Igneous Rocks, as the name indicates, are those types which have been formed by the cooling and consequent solidification of a once hot and fluid mass of rock material. The name means "fire formed". This liquid mass is known as a rock magma. This magma, in a measure, is like a solution containing in a dissociated condition the elements which, when the mass cools sufficiently, unite to form the various minerals that go to make up the resulting rock. The elements which form the chief constituents of the magmas of igneous rocks are oxygen, silicon, aluminum, iron, calcium, magnesium, sodium and potassium, named in the order of their abundance within the earth’s crust.
When a magma cools the elements within it unite to form various mineral molecules, which, when the point of supersaturation is reached, crystallize out to form the minerals of the rock. Certain compounds under similar conditions crystallize out of the fluid mass earlier than do others. In most igneous rocks there is a more or less definite order of crystallization for their mineral constituents which through study of them can be determined. In general, the more basic minerals or those which contain the smaller amounts of silica, which is the acid element in igneous rocks, are observed to crystallize from the melt first and the more acid minerals last. Among the commoner rock-making minerals the following would be the usual order of crystallization; iron oxides like magnetite first, then the ferro-magnesian minerals like pyroxene, next the plagioclase feldspars, then orthoclase and lastly quartz.
The type of minerals to be found in any igneous rock would depend chiefly upon the chemical composition of the original magma. If the magma was acid in character, i.e., had a high percentage of silica, the resulting rock would contain the more acid minerals and an abundance of free quartz. It would usually be light in color. If, on the other hand, the magma had a low percentage of silica, or in other words was basic in character, the resulting rock would contain the more basic minerals and would not show free quartz. It would also in general be dark in color.
In addition to the wide variation in. chemical and mineral composition shown by igneous rocks there is also a variation in their physical structure. This is dependent upon the mode of origin of the rock. If a rock has been formed from a magma buried at a considerable depth in the crust of the earth it must have cooled very slowly and taken a long period of time for its gradual crystallization and solidification. Under these conditions the mineral particles have had the opportunity, because of the slowness of crystallization, to grow to considerable size. A rock having such a deep-seated origin has, therefore, a coarse grained structure and the various minerals that go to form the rock can in general be differentiated and recognized by the unaided eye. Such rocks are commonly termed plutonic.
On the other hand, if, by volcanic forces, the magma has been extruded upon the surface of the earth or intruded in the form of dikes into the rocks lying close to the surface, its subsequent cooling and solidification go on quite rapidly. Under these conditions the mineral particles have little chance to grow to any size and the resulting rock is fine-grained in character. In some cases, indeed, the cooling has been too rapid to allow the separation of any minerals and the resulting rock is like a glass. Ordinarily the mineral constituents of such a rock are only to be definitely recognized by a microscopic examination of a thin section of the rock. Such igneous rocks are known as volcanic rocks.
An igneous rock, because of the mode of its formation, consists of crystalline particles which may be said to interlock with each other. In other words, it is a solid mass, and each mineral particle is intimately and firmly embedded in the surrounding particles. This structure will enable one ordinarily to distinguish an igneous from a sedimentary rock, the latter being composed of grains which do not interlock with each other but stand out, more or less, by themselves. A sedimentary rock is not as firm and coherent as an igneous rock. Further the texture of an igneous rock is the same in all directions and it forms a fairly uniform and homogeneous mass. This characteristic will enable one to distinguish an igneous from a metamorphic rock, since the latter shows a more or less definite parallel arrangement of its minerals and a banded structure.
Because of the almost infinite variation possible in the chemical composition of their magmas, and because of the various conditions under which they may form, igneous rocks show likewise a wide variation in character. The more common and important types, however, are very briefly described below.
The Granite Family
The Diorite Family
The Gabbro Family
The Diabase (Dolerite) Family
The Peridotite Family
The Serpentine Group
The Rhyolite Family
The Andesite Family
The Basalt Family
Pegmatite Dikes and Veins
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