heat to stand this? Can ebullition be maintained for the observed periods if the steam comes from ground-waters?

Many lavas probably do not carry a very large excess of heat above that necessary for liquefaction, for not a few of them contain crystals already forming, which shows that they are within the range of the temperatures of solidification of their constituents. The same conclusion is indicated by the limited fusing effects shown by the walls of dikes and sills. On the other hand, as already remarked, dikes and sills often show the effects of a rather rapid cooling from the walls. The method of flow often implies the same condition for the acidic lavas, since they usually behave as stiff, pasty masses of limited liquidity. On the other hand, the basic lavas, whose fusion-point is much lower, often flow freely and reach great distances before solidifying. The facts taken altogether imply that the average temperature of the lavas is not much above the fusing-point of the acidic lavas, while it may probably be very considerably above the fusing-point of basalt. For a rough estimate, with no pretensions to accuracy, it may be assumed that in an average case there are 500° Fahr. excess, but probably not 1000° Fahr. A computation based on even so rough an estimate as this may, by showing the order of magnitude of the thermal considerations, indicate their radical bearing. The average temperature of the ground-water of the upper two or three miles of the crust—the only portion through which water probably penetrates with sufficient freedom to be effective in this case—is probably less than 200° Fahr. The specific heat of rock appears to average somewhat less than 0.2. The temperature of the lava may be taken at 3000° Fahr. as a sufficiently high average. From these data it follows that if an amount of ground-water equal to five percent. of the volume of the lava entered the lava and was brought up to its temperature and then discharged, the temperature of the whole mass would be lowered 550° Fahr. It is therefore evident that only a small percentage of surface-water can pass through the lava consistently with its continued fluidity.