The practice of rock classification serves as a fundamental organizational tool in geology, allowing scientists to systematically categorize the solid materials that make up the Earth’s crust. This systematic approach provides a standardized framework for analyzing a rock’s physical and chemical properties. Understanding how rocks are classified is a prerequisite to interpreting Earth’s history, as the type of rock present indicates the past processes and environments that created it.
Defining the Three Main Rock Families
The broadest and most fundamental classification of rock begins with the process of its formation, which divides all rock into three distinct families.
Igneous rocks are formed from the cooling and solidification of molten material, known as magma when beneath the surface and lava when erupted onto the surface. This process of crystallization from a liquid state creates a dense, interlocking network of mineral grains.
Sedimentary rocks originate from the accumulation and consolidation of fragments of pre-existing rock, mineral grains, or biological debris. These materials are deposited in layers by water, wind, or ice and are subsequently compressed and cemented together over time.
Metamorphic rocks are created when any pre-existing rock—igneous, sedimentary, or another metamorphic rock—is subjected to intense heat and pressure deep within the Earth’s crust. This process alters the rock’s mineral composition and texture without causing it to fully melt.
Classifying Igneous Rocks by Texture and Composition
Igneous rocks are classified based on two primary characteristics: texture, which reveals cooling history, and mineral composition, which indicates chemical makeup. Texture is determined by the size and arrangement of mineral crystals, a direct consequence of the rate at which the molten material cooled.
Intrusive rocks, such as granite, form when magma cools slowly deep beneath the surface, allowing crystals to grow large enough to be seen (phaneritic texture). Extrusive rocks like basalt cool rapidly on the Earth’s surface, resulting in a fine-grained, or aphanitic, texture where individual crystals are microscopic. A third texture, porphyritic, develops when magma cools in two stages, creating large crystals called phenocrysts embedded within a fine-grained groundmass.
Chemical composition is divided into felsic and mafic categories based on silica content and mineral color. Felsic rocks, like rhyolite, are rich in silica, potassium, and sodium, resulting in lighter-colored minerals such as quartz and feldspar. Mafic rocks, such as gabbro, contain lower amounts of silica but are rich in magnesium and iron, often giving them a darker color. Intermediate compositions, such as andesite, fall between these two extremes, exhibiting a blend of light and dark minerals.
Classifying Sedimentary Rocks by Origin
Sedimentary rocks are classified into three distinct groups based on the origin of the material from which they formed. The most abundant group is clastic sedimentary rock, composed of fragments of weathered rock and minerals known as clasts. Classification is primarily based on the size of these clasts.
Clastic rocks composed of coarse, gravel-sized fragments are called conglomerate (if rounded) or breccia (if angular). Sandstone forms from sand-sized grains, while finer-grained rocks like siltstone and shale are comprised of silt- and clay-sized particles. The specific grain size provides clues about the energy of the ancient depositional environment.
The second group is chemical sedimentary rock, which forms when minerals precipitate directly out of water solutions. Evaporites, such as rock salt (halite) and gypsum, form when water evaporates, leaving behind dissolved mineral content. Chert is another chemical rock, composed of microcrystalline silica that precipitated from water.
The third category is organic or biochemical sedimentary rock, which forms from the accumulation of biological remains. Limestone, largely composed of calcite, often forms from the shells and skeletal fragments of marine organisms. Coal is a prime example, created from the burial and compression of vast quantities of plant material over millions of years.
Classifying Metamorphic Rocks by Structure and Parent Rock
Metamorphic rocks are classified using two main criteria: the presence or absence of a layered structure called foliation, and the composition of the original rock, known as the parent rock or protolith. Foliation develops when uneven pressure causes minerals to flatten and align perpendicular to the direction of the stress.
Foliated rocks are categorized based on the texture and degree of layering, which corresponds to the intensity of metamorphism. Slate exhibits the lowest grade, where fine-grained minerals align to split into thin, flat sheets. With increasing temperature and pressure, grain size increases, progressing through phyllite to schist, which has visible, wavy layers of mica.
The highest grade of foliation is found in gneiss, characterized by distinct alternating bands of light-colored, granular minerals and dark-colored, platy minerals. Non-foliated metamorphic rocks form under conditions where pressure is uniform or the parent rock is composed of minerals that do not easily align. These rocks have a massive, interlocking granular texture.
Marble, a classic non-foliated rock, forms when limestone is subjected to heat and pressure, causing the calcite grains to recrystallize into a denser mass. Similarly, quartzite is the metamorphic equivalent of sandstone, where quartz grains have fused together. The chemical make-up of the parent rock dictates the mineralogy of the resulting metamorphic rock; a quartz-rich protolith will always yield a quartz-rich metamorphic rock.