Igneous rocks form when molten material cools and solidifies. Intrusive igneous rock formation begins when magma cools entirely beneath the Earth’s surface. These rocks are also called plutonic rocks. This slow, subterranean cooling process creates a significant portion of the Earth’s continental crust, which is later exposed by erosion.
The Journey of Magma: From Mantle to Crust
Magma originates deep within the Earth, in the mantle or lower crust, where rock is subjected to intense heat and pressure. Melting is commonly triggered by three primary mechanisms: a decrease in pressure (decompression melting), the introduction of volatiles like water (flux melting), or the transfer of heat from existing, hotter magma bodies. For example, decompression melting often happens beneath mid-ocean ridges where the mantle rises.
Once formed, molten rock is substantially less dense than the surrounding solid rock, creating a powerful buoyant force. This buoyancy drives the magma upward through fissures and conduits in the overlying crust. The magma’s ascent continues until its density matches the surrounding rock, or the temperature drops enough to cause solidification.
The magma frequently stalls and pools in large chambers or smaller fractures beneath the surface, rather than reaching the surface to erupt as lava. This halting of the upward journey defines the formation of an intrusive body. The resulting mass of solidified magma is known as a pluton, which can range widely in size and shape depending on the volume of material and the structure of the host rock.
The Defining Process: Slow Cooling and Crystal Growth
The defining characteristic of intrusive rock formation is the extremely slow rate of cooling. Deep within the Earth, the surrounding rock acts as an efficient insulator, preventing rapid heat loss. This process can take tens of thousands to millions of years to complete, depending on the size and depth of the intrusion.
This prolonged cooling time allows mineral atoms to migrate slowly and bond together, forming large, well-structured crystals. The resulting texture is described as phaneritic, meaning the individual mineral grains are visible to the unaided eye. These crystals are interlocking, forming a dense and cohesive rock structure.
As the magma cools, different minerals crystallize sequentially based on their specific freezing temperatures. Minerals with higher crystallization temperatures, such as those rich in iron and magnesium, solidify first and have more space to grow larger. Minerals with lower temperatures, like quartz, crystallize last, filling the remaining spaces and completing the rock’s interlocking, coarse-grained structure.
Intrusion Structures: Shaping the Subsurface
The final shape of the solidified magma body, called a pluton, is classified based on its size and relationship to the surrounding host rock. The largest bodies are batholiths, which are massive, irregularly shaped intrusions with an exposed surface area greater than 100 square kilometers. These structures often form the cores of major mountain ranges.
Smaller, tabular intrusions are classified based on how they interact with the layering of the existing rock. A dike is a discordant intrusion, meaning it cuts across the bedding planes or layers of the host rock. In contrast, a sill is a concordant intrusion that forms when magma injects itself parallel to the existing rock layers, creating a horizontal or gently-dipping sheet.
A laccolith is similar to a sill because it is concordant, but the magma is viscous enough to push the overlying rock layers upward into a dome shape. These various structures, once exposed by millions of years of erosion, reveal the complex plumbing system that exists beneath the Earth’s surface. The geometry of these intrusions provides geologists with clues about the pressure and stress conditions present during their formation.
Common Examples of Intrusive Igneous Rocks
The composition of the original magma determines the specific type of intrusive rock that forms. Granite is the most recognized and abundant intrusive rock, characterized by its light color and felsic composition, rich in silica, quartz, and alkali feldspar. Its durability makes it a widely used material in construction and architecture.
At the other end of the compositional spectrum is gabbro, a dark-colored, mafic rock that is low in silica but high in iron and magnesium. Gabbro is primarily composed of pyroxene, olivine, and calcium-rich plagioclase feldspar.
Diorite represents an intermediate composition, often displaying a speckled, “salt and pepper” appearance. This appearance is due to a mix of light-colored plagioclase feldspar and dark minerals like hornblende and pyroxene.