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A Pluton is a monolithic body of rock that was formed by magma solidifying underneath the surface of the Earth. Plutons come in all shapes and sizes; some mountain ranges are essentially formed from large plutons, while other plutons may be a fraction of a centimeter wide.

Not all magma solidifies into plutons. When magma breaks through the surface of the Earth's crust it is called lava, and it solidifies into volcanic rocks ('extrusive igneous rock'). When magma solidifies below the surface it forms plutonic rock ('intrusive igneous rock'), and the structures it forms are called plutons. Any intrusive igneous rock may be called a pluton, although often the the term is used to refer to very large masses of rock.

Because plutons form beneath the ground, the molten magma cools more slowly than it does in volcanic formations. This means that the minerals therein have time to form larger crystals. Thus, plutons are often formed of rocks with visible grains (a phaneritic texture), such as granite, diorite, and gabbro. These rocks are formed from exactly the same minerals as volcanic rocks; for example, basalt is a volcanic rock that forms from the same magma as the plutonic gabbro. This can give us important information on how ancient rock formations were formed.

Obviously, plutons generally form in areas of volcanism, which is to say, at the boundaries of tectonic plates. Plutons may form by magma pushing into existing rock, in which case it often follows fractures in the country rock. They may also form by melting into the country rock, forming new magma which then cools in place. Larger plutons are generally formed by multiple intrusions of magma, each one adding to the previous formation; this can go on for thousands of years. A large pluton does not cool quickly, and it may take centuries for a pluton to fully solidify.

Types of plutons:

  • Batholiths: Large plutons, with greater than 100km2 surface area. These are usually formed from multiple intrusions of magma, with new flows of magma pushing in over time. Some batholiths, such as the coastal batholith of Peru, took tens of millions of years to reach their current size, and are composed of hundreds of separate magma intrusions. Batholiths are often formed in part when magma melts the country rock, and it re-solidifies as plutonic rock. Batholiths may be pushed up by tectonic movement, and be exposed when the upper layers of the crust erode. This is how the Sierra Nevadas were formed.

  • Dikes: A pluton forming within a vertical fracture, that is, a fracture cutting perpendicular to the existing rock layers. These may be nearly any size, and often appear in groups in volcanic areas, a phenomenon known as a dike swarm. Large circular dikes (+/- 1 km) often form at divergent plate boundaries; this are knowns as ring dikes.

  • Sills: A pluton formed in a horizontal fracture, that is, a fracture between existing rock layers. It sometimes happens that the country rock becomes so deformed that a sill may actually form along a vertical fracture, but as long as it occurs between preexisting layers of rock, rather than across, it is a sill, not a dike. These often form in sheets kilometers long, although they are often comparatively thin.

  • Laccoliths: A sill that forms in a dome shape. These push up the overlaying strata as they form, and may form hills on the surface.

  • Lopoliths: A type of laccolith in which the dome is upside-down, with a flat or concave upper surface and a curved bottom. These are usually formed from mafic rock, which implies that this formation is influenced by the greater weight of these rocks.
