Igneous rocks were once hot liquids that had a range of chemical compositions. On coming near the surface, the liquids cooled at different rates. Very fast cooling results in glasses (obsidian). Slower cooling, typical of most volcanic rocks, results in small grain sizes. 4 Porphyritic texture illustrated by large, visible crystals found within a fine-grained matrix in igneous rock. Some igneous rocks have a mixture of large crystals within a fine-grained matrix. Such a texture is called porphyritic .