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The basic threefold classification of rocks is igneous, sedimentary, and metamorphic. The textural, mineralogical,and geochemical characteristics of these rocks lead to specific nomenclature for describing and classifying them; these are summarized in Figures 2.8, 2.9, 2.10, and 2.11 for more common rocks.

Igneous rocks crystallize (a process sometimes called solidification) from a molten material (called a melt or magma); the rock is composed of interlocking minerals. Magmas are generated from partial melting of mantle material or of rocks deep in the crust. If this melt flows out and cools to form a rock at the surface of the Earth, it is called volcanic or extrusive. If the melt cools and solidifies inside the Earth, it is called plutonic or intrusive.

Figure 2.8 Simple and generalized classification diagram for igneous rocks, based on dominant rock forming mineral composition. On the right are igneous rocks with high magnesium and iron content, which are termed “mafic”, such as extrusive basalt with fine grained crystal sizes or intrusive gabbro with coarse crystal sizes. Basalt will be composed predominantly of plagioclase and pyroxene, with variable amounts of olivine, amphibole, and biotite as well as other lesser minerals such magnetite, depending on the specific geological setting. Schematic from Wikimedia Commons.

Figure 2.9 Simple and generalized classification diagram for sedimentary rocks. First order classification is carried out on texture, and then on grain sizes and compositions of the material. Schematic from ESProjects under Creative Commons.

Figure 2.10 Simple diagram for metamorphic rocks based on changing temperature (T) and pressure (P) conditions as a function of depth. The green dashed line (a) depicts the typical path of a mudrock being buried and enduring prograde (increasing P and T) metamorphism, transforming into slate, then subsequently into phyllite, schist, gneiss, and, finally, starting to melt as it becomes a migmatite. The yellow line (b & e) indicates conditions in proximity to volcanic centers, while (d) represents the region immediately adjacent to igneous magma and rocks at shallow depths, often termed “contact metamorphism”. The blue dashed line (c) represents the low‐temperature / high‐pressure path that a mudrock might take if subducted alongside cold basalts at a convergent margin. The red dashed line at high temperatures indicates a region where granitic rocks will start to melt. Earle (2015) / CC BY 4.0.

Sedimentary rocks form by several processes generally tied to physical erosion, transport and redeposition, chemical precipitation, or biological precipitation. Physical erosion and weathering of an existing rock can form a clastic sedimentary rock, such as a sandstone, siltstone, or mudstone. These rocks are composed of the fragments and grains of the rock(s) that were being eroded to form the sediment. Chemical precipitation at the Earth’s surface can occur when a body of water such as a lake or inland sea undergoes sufficient evaporation to form layers of evaporitic minerals, such as salt. Biological precipitation of minerals includes the production of coral reefs, sediments composed of shells, and deposition of plant material in swamps to form coal.

The unconsolidated sediments themselves are transformed into rocks via a process called diagenesis or lithification, which physically and chemically cements the sedimentary grains together. Like metamorphism, this process involves heat, pressure, and percolating fluids but not to such a degree that the rock’s mineralogy or structure is drastically transformed.

Metamorphic rocks are formed by the modification or alteration of preexisting rocks (igneous, metamorphic, and sedimentary) via a geological process termed metamorphism. The processes that transform or metamorphose rocks involve heat and/or pressure and very often fluids percolating through the subsurface. Rocks can be compressed and new minerals may be generated that are more stable under the new temperature and/or pressure conditions. Pressure is often the result of compressional tectonic forces generated when plates collide; this can also generate heat. In addition, pressure and temperature will increase with depth into the Earth’s crust. Just as increasing pressure and temperature can result in new metamorphic minerals forming, decreasing these pressure and temperature conditions can also lead to mineralogical changes and therefore metamorphism. Generally speaking, when a rock is experiencing increasing temperature and pressure changes it is termed “Prograde metamorphism” and during decreasing conditions it is termed “Retrograde metamorphism”. In the context of gem formation, some gemstones require high temperatures and pressures to become stable and allow growth. For example, marble‐hosted sapphires and rubies generally form at pressures above 5 kilobars (~20 km depth) with temperatures reaching over 600°C, often the result of continent–continent collisional zones.

Figure 2.11 Simple diagram for metamorphic rock descriptions based primarily on texture (foliated vs. nonfoliated). Schematic from ESProjects under Creative Commons.

Figure 2.12 This schematic of the Rock Cycle illustrates some of the most common pathways or process that geological materials are subjected to. Examples include partial melting of mantle material to form magma; magma crystallizing to form igneous rocks; weathering and erosion of igneous and metamorphic rock to produce sediments that lithify to form sedimentary rocks; some igneous and sedimentary rock undergo tectonic burial and metamorphism to form metamorphic rock. Spencer Sutton / Science Source.

The rock cycle (Figure 2.12) is a concept that describes how rocks can be transformed by various Earth processes into other rock types in the threefold classification. Note that not every rock has to pass through each of the stages in the rock cycle. For example, sedimentary rocks can be weathered into sediments without being subjected to metamorphism or melting, igneous rocks can be metamorphosed, and metamorphic rocks can undergo multiple stages of metamorphism.