Texture of Igneous Rock

Now that you have determined the color index of your igneous rock you must determine its texture

The texture of an igneous rock is a measure of crystal size within the rock and therefore a record of the cooling history of the rock itself.

Igneous textures are described by a number of terms. The phaneritic texture describes coarse grains or rocks that contain mostly large crystals. A rock with an aphanitic texture has a fine grain; it contains mostly small crystals. Observationally, a rock that exhibits a phaneritic texture would have visible crystals, as seen below. The porphorytic texture describes a rock that exhibits both phaneritic and aphanitic textures.

Aphanitic Texture	Phaneritic Texture	Porphorytic Texture

Other igneous textures that are typical of surface formation are associated with volcanic activity.

Thesetextures are pyroclastic, vesicular, or glassy textures. Pyroclastic textures result from the compaction of volcanic ash and explosive debris. The resulting rock is formed when the ash and debris is welded together by the immense heat contained in the debris. As a result this texture typically has a gritty feel, and the rock contains tiny bits of fragmented material.

Pyroclastic	Vesicular	Glassy

A vesicular texture is caused when dissolved gases and other volatile components of a magma erupt from the liquid portion due to a

decrease in pressure. This causes the magma to foam up, much like what would occur when

the cap is removed from a shaken soda bottle. The rock that results from the cooling frothed magma would be riddled with hole-like structures called vesicles.

Finally, when magma cools so quickly that the molecular components have no time to form even the smallest crystals a glassy texture will result.

Once you have determined the texture of the rock you wish to identify click the appropriate link in the menu on the left.