Igneous rocks are those formed within the Earth at temperatures that are sufficiently high to produce a liquid component. As the liquid component increases it moves towards the surface because of its lower density than the enclosing solid rock. Many factors determine whether the partly molten rock (magma) remains within the crust (to form rocks such as granite) or whether it reaches the surface and is extruded as a lava. Temperature, fluid content (volatiles) and bulk composition of the magma are factors that strongly influence the behaviour of the magma.
Because of the infinite variables that control magma compositions no two igneous rocks are ever identical even though there might be some visual resemblance. Small variations in the mineral composition and proportions of the source material (often controlled by the degree of melting) will be reflected in the final product. If the magma generated in the “melting pot” becomes contaminated by crustal material (such as a descending slab) or coalesces with other melting pots, or traverses pockets of melt at higher levels during its rise to the surface its original composition will be modified and will be reflected in the eventual crystalline rock. Research into the different stages of rock formation is a fascinating science.
There are two principal igneous rock classes – those that remain under the crust when they crystallize (plutonic such as granites) and those that come out of the crust (volcanic such as lavas). Within each group there are two major sub-classes depending on their bulk composition, that is, felsic (quartz-bearing) and mafic (little or no quartz). These two sub-classes are generally reflected in colour with the former being light and the latter being dark. Some classifications include a more-nebulous intermediate group.
Further subdivisions of igneous rocks can be made on the basis of texture. Textures reveal much of the crystallization history of each rock and are a fundamental aspect of rock description. Some rocks crystallize very rapidly (e.g. when hot basalt lava erupts into sea-water) whereas other may crystallize very slowly in the presence of abundant fluid and produce crystals up to several meters in length (pegmatites). Some textures reveal a mixture of crystals and original melt and others contain minerals and inclusions that have been incorporated during ascent or retained from the source of melting.