Volcanic rocks form from volcanic lava near the surface of the earth. They differ from igneous rocks which form from magma below the surface of the earth. The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy mineral pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite.
The lavas, having cooled rapidly in contact with the air or water, are mostly finely crystalline or have at least fine grained ground mass representing that part of the viscous semicrystalline lava flow which was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the steam and other gases, which they contained in great quantity were free to escape.
Many important modifications arise from this, the most striking being the frequent presence of numerous steam cavities often drawn out to elongated shapes subsequently filled up with minerals by infiltration. As crystallization was going on while the mass was still creeping forward under the surface of the Earth, the latest formed minerals in the ground mass are commonly arranged in subparallel winding lines following the direction of movement and the larger early minerals which had previously crystallized may show the same arrangement.
Most lavas have fallen considerably below their original temperatures before they are emitted. In their behavior they present a close analogy to hot solutions of salts in water, which, when they approach the saturation temperature, first deposit a crop of large, well formed crystals and subsequently precipitate clouds of smaller less perfect crystalline particles.
In igneous rocks the first generation of crystals generally forms before the lava has emerged to the surface, that is to say, during the ascent from the subterranean depths to the crater of the volcano. It has frequently been verified by observation that freshly emitted lavas contain large crystals borne along in a molten, liquid mass. The large, well-formed, early crystals are said to be porphyritic, which means that there is a large difference between the size of the tiny matrix crystals and other much larger crystals. The smaller crystals of the surrounding matrix or ground mass belong to the post effusion stage.
More rarely lavas are completely fused at the moment of ejection. They may then cool to form a finely crystalline rock, or if more rapidly chilled may in large part be non crystalline or glassy, such as obsidian, tachylyte, pitchstone. A common feature of glassy rocks is the presence of rounded bodies consisting of fine divergent fibres radiating from a center. They consist of imperfect crystals of feldspar, mixed with quartz or tridymite. Similar bodies are often produced artificially in glasses which are allowed to cool slowly. Rarely these spherulites are hollow or consist of concentric shells with spaces between. Perlitic structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling.
The porphyritic minerals are not only larger than those of the ground mass, but as the matrix was still liquid when they formed they were free to take perfect crystalline shapes, without interference by the pressure of adjacent crystals. They seem to have grown rapidly, as they are often filled with enclosures of glassy or finely crystalline material like that of the ground mass.
Microscopic examination of the larger crystals of the phenocrysts, which are the larger fragments in the rock, often reveals that they have had a complex history. Very frequently they show layers of different composition, indicated by variations in color or other optical properties. Augite may be green in the center surrounded by various shades of brown, or they may be pale green centrally and darker green at the periphery. In the feldspars the center is usually richer in calcium than the surrounding layers, and successive zones may often be noted, each containing less calcium than those which lie within it.
Phenocrysts of quartz, instead of sharp, perfect crystalline faces, may show rounded corroded surfaces, with the points blunted and irregular tongue like projections of the matrix into the substance of the crystal. It is clear that after the mineral had crystallized it was partly again dissolved or corroded at some period before the matrix solidified.