The names and terminology applied to igneous rocks can be quite confusing for geology students. Various terms may refer to rock texture, mineral constituents, or chemical composition. Many names with vague or poorly defined meanings have been applied over the years to the great variety of rocks formed by cooling from magma or lava. Texture refers to the size of crystals, presence of glass, and porosity of the rock.
Texture is determined primarily by how the magma or lava cooled.
Following are common textural terms for igneous rocks. This texture forms by slow cooling of magma deep underground in the plutonic environment. The large crystals are called phenocrysts . They typically occupy veins or layers within a larger plutonic body. The large crystals form by slow cooling of magma.