STRUCTURAL FEATURES OF IGNEOUS ROCKS.
Certain structural features of igneous rocks have been mentioned in treating of their origin in the previous chapter. When a great flow of lava spreads out upon the surface, there is no internal lamination or stratification, and the resulting rock is usually classified as massive rather than stratified; but when a succession of flows occur, each individual flow forms a layer, and the series as a whole becomes stratiform. The successive flows are not usually coextensive. If the later flows of the closing stages of a period of vulcanism fail to reach as far as the earlier ones, a terraced or step-like aspect is given to the region, whence the name trap-rock (trappe, steps) is derived. Such lava sheets, especially if of basalt, often assume a columnar structure in cooling, the columns being rude six-sided prisms standing at right angles to the cooling surfaces (Figs. [379] and [380]). This phenomenon is usually best developed where the sheet is intruded between layers of preexisting rock in the form of sills. The formation of the columns is sometimes regarded as a variety of concretionary action, but more commonly as a result of contraction. The former is suggested by the ball-and-socket ends of the sections of some columns ([Fig. 382]). The development of the columns by contraction may be explained as follows: The surface of the homogeneous lava contracts about equally in all directions on cooling. The contractile force may be thought of as centering about equidistant points. About a given point, the least number of cracks which will relieve the tension in all directions is three ([Fig. 383]). If these radiate symmetrically from the point, the angle between any two is 120°, the angle of the hexagonal prism. Similar radiating cracks from other centers complete the columns ([Fig. 384]). A five-sided column would arise from the failure of the cracks to develop about some one of the points ([Fig. 385]).
Fig. 380.—Columnar structure, obsidian cliff, Yellowstone Park. (Iddings, U. S. Geol. Surv.)
When lava is forced into crevices or rises to the surface through fissures, and the residual portion solidifies in them, it gives rise to dikes, as illustrated in Figs. [2] and [417] (not a true dike). Dikes are sometimes affected by columnar structure. In this case, as in all others, the columns are likely to be at right angles to the cooling surface. Lava solidifying in the passageway leading from the interior of a volcano gives rise to a neck or plug. If the lava is forced between beds of rock in the form of a sheet, and solidifies there, it is called a sill. If, after rising to a certain point in the strata, the lava arches the beds above into a dome, and forms a great lens-like or cistern-like mass, it constitutes a laccolith ([Fig. 334]). If an intrusion of the laccolithic type faults the overlying beds instead of arching them, and especially if the vertical dimension of the intruded mass be great in comparison with its lateral dimensions, its shape is more like that of a plug or core. Such an intruded core is a bysmalith[213] ([Fig. 124]). Between the bysmalith and the laccolith there are various gradations, just as between the laccolith and the sill. When lava forces aside the rocks at considerable depths or absorbs them by solution or by “stoping,” and then solidifies in great masses of irregular or undetermined forms, these masses are called batholiths.
Fig. 381.
Fig. 382.
Fig. 383.
Fig. 381.—Sections of columns from Giant’s Causeway, coast of Ireland.
Fig. 382.—Ball-and-socket joints in columns of basalt. (Scrope.)
Fig. 383.—Diagram to illustrate the first stages in the formation of hexagonal columns by contraction.
Fig. 384.
Fig. 385.
Fig. 384.—The completion of the hexagonal columns.
Fig. 385.—Diagram to illustrate the development of five-sided columns.
Volcanic cones are familiar structures built up about the vents of active volcanoes, and will be discussed under vulcanism.