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. The name "plutonic" refers to Pluto, Roman god of wealth and the underworld.

The main way to tell a plutonic rock is that it's made of tightly packed mineral grains of medium size (1 to 5 millimeters) 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. They have large mineral grains because they cooled over a very long time period (tens of thousands of years or longer), which allowed the individual crystals to grow large. The grains 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 millimeter, 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.

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. They are classified according to various triangular diagrams, starting with one based on the content of quartz and the two types of feldspar (the QAP diagram).

Producers of building stone classify all plutonic rocks as commercial granite.

A body of plutonic rock is called a pluton.

Pronunciation: plu-TONN-ic