The rhyolites are the volcanic equivalents of the granites, or, in other words, rocks produced by the consolidation of a granite magma under volcanic conditions. They are highly acid, containing, in most cases, free silica in the form of quartz. Having been poured out as sub-aerial or submarine lavas, they usually present marked fluidal, spherulitic and perlitic structures. Occasionally they are compact, but more frequently they have a porphyritic structure. The phenocrysts are quartz and feldspar, imbedded in a light-colored trachytic groundmass, which is mainly composed of microlites of feldspar.
Prisms of augite and hornblende, plates of biotite and granules of magnetite also occur. A glassy base is present in variable proportion, according to the conditions that prevailed during cooling. If the proportion of glass is so large that the crystals are subordinate, the rocks are known as obsidians.
Accessory minerals are magnetite, tridymite, apatite, cordierite, garnet, sphene and allanite. The quartz is sometimes bi-pyramidal, but oftener without definite shape. Though usually pellucid and colorless, it has frequently a dark-colored or smoky hue. In some rhyolites the conditions of eruption have been such as to prevent the separation of quartz; but the high percentage of silica shown by chemical analysis facilitates the correct diagnosis of such rocks. The feldspar is usually sanidine a glassy variety of orthoclase. It occurs frequently in large tabular crystals, which are glassy, clear and much fissured; but also in smaller and less regular grains. In addition to the orthoclase, there is often present a triclinic feldspar. This is sometimes oligoclase, but more frequently albite, soda-microcline or anorthoclase.
In some rhyolites, the sole porphyritic constituent is a soda-feldspar, having the same glassy habit as the potash -feldspar (sanidine). According to the predominance of potash-feldspar or soda-feldspar, the rhyolites may be divided into potash-rhyolites and soda-rhyolites. The latter comprise the more acid pantellerites. Fluxion -structure is often shown in the groundmass by the beautiful wavy lines of flow which sweep round the larger embedded grains. The same structure is also occasionally brought out under the microscope by the strict parallelism of the feldspar microlites, or by bands of crystallites and trichites in the glassy rhyolites and obsidians.
The identity of many so-called felsites with the rhyolites has been demonstrated by the researchers. Many of these rocks, consisting of a cryptocrystalline aggregate of quartz and feldspar (felsitic matter), in which porphyritic crystals of quartz and feldspar are often embedded, were found to possess traces of perlitic, spherulitic and fluidal structures. These structures, being characteristic of rocks that have consolidated in a vitreous or semi-vitreous condition, indicated the true nature of the rocks that possess them. They are, in fact, ancient flows of rhyolitic ash flow tuffs that have subsequently welded and acquired a cryptocrystalline structure by a gradual process of devitrification.
Many rocks initially suspected to be rhyolites when first seen in the field are later demonstrated under examination in the laboratory to be welded rhyolitic ash flow tuffs that were erupted explosively. Tuffs are rock composed of fragments of volcanic origin which have been thrown out during an eruption, and have consolidated into rock around the volcano. Like the rhyolites, they contain porphyritic crystals of both unstriped and striped feldspar, and their chemical composition shows that they may be separated into a potash type and a soda type. In expression of their ask flow tuff nature, these rocks are found inter-bedded with sedimentary strata, in association with other tuffs and breccias, which were doubtless a product of the same volcanic activity. A nodular character is very common in rhyolitic lavas. The nodules vary in size from a small marble to a man's fist; in some cases they are even larger. Probably in many instances the nodules are enlarged spherulites; but they have also been ascribed to contraction on cooling as in true perlitic structure, and to the infilling of original vesicles with quartz and agate by infiltration.
Because of their very fine-grained structure, many volcanic rocks cannot in general be readily told apart without a microscope. A number of different types are recognized, the distinction between them being based, however, chiefly upon microscopic study. In the field only an approximate classification, depending upon whether the rock is light or dark in color, can be made. Light colored roacks often go by the term “felsic rocks”. They normally and commonly show light shades of color; white, which is not very common, light to medium gray, light pink or red to dark red, pale yellow or brown, purple or light green. With the lens it can be frequently seen that they consist of minute mineral grains, too small for reliable determination, and the texture is then very fine granular. In other cases the grains may be entirely too fine to be seen; the rock has then a dense, horn-like or flinty aspect, appearing like a homogeneous substance. In this latter case it is very apt to have a smooth conchoidal fracture. In other cases, especially in surface lavas, the texture is more or less porous and the fracture surface of the rock rough and hackly, with a harsh feeling.
A pronounced cellular or vesicular structure, common in basalt and in glassy rocks, is not very common in this group. The surface lavas not infrequently show fluidal bandings and streaks, more or less flat lenticular, and often curved or curled, due to flowage, and often clearly brought out on weathered surfaces.
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