The texture of an igneous rock means the size, shape, and mode of aggregation of its constituent mineral particles. Texture is a very important means of determining the circumstances under which the rock was formed, and hence great attention is paid to it. Since texture responds so accurately to the circumstances of solidification, rate of cooling, pressure, etc., all the varieties shade into one another by imperceptible gradations and form a continuous series. Nevertheless, it is necessary to distinguish and name the more important kinds.
Among the igneous rocks are found four principal types of texture, with several minor varieties: -
Here the rock is a glass or slag, without distinct minerals in it, though the incipient stages of crystallization, in the form of globules and hair-like rods, are often observable with the microscope. (See Fig. 24, p. 75.) When the glass or slag is made frothy by the bubbles of escaping steam and gas, the texture is said to be vesicular, scoriaceous, or pumiceous (see Figs. 27, 210), according to the abundance of the bubbles. These are varieties of the glassy texture, though other kinds may also be vesicular. A vesicular rock in which the steam-holes have been filled up by the subsequent deposition of some mineral is called amyg-daloidal, a term derived from the Greek word for almond.
The Compact (Or Felsitic) texture is characterized by the formation of exceedingly minute crystals, too small to be seen by the unassisted eye, giving the rock a homogeneous but stony and not glassy appearance. If the crystals are too minute to be identified even by the aid of the microscope, the rock is said to be cryptocrystalline, and when such identification can be made, it is called micro crystalline.
In rocks of this texture are large, isolated crystals, called phenocrysts, embedded in a ground mass, which may be glassy or made up of small crystals. The phenocrysts may have sharp edges and well-formed faces, or they may have irregular and corroded surfaces. The porphyritic texture indicates two distinct phases of crystallization. The first is the formation of the phenocrysts, which remain suspended in the molten mass, or magma, and are often corroded and partially redissolved (resorbed) by it. These crystals are said to be of intratelluric origin, because formed before the eruption of the lava, and such crystals are showered out of certain active volcanoes at the present time. Stromboli (see p. 75), for example, ejects quantities of large and perfect augite crystals. There is reason to believe, however, that not all phenocrysts are thus intratelluric, but that the first phase of crystallization sometimes takes place after the ejection of the molten mass. The second phase consists in the formation of the ground mass, which may be glassy, finely crystalline, or both.
Mineral particles having distinct crystalline form are called idiomorphic.
In this texture the rock is wholly crystalline, without ground mass or interstitial paste. The component grains, which may be fine or very coarse, are of quite uniform size, and as the crystals have interfered with one another in the process of formation, they have rarely acquired their proper crystalline shape. Such grains are said to be allotriomorphic.
An additional texture which should be mentioned is the frag-mental. This is represented by the accumulations of the frag-mental products ejected by volcanoes (see p. 79), agglomerates, bombs, lapilli, ashes, etc. Many such materials accumulate in bodies of water and are there sorted and stratified and, it may be, mingled with more or less sand and mud and other sedimentary material. Rocks formed in this manner partake of the nature of both the igneous and sedimentary classes, and may be regarded as a series intermediate between the other two and in a measure connecting them. These rocks will here be treated as a special subdivision, under the name of pyroclastic rocks.
Fig. 144. - Slab of polished porphyry, natural size. Phenocrysts of felspar.
In our studies of the products of modern volcanoes, we saw that the same molten mass will give rise to rocks of very different appearance in its different parts, according to the circumstances of rapidity of cooling, pressure, etc. We may now express this in somewhat more general form and say that the texture of an igneous rock is determined by the several factors which affect the molten mass during consolidation. Of such factors may be mentioned the chemical composition, temperature, rate of cooling, degree of pressure, and the quantity present of dissolved vapours and gases, which are called mineralizers.
Fig. 145. - Hand specimen of granite, natural size.