Explain the texture of igneous rocks?

There are four main textures of igneous rocks which are used to classify them. The rate of cooling and crystal growth account for the differences in the textures, as well as different mineral types that comprise the numerous igneous rocks of the Earth.

Texture describes the over-all appearance of a rock based on its internal arrangement of interlocking crystals. Texture is very important because it reveals a lot about the environment and depth in which a particular rock sample has formed. There are four main textures of igneous rocks.

Aphanitic rocks, image A, are ones which cooled near the surface of the Earth. They have fine mineral grains too small to see by the unaided eye.

Phaneritic rocks, image B, form at great depths and exhibit a coarse grain mineral texture. Granite is a common example of a Phaneritic rock found in many mountain ranges.

Porphyritic rocks, image C, are rocks which formed at depth but were extruded at the surface suddenly. This causes a rock with large crystals suspended in a matrix of fine texture.

Glassy rocks, image D, from lava cooling rapidly in the air. They have a glassy, often black, appearance. One common example of a glassy rock is the volcanic rock obsidian, which is also known as "black volcanic glass".