Aphanitic Rock

Aphanitic rock is igneous rock in which the grain or crystalline structure is too fine to be seen by the unaided eye. Such rock is formed when the material solidifies at or near the surface so that the cooling is rather rapid. Such rocks are termed "extrusive" rocks. Under these conditions, there is not enough time for the growth of large crystals. Basalt from surface lava flow often exhibits an aphanitic texture.

Since the crystals of individual minerals cannot be easily resolved for classification, aphanitic rocks are classified in general terms like light, intermediate or dark in color.

The presence of voids called vesicles is common in aphanitic rock since the condition of cooling rapidly may be associated with the upper portion of lava flows. These vesicles caused by gases escaping from these lava flows will be most numerous in the upper portions of the flows.

Phaneritic Rock

Phaneritic rock is igneous rock with large, identifiable crystals of roughly equal size. Such crystals are characteristic of rocks which solidified far below the surface so that the cooling was slow enough to enable the large crystals to grow. Such rocks are termed "intrusive" rocks. When such rocks are found on the surface, this can be taken to imply that the overlying material has been removed by erosion.

Porphyritic Rock

Porphyritic rock is igneous rock which is characterized by large crystals surrounded by a background of material with very small crystals. The scenario for the production of such rocks involves the formation of certain types of mineral crystals over a long period deep in the earth. Because of differences in melting temperatures and growth rates, the surrounding material may not have appreciably crystallized. If this material is suddenly ejected from the surface, as in a volcano, then the surrounding material will solidify rapidly to form small crystals in the spaces between the large ones.

In such rocks the large crystals are called phenocrysts while the surrounding material is called groundmass. The entire collection of material is called a porphyry.

Glassy Rock

When molten rock is suddenly ejected from a volcano, it may be cooled so rapidly that organized crystal formation cannot occur. This results in igneous rock which has no internal structure. It has a glassy appearance and produces no planes or crystal symmetry when broken.

Obsidian is a common natural glass occuring in lava flows.

Pyroclastic Rock

Sometimes material is violently ejected from volcanoes and then reassembled into igneous rocks from this material. The material may range from fine dust or fine hair-like strands to large molten blobs. The consolidation of such material into rocks produces what is called the pyroclastic texture.