Igneous rocks form from the cooling and solidification of molten material originating deep within the Earth. This crystallization process occurs in two distinct environments: deep beneath the crust (intrusion) or directly on the planet’s surface (extrusion). The location where the material cools dictates the rock’s final texture and structure.
Intrusion: Subsurface Magma Cooling
Intrusion involves the cooling and crystallization of magma deep inside the Earth’s crust. This molten rock remains underground, surrounded by pre-existing rock, known as country rock, which acts as a thermal insulator. The insulation and pressure allow the magma to cool extremely slowly, often over thousands or millions of years.
This protracted cooling permits the formation of large, easily visible mineral grains within the rock structure. Geologists refer to this coarse-grained appearance as a phaneritic texture, where individual crystals can be identified without magnification. Granite, formed from silica-rich magma, is a common example of an intrusive rock exhibiting this texture.
Intrusions are categorized based on their depth of formation; plutonic rocks solidify at great depths, while hypabyssal rocks form closer to the surface. The resulting solidified bodies are collectively called plutons, which take on various shapes relative to the country rock they invade. Massive, irregularly shaped intrusions with an exposed area greater than 100 square kilometers are termed batholiths.
Other common intrusive structures include dikes, which are tabular bodies that cut vertically across the layering of the surrounding rock, and sills, which are sheets of magma that intrude horizontally and parallel to the existing rock layers. These intrusions must force their way into the surrounding rock, sometimes filling existing fractures or even partially melting the rock around them. The presence of these deep-seated bodies is often associated with the core structures of mountain ranges.
Extrusion: Surface Lava Flow
Extrusion occurs when molten material is erupted onto the Earth’s surface, representing a fundamental shift in the cooling environment. Once underground magma breaches the surface, it is referred to as lava. This surface environment exposes the lava to the atmosphere or water, causing it to solidify quickly.
The rapid cooling rate limits the time available for mineral crystals to grow to a visible size. Consequently, extrusive rocks exhibit an aphanitic texture, meaning the crystals are microscopic. In cases of exceptionally fast cooling, such as when lava is quenched by water, the material may solidify without forming any crystalline structure, resulting in volcanic glass like obsidian.
During an eruption, the drop in pressure allows dissolved gases (volatiles) to rapidly escape from the molten rock. If the lava solidifies before these gas bubbles fully escape, the resulting rock contains numerous holes, called vesicles, leading to a porous or vesicular texture. Basalt, the most abundant extrusive igneous rock, forms extensive lava flows and plateaus exhibiting these textures.
Extrusive features also include pyroclastic debris, which are rock fragments ejected violently into the air during explosive eruptions. These fragmented materials, such as ash and cinders, then settle and consolidate to form layers around the vent, contributing to the structure of volcanic cones. Lava flows themselves can form structures like ropy pahoehoe or blocky ʻaʻā surfaces, depending on the viscosity and cooling of the lava.
Key Geological Differences and Resulting Structures
The difference between intrusion and extrusion is defined primarily by the location of solidification, which dictates the rate of cooling and the resulting rock texture. Intrusive rocks form beneath the surface, while extrusive rocks solidify at or near the surface. This positional difference governs the thermal environment in which crystallization occurs.
The cooling rate is the most significant distinction. Magma cools slowly underground due to insulating country rock, while lava cools rapidly upon exposure to the atmosphere. This difference directly controls the final crystal size and texture. Intrusion produces coarse-grained rocks with a phaneritic texture, such as granite, because crystals have time to grow large.
In contrast, extrusion yields fine-grained rocks with an aphanitic texture, like basalt, or volcanic glass where crystals are non-existent. A final difference lies in how these rocks are exposed to the atmosphere over geologic time. Extrusive rocks are immediately subjected to weathering and erosion upon formation.
Intrusive rocks are formed deep below and only become exposed at the surface after millions of years of uplift and erosion of the overlying material. This removal eventually reveals former subsurface structures, such as batholiths, which appear as massive, resistant landforms. The hardness of these plutonic rocks means they erode more slowly than the surrounding rock, standing out as notable features like El Capitan.