Plutonic Rocks Definition and Examples

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 Earths 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.   What Does "Plutonic" Mean? The name plutonic refers to Pluto, Roman god of wealth and the underworld; plutos 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 The main way to tell a plutonic rock is that its 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 an 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. The Most Common Rocks on Earth 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 dont 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 theres 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  Ã¢â‚¬â€¹gabbro  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. There Are About a Dozen Major Types 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).