Porphyritic rock is a texture of igneous rock with two distinctly different crystal sizes due to two periods of cooling occurring at different rates. Larger crystals are called phenocrysts, while the smaller grains are known as groundmass or matrix.
The size, shape and arrangement of crystals in an igneous rock determines its texture. Extrusive volcanic rock tends to cool quickly near the Earth's surface, resulting in larger crystals. Intrusive volcanic rocks cool slowly, tending to have smaller crystals. Phaneric rock contains crystals of the same size, resulting from one cooling period. In addition to texture, igneous rock is classified by color and mineralogy.