Intrusive rocks, also known as plutonic rocks, are a type of igneous rock formed when molten material called magma solidifies deep beneath the Earth’s surface. Magma is forced into pre-existing rock layers and cools slowly to a solid state within the crust. This underground solidification contrasts with extrusive rocks, which cool quickly on the surface from lava.
The Magma Source and Subsurface Environment
Magma, the precursor to all igneous rocks, originates from the partial melting of existing rock, either in the Earth’s mantle or the lower crust. Melting can be triggered by a decrease in pressure, an increase in temperature, or the introduction of water that lowers the rock’s melting point. This buoyant, less dense molten rock then begins to rise toward the surface, pushing its way into the overlying crust.
The environment where intrusive rocks form is characterized by immense pressure and high temperatures. As the magma rises, it forces itself into cracks, faults, and weak zones within the surrounding host rock, which geologists call country rock. This deep burial provides a thick insulating blanket, ensuring the slow cooling process required for intrusive rock formation. The intrusion process involves the magma displacing the country rock, sometimes by melting it or by breaking off and enveloping fragments known as xenoliths.
The Slow Cooling Process and Resulting Texture
The formation of intrusive rocks is dictated by the extremely slow rate at which the trapped magma loses its heat. Because the surrounding country rock acts as a highly effective insulator, the magma may take thousands to millions of years to completely solidify, especially in large bodies. This extended cooling time is the single most important factor determining the final texture of the rock.
The slow loss of thermal energy allows atoms within the molten material ample time to migrate and bond into orderly crystal structures. Consequently, the individual mineral grains grow to a relatively large size, often visible to the naked eye. This characteristic large-grained texture is scientifically termed phaneritic.
The common intrusive rock granite is a classic illustration of this texture, displaying interlocking crystals of quartz, feldspar, and mica. The large crystal size contrasts with extrusive rocks, which cool rapidly on the surface, resulting in a glassy or very fine-grained texture. The size of the crystals serves as a direct record of the prolonged, deep-seated conditions under which the rock formed.
Common Intrusive Geometries
Once the magma solidifies, the resulting intrusive rock body takes on various geological shapes and forms called plutons. The geometry of these bodies is classified based on their size and their relationship to the layering of the surrounding country rock.
One of the largest forms is the batholith, which is a massive, irregularly shaped pluton with an exposed surface area greater than 100 square kilometers. Smaller, similarly shaped bodies are known as stocks. These large masses typically cut across the layering of the host rock, making them discordant intrusions.
Other common forms are tabular, sheet-like intrusions classified by their orientation relative to the country rock’s layers. A dike is a discordant tabular body that cuts across existing rock layers or structures at a steep angle. In contrast, a sill is a concordant tabular intrusion that is injected parallel to the existing layering, often between beds of sedimentary rock.