Vast amounts of steam are, as we have seen, emitted from volcanoes, and comparatively small quantities of other vapors, such as various acid and sulphurous gases. The rocks erupted from volcanoes differ widely in chemical composition and in texture.
Fig. 220. Cellular Lava
Acidic and basic lavas. Two classes of volcanic rocks may be distinguished,—those containing a large proportion of silica (silicic acid, SiO2) and therefore called acidic, and those containing less silica and a larger proportion of the bases (lime, magnesia, soda, etc.) and therefore called basic. The acidic lavas, of which rhyolite and thrachyte are examples, are comparatively light in color and weight, and are difficult to melt. The basic lavas, of which basalt is a type, are dark and heavy and melt at a lower temperature.
Scoria and pumice. The texture of volcanic rocks depends in part on the degree to which they were distended by the steam which permeated them when in a molten state. They harden into compact rock where the steam cannot expand. Where the steam is released from pressure, as on the surface of a lava stream, it forms bubbles (steam blebs) of various sizes, which give the hardened rock a cellular structure ([Fig. 220]), In this way are formed the rough slags and clinkers called scoria, which are found on the surface of flows and which are also thrown out as clots of lava in explosive eruptions.
On the surface of the seething lava in the throat of the volcano there gathers a rock foam, which, when hurled into the air, is cooled and falls as pumice,—a spongy gray rock so light that it floats on water.
