Plutonic Rocks

Tonalite, holocrystalline magmatic rock
De Agostini Picture Library/De Agostini Picture Library/Getty Images

Plutonic rocks are igneous rocks that solidified from a melt at great depth. Magma rises, bringing minerals and precious metals such as gold, silver, molybdenum, and lead with it, forcing its way into older rocks. It cools slowly (tens of thousands of years or longer), underneath Earth's crust, which allows the individual crystals to grow large by coalescing, like with like; thus, plutonic rock is coarse-grained rock. The rock is later exposed by erosion. A large body of this type of rock is called a pluton. Hundreds of miles of plutonic rock are batholiths

The name "plutonic" refers to Pluto, Roman god of wealth and the underworld; pluto's origins also come from "wealth," or "rich one," which could refer to the precious metals present in the Earth and in rocks. Gold and silver are found in veins in plutonic rocks, which are formed from the intrusions of the magma.

In contrast, volcanic rocks are formed by magma above ground. Their crystals are evident only through an examination under a microscope.

The dwarf planet Pluto, however, is mostly ice made up of frozen nitrogen, methane, and carbon dioxide, though it may have a rocky core that contains some metals. 

How to Identify Plutonic Rocks

The main way to tell a plutonic rock is that it's made of tightly packed mineral grains of medium size (1 to 5 mm) or larger, which means that it has phaneritic texture. In addition, the grains are of roughly equal size, meaning that it has equigranular or granular texture. Finally, the rock is holocrystalline—every bit of mineral matter is in a crystalline form, and there is no glassy fraction. In a word, typical plutonic rocks look like granite. In fact, producers of building stone classify all plutonic rocks as commercial granite.

Different Types

Plutonic rocks are the most common rocks on Earth and form the basis of our continents and roots of our mountain ranges.

The large mineral grains in plutonic rocks don't generally have well-formed crystals because they grew crowded together—that is, they are anhedral. An igneous rock from a shallower depth (with grains smaller than 1 mm, but not microscopic) may be classified as intrusive (or hypabyssal), if there's evidence that it never erupted onto the surface, or extrusive if it did erupt. As an example, a rock with the same composition could be called ​gabbro if it were plutonic, diabase if it were intrusive, or basalt if it were extrusive. Whereas plutonic rocks form continents, basalt lies in the crust underneath the oceans.

The name for a particular plutonic rock depends on the mix of minerals in it. There are about a dozen major plutonic rock types and many more less common ones. In ascending order, four types include gabbro (dark in color, not much silica), diorite (an intermediate amount of silica), granite (68 percent silica), and pegmatite. Types are classified according to various triangular diagrams, starting with one based on the content of quartz (which is pure silica) and the two types of feldspar (which is quartz with impurities). See the QAP diagram for more information.