Igneous rocks form when a volcano erupts and the hot lava cools. Igneous rocks have many different textures. The texture is determined by how quickly the rocks cool. Those that form on or near the surface tend to cool rapidly, giving the rock a fine grain, such as basalt, the most common rock on the ocean floor. Those igneous rocks that form deep below the surface tend to cool more slowly and, as a result, tend to be tough with a coarse grain, such as granite.