Igneous rocks, one of the three main rock types, form directly from the cooling and solidification of molten rock material. This melt is referred to as magma when it is beneath the Earth’s surface and lava once it erupts onto the surface. The classification of these rocks is necessary because the source material can vary widely in chemical makeup, and the environment in which the cooling occurs can drastically alter the final rock structure. Geologists use a multi-pronged approach that considers the rock’s origin, its physical texture, and its internal chemical components to accurately categorize the vast diversity found in the Earth’s crust.
The Fundamental Basis: Origin and Cooling Environment
The most fundamental division of igneous rocks is based on where the molten material solidifies, which dictates the rate of cooling. Magma that cools and crystallizes deep beneath the Earth’s surface forms intrusive, or plutonic, igneous rocks. Because the surrounding rock acts as an insulating blanket, this slow cooling process allows for the growth of large, interconnected mineral crystals.
Rocks that form from lava erupted onto the surface or near the surface are termed extrusive, or volcanic, igneous rocks. When the molten material is exposed to the relatively cool atmosphere or water, it solidifies almost instantly. This rapid quenching restricts the time available for mineral crystal growth. Granite (intrusive) and basalt (extrusive) share a similar chemical composition but appear completely different due to their distinct cooling histories.
Classification by Texture: The Role of Crystal Size
Texture in igneous rocks refers to the size, shape, and arrangement of the mineral grains, which is directly controlled by the cooling rate. The phaneritic texture is characteristic of intrusive rocks, where slow cooling allows crystals to grow large enough to be easily visible to the naked eye. Conversely, the aphanitic texture is found in extrusive rocks that cooled rapidly, resulting in microscopic crystals too small to be individually distinguished without magnification.
When lava cools extremely quickly, such as through contact with air or water, it may not form any crystalline structure at all, resulting in a non-crystalline, glassy texture. Obsidian is the most well-known example of this volcanic glass. A porphyritic texture indicates a two-stage cooling history, where large crystals (phenocrysts) formed slowly deep underground before the magma was rapidly erupted, solidifying the remaining melt into a fine-grained matrix (groundmass).
Classification by Chemical and Mineral Composition
Beyond the physical appearance, igneous rocks are systematically classified by their chemical and mineral composition, focusing primarily on the percentage of silica (\(\text{SiO}_2\)) present. The amount of silica dictates which specific minerals will crystallize and, consequently, the rock’s overall density and color. This compositional classification divides igneous rocks into four primary groups: felsic, intermediate, mafic, and ultramafic.
Felsic rocks are defined by their high silica content, typically greater than 63% by weight. They are predominantly composed of light-colored minerals such as quartz and potassium feldspar, making the rock light in color and relatively low in density. Conversely, mafic rocks have a lower silica content, generally ranging between 45% and 55%. These rocks are rich in magnesium and iron (ferromagnesian) minerals, such as pyroxene and olivine, causing them to be dark in color and higher in density.
Intermediate rocks fall chemically between the felsic and mafic extremes, featuring a moderate silica content and containing a mix of both light and dark minerals. Ultramafic rocks represent the lowest end of the silica spectrum, containing less than 45% silica. They consist almost entirely of dense, dark ferromagnesian minerals like olivine and pyroxene. Combining the cooling-dependent texture with the silica-dependent chemical composition accurately determines the rock type, such as pairing felsic composition with a phaneritic texture to identify granite.