Igneous rocks are formed from the solidification and crystallization of molten rock material. The primary distinction between extrusive and intrusive types rests entirely on the location where this process takes place: intrusive rocks form beneath the Earth’s surface, while extrusive rocks solidify on or very near the surface. This difference in formation environment drives all subsequent physical characteristics used to identify these rocks.
Formation Environment and Cooling Rate
The molten material deep beneath the crust is referred to as magma; when it reaches the surface, it is called lava. Intrusive rocks, sometimes called plutonic rocks, form when magma becomes trapped within the Earth’s crust. Because this magma is surrounded by pre-existing rock, it is highly insulated. This insulation leads to an extremely slow cooling process that can take thousands or even millions of years.
When magma erupts onto the surface through a volcano or fissure, it becomes lava and cools to form extrusive rocks, also known as volcanic rocks. The surface environment, exposed to the atmosphere or water, causes the lava to solidify almost instantly. This rapid cooling limits the time available for mineral components to organize themselves. This difference in cooling duration—millions of years versus hours or days—determines the final appearance of the rock.
The Resulting Rock Texture
The pace of cooling dictates the size of the mineral grains, which is the most observable difference between the two rock types. Intrusive rocks cool slowly, allowing mineral molecules sufficient time to migrate and lock into large, interlocking crystals. This results in a coarse-grained texture, called phaneritic, where individual mineral grains are easily visible to the naked eye.
Extrusive rocks cool so quickly that mineral grains have little time to grow. This rapid solidification produces a fine-grained texture, known as aphanitic, where crystals are microscopic and cannot be discerned without magnification. If cooling is exceptionally fast, such as when lava meets cold water, the material may quench before any crystalline structure forms. This results in a glassy texture, characterized by an amorphous solid like obsidian.
Key Examples of Each Type
Granite is the most common example of an intrusive igneous rock, characterized by its visible grains of quartz, feldspar, and mica. This coarse-grained rock is often exposed in mountain ranges where millions of years of erosion have removed the overlying material. Gabbro and diorite are other common intrusive rocks; gabbro often forms a major component of the lower oceanic crust.
Basalt is the most abundant extrusive igneous rock on Earth, making up much of the oceanic crust and volcanic islands like Hawaii. It is typically dark-colored and fine-grained, reflecting its rapid cooling history on the surface. Rhyolite is the fine-grained equivalent of granite, sharing a similar chemical composition but displaying a microscopic crystal structure. Obsidian and pumice are examples, with obsidian being a volcanic glass and pumice a lightweight rock containing trapped gas bubbles.