1911 Encyclopædia Britannica/Basalt

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BASALT, in petrology, one of the oldest rock names, supposed to be derived from an Ethiopian word basal, signifying a stone which yields iron; according to Pliny, the first basalts were obtained in Ethiopia. In current usage the term includes a large variety of types of igneous rock belonging to the basic subdivision, dark in colour weathering to brown, and comparatively rich in magnesia and iron. Some basalts are in large measure glassy (tachylites), and many are very fine grained and compact; but it is more usual for them to exhibit porphyritic structure, showing larger crystals of olivine, augite or felspar in a finely crystalline groundmass. Olivine and augite are the commonest porphyritic minerals in basalts, the former green or yellowish (and weathering to green or brown serpentine), the latter pitch-black. Porphyritic plagioclase felspars, however, are also very common, and may be one or two inches in length, though usually not exceeding a quarter of an inch; when fresh they are dark grey with smooth lustrous cleavage surfaces; when decomposed they become turbid, and assume grey or greenish shades. Basaltic lavas are frequently spongy or pumiceous, especially near their surfaces; and, in course of time, the steam cavities become filled with secondary minerals such as calcite, chlorite and zeolites. Another characteristic of this group of rocks is the perfection with which many of them show prismatic or columnar jointing, a structure often called "basaltic jointing."

The minerals of basaltic rocks have a fairly uniform character throughout the whole group. In microscopic section the olivine is pale green or colourless, and is very frequently more or less altered to serpentine. The secondary mineral begins to form upon the surfaces and along the cracks of the olivine, gradually producing a mesh-work in the interstices of which small kernels of olivine remain; and when the process is completed the mesh structure persists in the resulting pseudomorph, giving a clear indication as to its history. The augite is mostly brown, often with a purplish tinge, hardly at all dichroic, but frequently showing zonal or hour-glass structure, and various types of twinning. It weathers to chlorite, uralite, calcite, &c. The plagioclase felspar, if fresh, is transparent and appears simple in ordinary light, but when polarized breaks up into a series of bars of different colours owing to its complex twinned structure. Practically all varieties of this mineral from anorthite to albite are known to occur in basalt, but by far the commonest species are bytownite and labradorite. Weathering destroys the limpid character of the fresh mineral, producing turbid pseudomorphs containing epidote, calcite, white micas, kaolin, &c. When these minerals occur as phenocrysts their crystalline outlines may be very perfect (though, especially in the olivine, corrosion and partial resorption may give rise to rounded or irregular forms).

In the groundmass, or second generation of crystal, not only are the ingredients smaller, but their crystals are less perfect; yet in many basalts small lath-shaped felspars and minute prisms of augite, densely crowded together, form the matrix. With these there may be a greater or less amount of brown, isotropic glass. Olivine rarely occurs as an ingredient of the groundmass. In the vitreous basalts sometimes very few crystallized minerals are observable; the greater part of the rock is a dark brown glassy material, almost opaque even in the thinnest sections, and generally charged with black grains of magnetite, skeleton crystals of augite or felspar, spherulites, perlitic cracks, or steam vesicles. In other basaltic rocks no glassy material appears, but the whole mass is thoroughly crystallized; rocks of this nature are generally known to British petrologists as dolerites (q.v.). Till recent years it was widely believed by continental geologists that the pre-Tertiary basalts differed so fundamentally from their Tertiary and recent representatives that they were entitled to be regarded as a distinct class. For the older rocks the names anamesite, diabase porphyrite, diabas-mandel-stein, or melaphyre were used, and are still favoured by many writers, to indicate varieties and states of more or less altered basalts and dolerites, though no longer held to differ in any essential respects from the better preserved basalts. Still older is the term trap, which is derived from a Swedish word meaning "a stair," for in many places superposed sheets of basalt weather with well-marked step-like or terraced features. This designation is still used as a general term for the whole suite of basaltic rocks by many geologists and travellers (e.g. trap-dikes, the "traps" of the Deccan).

In the early years of the 19th century a great controversy convulsed the geological world as to the origin of the older basalts or "floetz-traps." Werner, the Saxon mineralogist, and his school held them to be of aqueous origin, the chemical precipitates deposited in primeval seas, but Hutton and a number of French geologists maintained that they were really volcanic rocks emitted by craters now extinct (see Geology: Historical).

Of the less common minerals of basalt, a few may be mentioned. Black hornblende, dark brown in thin sections, and often corroded, is not uncommon, especially in intrusive basalts. Hypersthene occurs also, usually replacing olivine. Black mica (biotite) is not infrequently to be seen. Sapphire, garnet and zircon are rare. Minerals of the felspathoid group occur in a large number of basaltic rocks; nepheline and leucite are the most common, but haüyne is occasionally present. If nepheline entirely replaces felspar, the rock is known as nepheline-basalt; if the replacement is only partial the term nepheline-basanite is used. Similarly there are leucite-basalts and leucite-basanites. The nepheline is in small six-sized prisms, and usually cannot be detected with the unaided eye. Even with the help of the microscope nepheline basalts are not always easy to determine, as the crystals may be exceedingly small and imperfect, and they readily decompose into analcite and zeolites. In some cases only the presence of an anisotropic substance, with weak double refraction and readily attacked by acids (the so-called "nephelinitoid"), can be made out. This substance may be imperfectly crystallized nepheline, or a peculiar glass which is rich in soda. Most nepheline basalts are fine grained, very dark coloured rocks, and belong to the Tertiary period. They are fairly common in some parts of Germany and occur also in Tripoli, Asia Minor, Montana, Cape Verde Islands, &c. Leucite-basalts contain small rounded crystals of leucite in place of plagioclase felspar. Rocks of this group are well known in the Eifel, and other volcanic districts in Germany, also in Bohemia, Italy, Java, Montana, Celebes, &c. The minerals haüyne, nosean, sodalite and melilite tend to occur with some frequency in nepheline and leucite-basalts, though rare in ordinary basalts. Melilite, a lime-alumina-silicate, is characteristic of certain very basic rocks, the melilite-basalts. It is pale yellow or colourless in thin sections, and yields peculiar and characteristic dark blue polarization colours. This rare group of rocks is known to occur in Bohemia, Swabia and South Africa. Perofskite, in small dark brown cubic crystals, is a constant accessory in these rocks. The augite is usually violet coloured, and shows zonal and hour-glass structures. Green augite may occur in the nepheline-basalts, and aegerine (soda-iron-augite) is occasionally found in them.

The distribution of basalts is world-wide; and in some places they occur in immense masses, and cover great areas. In Washington, Oregon, and Idaho many thousands of square miles are occupied by basaltic-lava flows. In the Sandwich Islands and Iceland they are the prevalent lavas; and the well-known columnar jointed basalts of Skye, Staffa, and Antrim (Giant's Causeway) form a southward extension of the Icelandic volcanic province, with which they are connected by the similar rocks of the Faeroe Islands. In the Deccan in India great basaltic lava fields are known; and Etna and Vesuvius emit basaltic rocks. In older geological periods they were not less common; for example, in the Carboniferous in Scotland.

(J. S. F.)