Plutonic rocks are composed of large minerals because they cooled slowly within the crust and upper mantle of the Earth. This gives the minerals time to grow larger. Large minerals are referred to having a phaneritic texture. For example, magma that cools slowly creates rocks like granite, which have large minerals that can be seen with the naked eye.

Plutonic rocks have other textures that can help identify them. Because the magma chamber is a fluid there can be movement of the liquid. As the minerals start to precipitate out of the melt, they can form patterns as the fluid moves. Sometimes the magma cools into rock with this pattern. For example, a schlieren structure is due to this movement. It is a concentration of darker minerals in a wavy pattern.

Granite

Schlieren structure