Volcanic landforms are the most readily recognized representation of igneous rocks. These features remain hidden from sight until erosion removes the overlying rocks. The characteristics of the intrusions are largely controlled by the volume of magma involved and the character of the surrounding rocks. Magma moving within the earth's crust will often follow the path of least resistance such as fractures. Plutonic rocks can be distinguished from volcanic rocks by texture .
Texture is most readily apparent as grain size .
In contrast, plutonic rocks cooled slowly under relatively warm conditions and the mineral grains had sufficient time to grow large. Plutonic rocks equivalent to basalt, andesite, and rhyolite, are gabbro , diorite , and granite , respectively. Keep in mind that the same minerals are present in both volcanic and plutonic igneous rocks formed from the same magma. Magma that rises to the surface in stages may contain two discrete grain sizes and this texture is termed a porphyry. Large grains form initially at depth but are surrounded by a fine grained matrix formed when the remainder of the magma cools near the surface.