Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The philosophy of this ascensive action, taken as a whole, is simple. In the effort at concentration under the powerful action of the earth’s gravity, the material of high specific gravity is urged more strongly toward the center, volume for volume, than that of less specific gravity, and as gravity is perpetually active, it follows that whenever any movement, molecular or molar, takes place which permits a readjustment of the positions of the two kinds of matter, the heavier sinks toward the center and the lighter rises, or at least tends to do so. So also where there are stress-differences, the mobile matter tends to flow from the regions of greater stress toward those of lesser stress. In so far as any portion of the interior becomes liquid, it is free to move up or down according to the balance of stress brought to bear upon it, and adapts itself to any line of least resistance available to it. As a natural result, therefore, the portion of the interior which becomes fluid most largely participates in the outward movement. In so far as molecular action permits a readjustment of material, there is a tendency, even in the solid state, for the lighter material to move upwards and the heavier downwards, and for the more stressed portions to move toward points of less stress; but this takes place with extreme slowness. In so far as the materials of the interior diffuse themselves through each other, the same laws hold good, but they are modified by the special principles that control diffusion. The outward diffusion of interior gases may be a factor of appreciable importance, but this cannot be affirmed at present.

Phases of vulcanism.—The forcing of fluid rock outward assumes two general phases, which, however, merge into each other; and these main phases take on various sub-phases. The first phase embraces those outward movements of fluid rock which do not reach the surface. The lavas, after ascending to the vicinity of the surface, intrude themselves into the outer formations of the earth and congeal underground (plutonic). The second phase embraces those outward movements in which the fluid rock reaches the surface and gives rise to eruptive phenomena (volcanic). The first is intrusive, the second extrusive; the first constitutes irruptions, the second eruptions.[275] The fundamental nature of the two is the same, but the extrusions usually take on special phases because of the relief of pressure at the surface of the earth, and because of the action of surface-waters in contact with the heated lavas. Just where the lavas come from, and how they find their way through the deep-lying compact zone below the zone of fracture, may better be considered later. When they reach the zone of fracture, they usually either take advantage of fissures already formed, or force passageways for themselves by fracture. There is little evidence that they bore their way through the rocks by melting, though they appear to round out their channels in some way into pipes, ducts, and other tubular forms when they flow through them for long periods of time.

1. Intrusions.

Fluid rock forced into fissures and solidified there forms dikes; forced into chimney-like passages, it forms pipes or plugs; insinuated between beds, it forms sills; bunched under strata so as to arch them upwards, it forms laccoliths; massed in great aggregations underground, it constitutes batholiths, as already described (pp. [394] and [500]). Lavas sometimes crowd aside the adjacent rocks so far as to cause them to take a concentric form about the intruded mass. This is not uncommon in the oldest formations, and is probably not infrequent in the deeper horizons where the pressures are very great. Some part of this may, however, be due to later deformations. Nearer the surface, usually, the beds are merely lifted as in forming the sills, or are bowed upwards, as in the laccoliths, or faulted as in bysmaliths ([p. 500]).

The heating action on the adjacent rock varies greatly with the mass and temperature of the intruded lava. Thin dikes and sills often produce little effect, while greater and hotter masses notably metamorphose the adjacent rock. In some cases marked effects are due to a thin stream of lava flowing through a fissure for a long period, and so maintaining a high temperature. In the least effective cases, the adjacent rock usually shows some signs of baking. In the marked cases, there is more or less new crystallization. The surrounding rock commonly shows some evidence of material derived from the lavas; less often the lava shows some evidence of having received material from the adjacent rock. But since the lavas do not usually bore their way through the strata in the zone of fracture, nor melt the adjacent rock, the constitution of the lavas is not appreciably changed by the kinds of rock which they penetrate. On the other hand, the intrusions often show the effects of rather rapid cooling by contact with the adjacent rock, (a) by a less coarse crystallization near the rock-walls, and sometimes (b) in a segregation of the material.

2. Extrusions.

When molten rock is forced to the surface it gives rise to the most intense and impressive of all geological phenomena. The energies acquired in the interior under great compression here find sudden relief. Occluded gases often expand with extreme violence, hurling portions of the lavas to great heights and shattering them into fragments constituting “smoke,” ash, cinders, bombs, and other pyroclastic material. Much of the explosive violence of volcanoes has been attributed to the contact of surface-waters with the hot rising lava, but the function of this kind of action has probably been exaggerated.

There are two phases of extrusion often quite strongly contrasted. The one is explosive ejection, often attended with great violence; the other, a quiet out-welling of the lava, with little more than ebullition. More or less closely related to these differences are two classes of conduits, (a) the one, great fissures, out of which the lava pours in great volume and spreads forth over wide tracts, often in broad thin sheets; (b) the other, restricted openings, often pipes, ducts, or limited fissures, from which the extrusion is usually much less abundant, and hence it more largely congeals near the orifice, forming cones. Flows from the former constitute massive eruptions; those from the latter, the more familiar volcanic eruptions. There is no radical difference between them, and the two classes blend. The extent of the spreading of lava into thin sheets is due more to the mass and the fluidity than to the form of the outlet. The stupendous outflows of certain geologic periods appear to have issued mainly from extended fissures, doubtless because these better accommodated the outbursting floods.

a. Fissure eruptions.—The chief known fissure eruptions of recent times are the vast basaltic floods of Iceland. Most of the eruptions of historic times are of the volcanic type; but at certain times in the past there were prodigious outpourings, flow following flow until layers thousands of feet thick covering thousands of square miles were built up. One of these occurred in Tertiary times in Idaho, Oregon, and Washington, where some 200,000 square miles were covered with sheets of lava, aggregating in places 2000 feet or more in thickness. Earlier than this, in Cretaceous times, there were enormous flows on the Deccan plateau of India, covering a like area to a depth of 4000 to 6000 feet. Still earlier than this, in Keweenawan times, an even more prolonged succession of lava-flows covered nearly all the area of the Lake Superior basin, and extended beyond it, and built up a series of almost incredible thickness, the estimates reaching 15,000 to 25,000 feet. In these cases there is little evidence of explosive or other violent action. There are few beds of ash, cinders, and similar pyroclastic material. The inference is, therefore, that the lavas welled out rather quietly and spread themselves rather fluently over the surrounding country. For the most part these wide-spreading flows are composed of basic material, which is more easily fusible and more highly fluent at a given temperature than the acidic lavas. The latter are more disposed to form thick embossments near the point of extrusion.

Massive outflows of this class constitute by far the greatest phenomena of the extrusive type, though they are not now the dominant type. It has been sometimes thought that the more local volcanic type of extrusion followed the more massive fissure type as a phase of decline; but this has not been substantiated.