Plutonic rocks can form in large or small magma chambers inside the crust and upper mantle of the Earth. These chambers never reach the surface, unless they have cooled and the surface rocks are eroded away. The term pluton refers to large and small chambers. A batholith is the largest chamber, while a laccolith is a smaller pluton. Magma cuts into what is called the country rock. It can intrude either in the same direction (a sill) or across the beds (dike).

Sometimes the molten rock inside the chambers may be chemically different. This would create different types of minerals as the rock cools. For example, molten rock rich in silica (SiO₂), aluminum (Al), potassium (K), and calcium (Ca) tends to make light-colored minerals. This would be similar to granite. In contrast, gabbro is a rock from magma rich in iron (Fe) and magnesium (Mg), with green to black minerals.