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Igneous Textures This term is applied to igneous rocks in order to discribe their appearance. This allows geologist to determine how a particular igneous rock formed and under what conditions it formed. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.

Phaneritic (phaner = visible) Also called intrusive igneous rocks. When a magma cools it does so very slowly, thus giving the minerals inside the magma time to grow and form large complex arrangements. Because of this, the minerals in a phaneritic igneous rock look large and you can see each individual crystal with your naked eye. Examples of phaneritic igneous rocks are; Gabbro, Diorite, Granite.

Aphanitic (a = not, phaner = visible) In contrast to phaneritic rocks, aphanitic igneous rocks form from lava which is extrusive (thus the name extursive igneous rocks). Because extrusive rocks make conact with the atmosphere and/or water they cool quickly, so the minerals do not have time to form large crystals. You can not see the individual crystals in an aphanitic igneous rock. Examples of aphanitic igneous rock; Basalt, Andesite, Rhyolite.

Porphyritic During cooling of a magma, the composition of the material maybe such that different minerals require different cooling times. Which will lead to some minerals cooling faster than others within the magma camber, resulting in porphyritic texture. Simply, you have an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Another way to get porphyritic texture is, a magma will start cooling, so large crystals form, then the magma is erupted by its volcano and thus the rest cools quickly forming porphyritic texture.

Glassy During some volanic eruptions, the lava is ejected so far into the atmosphere, where it cools so rapidly the result is glassy texture, this cooling is faster than what is required for aphanitic texture. Example; Obsidian.

Pyroclastic Lava ejected from its volcano may pick up fragments of rock. Igneous rocks contaning fragments have pyroclastic texture.

Pegmatitic During magma cooling some minerals may grow so large that they become massive (the size ranges from a baseball to a car. This is called pegmatitic texture. This is usually associated with large plutons or veins.

References: E.J. Tarbuck and F.K. Lutgens, Earth An introduction to physical geology. seventh edition, Prentice Hall, 2002.