Crystalline rocks with mineral grains that cannot be distinguished from one another without magnification have an aphanitic igneous texture. Igneous rocks form by crystallization of minerals from liquid magma rising into the upper portion of Earth's crust from the lower crust and underlying mantle. Igneous rock texture indicates the rate of magmatic cooling. Crystallization takes place either slowly in deeply buried intrusions called plutons, or rapidly at the earth's surface where magma has been extruded as lava by volcanic activity. Igneous rocks are therefore classified as either intrusive (plutonic) or extrusive (volcanic). Slow, undisturbed cooling in a well-insulated pluton is conducive to orderly arrangement of atoms and molecules into large, well-formed crystals. Rapid cooling from a lava flow is not. Intrusive igneous rocks thus have coarse-grained, or phaneritic, textures with visible crystals, and extrusive igneous rocks have fine-grained, or aphanitic, texture. Volcanic glass, called obsidian, forms when lava is quenched and solidified so quickly that the silicate ions in the melt form no orderly atomic structure.

Texture indicates the rate at which an igneous rock cooled, but it has no relationship to the chemical or mineralogical composition of the rock. Aphanitic, extrusive, igneous rocks therefore have coarse-grained, intrusive counterparts with the same chemical and mineral composition. For example, the silica-rich extrusive rock, rhyolite, common in continental volcanic regions, is the fine-grained equivalent of intrusive granite. Both rock types are composed mainly of the silicate minerals quartz and orthoclase feldspar, but the crystals in the rhyolite are too small to see without a microscope. Basalt, the iron and magnesium-rich extrusive igneous rock that comprises the majority of the sea floor, has the same composition as the intrusive rock gabbro. The intermediate-composition extrusive igneous rock, andesite, is common in volcanic arcs above subduction zones, and its coarse-grained equivalent, diorite, is found in plutons along these same convergent plate tectonic margins.

See Also

Pluton and Plutonic Bodies