Rocks once formed in any of the above ways are being constantly altered in character by the various processes of nature. Those exposed on the surface are weathered to pieces, and the fragments are transported by wind or water to accumulate elsewhere as sedimentary rocks. Those buried deep beneath the surface are affected by the high temperature and pressure of the depths of the earth and thus metamorphosed. For instance a granite exposed on the surface is slowly weathered, some parts being carried away in solution by the rain water, others less soluble remaining as grains of quartz, mica or kaolin. These are transported by water and sorted, the finer kaolin being carried to still and deep water, the quartz and mica accumulating in some lowland as sand. This sand will in time be cemented to a sandstone, later slowly buried beneath the surface. If buried deep it will feel the effect of the interior temperature, which increases as one goes down at the rate of one degree F. for every 50 feet. If this should be in a region where folding and mountain-making takes place, the material under the folds would be melted (because of the relief from pressure which would permit the high temperature to act freely) and become igneous rock, either coming to the surface as lava, or remaining below the surface and making a granite or similar rock; while the sedimentary material not melted but near enough to the molten material to be affected, would be metamorphosed, in this case to a quartzite. Much of the interest and profit in studying rocks, will come from the understanding which they will give as to the history of that particular part of the earth’s crust where they are found.
Igneous Rocks
Igneous rocks are those which have formed from material that has been melted, which involves temperatures around 1300° C.; or, if there is water in the original material, temperatures as low as 800° C. will suffice. Considering the increase of temperature to be a degree for every 50 feet downward, this involves the rocks having been at depths of 5 to 10 miles below the surface. While at such depths the temperature must be high enough to melt rocks, the great pressure of the overlying rocks seems to keep them solid; for we know that the center of the earth is solid, as is shown by a variety of observations, such as the rate at which earthquake waves are transmitted through the earth, the lack of tidal effects, etc. However, there is every reason to believe that if the pressure is removed from the rocks which are five to ten miles below the surface, there is heat enough at those depths to melt them. When the crust of the earth is folded, as when mountain ranges are formed, the areas under the arches or upward folds are relieved of pressure. Then those rocks, which are under the arches and are relieved, become molten. The molten magma may well up and fill the space beneath the arch where it would cool again very slowly; or, if there is fissuring during the folding, some of the molten material may be forced out through the fissures and pour out over the surface as lava. Another area in which pressures may be locally relieved is in the region of faulting, where the crust of the earth is broken into blocks, between which there are readjustments, some being tipped one way, some another, some uplifted. Here again there would be areas of relieved pressure and molten magmas would form, some of them solidifying in place, others rising to the surface.
The molten material is termed the magma, and when it reaches the surface, great quantities of water vapor and other gases escape: or these gases may even escape from magmas which do not reach the surface, rising through fissures. As these hot vapors pass through the fissures, they are cooled, and may deposit part or all of their dissolved compounds in the fissure, making veins. Lava is the magma minus the vapors. Magmas vary greatly from place to place, indicating that they are formed locally and do not come from any general interior reservoir, as has sometimes been suggested.
When the molten magmas escape to the surface, they are termed extrusive, and as they spread out in a layer this is termed a sheet. This rise and overflow may be quiet, and from time to time one outpouring may follow another making sheet after sheet. Or after one outpouring, the pressure below may cease for a time and allow the lava to solidify and make a cap or cover over the opening. Before more lava can rise, this cover must be removed. This usually happens in an explosive manner, the lava below, with the increasing pressure exerted by its expanding gases, finally exerting enough pressure, so that the cover is broken, or shattered and thrown in thousands of fragments into the air, as happened at Mt. Pelée on the Island of Martinique in 1902. The fragments thrown into the air are often termed volcanic ashes, though this is not a good word for them, for they have not been burned.
In case the molten magmas under the relieved areas do not reach the surface they are termed intrusive. Such magmas may remain in the space under a mountain fold, or be forced in fissures part way to the surface. When the magma is forced into more or less vertical cracks and there solidifies, and these are exposed by erosion, they are termed dikes. Sometimes the magmas have risen part way to the surface and then pushed their way between two horizontal layers of rock and there hardened, in which case they are termed sills, when uncovered. The Palisades along the Hudson River are the exposed edge of a sill. Again the molten magmas may well up and spread between two horizontal layers, but come faster than they can spread horizontally, and then the magma takes the form of a half sphere, and the overlying layers of rock are domed up over it. Such a mass is termed a laccolith. In all these cases the mass of igneous rock is only discovered when the overlying rocks have been eroded off. The great mass of molten magma under the arches of mountain ranges simply cools slowly into a granitic type of rock. These masses are exposed when the thousands of feet of overlying rock are eroded off. When these masses are exposed, if of but a few miles in extent, they are called stocks, but, if of many miles in length and breadth, they are batholiths, and are very characteristic of the heart of mountain ranges.
In all the above cases the exterior of the molten mass cools first, and forms a shell around the rest. The shell determines the size of the mass. As the cooling continues into the interior, it also solidifies, and as all rocks shrink on cooling, cracks develop, separating the mass into smaller pieces. There is usually no regularity about these cracks and the mass is divided into blocks from six inches to three feet in diameter. However, in some cases, especially in sills and laccoliths where the cooling is slower, the shrinkage may be marked by a regular system of cracks which bound the rock into more or less regular hexagonal columns. The Palisades and the Devil’s Tower in Wyoming (See [Plate 52]) show this structure. The Devil’s Tower is the remnant of a laccolith, all except the central core of which has been eroded away. All of the above terms have nothing to do with composition, but refer entirely to the manner of occurrence.
While the igneous rocks are classified according to their composition, the rate at which they cooled has much to do with their texture, and certain names apply to the texture. For instance when the molten lava cools very rapidly, there is no time for the formation of crystals, and the resulting rock is glassy or non-crystalline. If the cooling is slow as in large bodies, crystals have time to form and grow to considerable size as in granites. Between these all grades may occur; and one classification of igneous rocks expresses their rate of cooling, in such terms as the following.
Glassy—lavas which have cooled so quickly that they are without distinct crystallization, such as obsidian, pitchstone, etc.
Dense or felsitic—lavas which have cooled less rapidly, so that crystals have formed, but in which the crystals are too small to be identified by the unaided eye, such as felsite or basalt.