Igneous Rocks (also known as eruptive rocks, and mostly, when classified as to texture, included among massive crystalline rocks) form, with aqueous and metamorphic rocks, the three main groups into which all rocks are classified with reference to mode of origin. The name, suggesting origin in fire, is unfortunate, but they have all originated in the heated interior of the earth, and have consolidated by cooling from a molten, or at least pasty, condition.
In this respect they may be subdivided into three series - plutonic rocks, lavas, and tuffs. Plutonic or hypogene rocks are those that have consolidated ifar below the surface, and consequently under great pressure and at a slow rate, thus' becoming very perfectly crystalline. Lavas are rocks which, having been poured out by volcanic action, have cooled at or near the surface, and therefore comparatively rapidly and with imperfect crystallisation, being sometimes glassy, slaggy, or, as it is termed, vitreous. This group of volcanic rocks, however, passes by gradations into the plutonic series, some of the more compact, massive, and crystalline lavas that occur in thick sheets being formerly separated as an intermediate group known as trap or trappcan rocks, from the Swedish trappa, a stair, from their step-like outcrops. Tuffs are fragmentary volcanic rocks, formed by the dust and scoria of volcanoes, fragments blown from molten lava by explosive action, cooled separately, generally with a texture vitreous rather than crystalline, and sometimes compacted into a cement-like sedimentary rock, such as the black volcanic mud now accumulating in the Bay of Naples.
Chemically, igneous rocks consist of silicate of alumina, with smaller proportions of silicates of magnesia, lime, potash, and soda, usually with some oxide of iron and phosphate of lime, and with or without an excess of free silica crystallising as quartz (q.v.). From this point of view we obtain the most satisfactory primary grouping of the massive members of the group (i.e. the plutonic rocks and lavas), based mainly upon the percentage of silica (formerly known as silicic acid) which they contain, into four sections, acid, intermediate, basic, and ultra-basic. Acid rocks contain more than 66 per cent. of silica, some of which occurs as quartz, and their specific gravity is about 2-5; the intermediate rocks range in silica percentage from 55 to 65, but are without quartz, and have a specific gravity of about 26; the basic rocks contain 46 to 55 per cent. of silica, with considerable percentages of magnesia, lime, and iron, are heavy, having a specific gravity of 2-7 to 3, and are readily fusible; and the ultrabasic rocks contain less than 46 per cent, of silica, and have a specific gravity generally exceeding 3.
Mineralogically igneous rocks consist mainly of felspar (q.v.), orthoclase felspar with quartz being characteristic of the acid section, but only plagioclase occurring in the basic section, and no felspar at all, as a rule, in the ultra-basic. Hornblende (q.v.) is characteristic of the intermediate section, associated either with orthoclase or with plagioclase. Augite (q.v.) and olivine (q.v.), associated with the more basic felspars, such as labradorite, with magnetite, ilmenite, and often apatite, characterise the basic section; and olivine, pyroxenes, magnetite, and chromite, without felspar, charac- terise the ultra-basic. In some few cases felspar is replaced by nepheline, leucite, olivine, or serpentine.
The acid section includes the crystalline granites, eurites, felsites, and liparites, the latter being always volcanic, and the glasses obsidian, pitchstone, perlite, and pumice. The intermediate section comprises the plutonic syenites and diorites, and the volcanic trachytes, andesites, and phonolites. The basic section includes the basalts, diabases, and gabbros; and the ultrabasic, the peridotites, picrites, and serpentinites. Most of these rocks are separately described.