Igneous rocks form from the solidification of molten material, known as magma or lava. Plutonic rocks represent a major subclass of igneous rocks formed exclusively beneath the Earth’s surface. They are the crystalline product of magma that never reached the atmosphere, instead cooling deep within the crust. This formation environment defines their fundamental characteristics, distinguishing them from their volcanic counterparts, which cool rapidly on the surface.
Defining Plutonic Rocks
Plutonic rocks are scientifically known as intrusive igneous rocks. The term “plutonic” is derived from Pluto, the Roman god of the underworld, referencing the deep, subterranean origin of these rock types. Because this magma is trapped at depth, it is insulated by the surrounding rock, which allows for a protracted cooling and crystallization period. This slow process is responsible for the unique, coarse texture that characterizes nearly all plutonic rocks. They are among the most common rocks on Earth, forming the stable foundations of continents and the roots of mountain ranges.
The Formation Process
The defining feature of plutonic rock formation is the extremely slow rate of cooling, which occurs under immense pressure deep within the crust. Magma, a hot melt originating from the mantle, rises and pools in chambers or forces its way into fractures and layers beneath the surface. The surrounding crustal rock, often called the country rock, acts as an efficient insulator, preventing rapid heat loss from the molten material.
This insulation means that the magma can remain in a liquid state for thousands, or even millions, of years, allowing the crystallization process to proceed gradually. High pressure from the overlying rock layers also plays a role, helping to keep volatile components dissolved in the melt. The magma solidifies entirely underground, becoming fully crystalline, or holocrystalline, with no glassy material present.
Textures and Crystalline Structure
The slow, deep-seated cooling process directly results in the distinctive texture of plutonic rocks, known as phaneritic texture. The term phaneritic means the crystals are visible to the naked eye, often ranging from about one millimeter up to several centimeters in size.
As the magma cools, mineral nuclei form and slowly incorporate surrounding atoms from the melt, allowing the crystals to attain substantial size. These crystals grow and interlock with one another, creating a durable and dense rock structure. This phaneritic texture stands in contrast to the fine-grained, or aphanitic, texture of volcanic rocks, which cool rapidly on the surface and have crystals too small to be seen without magnification.
Common Types and Geological Structures
Plutonic rocks are classified based on their mineral composition, with common examples including granite, diorite, and gabbro.
Common Rock Types
Granite is a light-colored, felsic rock, rich in quartz and potassium feldspar, and is the most abundant plutonic rock in the continental crust.
Diorite is intermediate in composition, often having a “salt-and-pepper” appearance due to a mix of light-colored feldspar and dark amphibole or pyroxene minerals.
Gabbro is a dark-colored, mafic rock, composed mainly of calcium-rich plagioclase feldspar and pyroxene.
Geological Structures (Plutons)
These large bodies of intrusive rock are collectively called plutons, which form massive geological structures. The largest of these are batholiths, which are huge, irregular masses of plutonic rock with surface exposures greater than 100 square kilometers.
Smaller, tabular intrusions include dikes, which cut vertically across existing rock layers, and sills, which intrude horizontally between layers. These structures, once buried kilometers deep, are only exposed at the Earth’s surface after millions of years of uplift and erosion remove the overlying country rock. The Sierra Nevada mountains in California are a well-known example of a large batholith exposed by these geological forces.