Igneous rocks form from the cooling and solidification of molten rock material, known as magma beneath the surface and lava above it. As the molten material cools, chemical elements organize into ordered, repeating structures called crystals. Nearly all igneous rocks contain mineral crystals, but their visibility varies based on formation conditions. This variation in crystal size is the primary feature geologists use to understand the rock’s cooling history, which is determined by the rate at which the molten material lost heat.
The Role of Cooling Rate in Crystal Formation
The size of the crystals in an igneous rock directly records the cooling rate of the magma or lava. Solidification begins with nucleation, where microscopic solid clusters, or nuclei, form and act as seeds for crystal growth.
If the melt cools slowly, atoms have time to migrate and attach to these nuclei, allowing crystals to grow large and well-formed. This slow growth results in coarse-grained crystallization, characterized by a small number of large crystals.
Conversely, rapid cooling locks atoms into place quickly, preventing them from joining existing crystals. This leads to a high rate of nucleation, creating a vast number of tiny crystals with little time to grow. The resulting rock is composed of fine-grained crystals, often too small to see without magnification.
Intrusive and Extrusive Igneous Rocks
The cooling environment dictates the speed of heat loss, separating igneous rocks into two major categories.
Intrusive igneous rocks, also called plutonic rocks, solidify deep beneath the Earth’s surface. The surrounding rock acts as a thick insulator, causing the magma to cool very slowly over thousands or millions of years. This extended time allows for extensive crystal growth, producing rocks characterized by large, easily visible crystals. Granite, a common intrusive rock, exemplifies this texture with its interlocking quartz and feldspar crystals.
Extrusive igneous rocks, or volcanic rocks, form when lava erupts onto the surface or solidifies in shallow environments. Exposed to the atmosphere or water, the lava loses heat rapidly, often cooling completely within days or weeks. This quick solidification restricts the time for crystal growth, resulting in extremely small or microscopic crystals. Basalt, which forms much of the ocean floor, is a common extrusive rock whose fine-grained nature reflects its rapid cooling history.
Classifying Igneous Rocks by Crystal Texture
Geologists use specific terminology to classify the resulting crystal structure, or texture, of igneous rocks.
Rocks with crystals large enough to be seen with the unaided eye have a phaneritic texture. This coarse-grained texture is found in slow-cooled intrusive rocks, such as diorite and gabbro.
In contrast, rocks where the crystals are too small to be individually recognized without a microscope are defined as aphanitic. This fine-grained texture is characteristic of fast-cooled extrusive rocks like rhyolite and andesite.
A separate category is the glassy texture, which forms when lava cools so fast that crystallization is completely suppressed. Atoms do not have time to organize into any crystalline structure, resulting in an amorphous solid. Obsidian, a volcanic glass, is the most well-known example of an igneous rock with few or no mineral crystals.