Metamorphic rocks are created when existing rock material, known as the protolith, is transformed by changes in heat, pressure, and chemically active fluids deep within the Earth’s crust. This process, called metamorphism, fundamentally alters the rock’s mineral structure and physical appearance. The modern classification system for these rocks relies on two primary observable characteristics: the rock’s physical structure, or texture, and its chemical makeup, which is expressed through its mineral content.
The Primary Basis: Texture
The most immediate characteristic used to classify a metamorphic rock is its texture, which describes the size, shape, and arrangement of its mineral grains. Texture is broadly divided into two major categories: foliated and non-foliated. The presence or absence of foliation is a direct indicator of the type of pressure the rock experienced during its formation.
Foliated rocks exhibit a distinct parallel alignment of mineral grains, resulting in a layered or banded appearance. This alignment forms under differential stress, where pressure is applied more intensely from one direction than others. The degree of foliation increases with the intensity of metamorphism, leading to different rock types distinguished by the coarseness of their layering.
The lowest grade of foliation is slaty cleavage, seen in slate, where microscopic mica and chlorite crystals align to allow the rock to split into thin, flat sheets. With increasing temperature and pressure, the mineral grains grow larger and develop schistosity, a wavy foliation where platy minerals like mica are visible, giving the rock a sparkly sheen (schist). The highest grade is gneissic banding, where light-colored minerals (quartz and feldspar) and dark-colored minerals (biotite and hornblende) form alternating, coarse layers (gneiss).
In contrast, non-foliated rocks lack any parallel alignment or layered structure. These rocks typically form under confining pressure, where stress is applied equally from all directions, or when the protolith is composed primarily of minerals that are not platy or elongated. The minerals in these rocks are often granular and equidimensional.
A common example is quartzite, which forms from the metamorphism of quartz-rich sandstone. The original quartz grains recrystallize and fuse together, creating a dense, interlocking mosaic structure. Similarly, marble forms from the metamorphism of limestone, where the calcite or dolomite crystals recrystallize into larger, blockier grains, erasing any sign of the original rock structure.
Secondary Basis: Mineral Content
While texture provides the initial, broad categorization, the specific minerals present are necessary for the final naming and understanding of a metamorphic rock. Mineral content directly reflects the chemical composition of the original parent rock (protolith) and the chemical reactions that took place during metamorphism. The minerals found in the rock determine the final part of its name, often used as a prefix to the textural classification, such as “biotite-garnet schist.”
The chemical makeup of the protolith dictates which elements are available to form new minerals under heat and pressure. For instance, a shale protolith, rich in aluminum and silicon, will yield different mineral assemblages than a basalt protolith, which is rich in iron and magnesium. Minerals that are stable across a wide range of temperatures and pressures, such as quartz and feldspar, may persist from the protolith, while others chemically react to form new, more stable phases.
Geologists use the presence of specific index minerals to refine the classification and history of the rock. These minerals form only within narrow, predictable ranges of temperature and pressure. For example, the presence of chlorite indicates a low-grade, low-temperature metamorphic environment. Finding staurolite or kyanite in the same rock type, however, indicates the rock achieved a significantly higher pressure and temperature during its transformation.
Linking Classification to Formation Conditions
The combined data from a rock’s texture and mineral content allows geologists to reverse-engineer the rock’s formation environment. The observed physical and chemical changes are systematically linked to the intensity of metamorphism, known as the metamorphic grade. This grade is a measure of the maximum temperature and pressure the rock reached.
Metamorphic grade is a continuum, described as low, intermediate, or high. As a protolith like shale is subjected to increasing temperature and pressure, it changes in a predictable sequence. This progression is visible in the textural changes, moving from fine-grained slate at low grades, to schist at intermediate grades, and finally to coarse-banded gneiss at high grades.
This increase in grade also corresponds directly with the progressive appearance of index minerals, creating distinct mineral zones. For example, in the metamorphism of a mudrock, the index minerals appear in sequence: chlorite, followed by biotite, then garnet, and finally sillimanite, each marking a step up in temperature and pressure. The rock name often incorporates the highest-grade index mineral present, giving a concise summary of its history.
This concept is formalized in the metamorphic facies system, which groups rocks based on a predictable set of mineral assemblages that form under specific pressure and temperature fields. For instance, the Greenschist facies is defined by the presence of low-grade minerals like chlorite and is associated with low-to-moderate temperatures. Conversely, the high-temperature Granulite facies is characterized by water-poor minerals and represents conditions deep within the continental crust. Classifying a metamorphic rock by its texture and mineralogy is, in essence, classifying the pressure and temperature conditions of its birth.