Metamorphism is a geological process that transforms pre-existing rock into a new rock type without melting it. This transformation involves significant changes in the rock’s mineral composition, texture, or both. The original rock, known as the protolith, can be igneous, sedimentary, or older metamorphic rock. These changes occur in the solid state when the rock is subjected to different physical and chemical conditions, typically at temperatures starting around 150°C to 200°C deep within the Earth’s crust. This process creates denser, more compact rocks with a new internal structure.
Fundamental Drivers of Rock Change
Three primary agents drive the change in a rock’s structure and mineral composition deep beneath the surface. Temperature is a major force, providing the energy necessary for chemical reactions and the atoms’ movement within the crystal lattice. Heat allows existing mineral grains to recrystallize, where they rearrange and often grow into larger, more stable forms without the rock reaching its melting point. This thermal energy mostly originates from the Earth’s natural geothermal gradient, which increases temperature with depth, or from nearby intrusions of hot magma.
Pressure is the second major driver, and it manifests in two distinct ways. Confining pressure is the uniform force exerted equally on all sides of a rock body by the weight of the overlying material. This pressure causes the rock to become more compact and dense, reducing its overall volume. In contrast, directed stress involves an unequal force applied from specific directions, often associated with tectonic plate movement.
The third agent consists of chemically active fluids, primarily hot water rich in dissolved ions and volatiles like carbon dioxide. These fluids permeate the microscopic pore spaces and fractures within the rock. Water acts as a catalyst, significantly increasing the rate at which metamorphic reactions occur by facilitating the transfer of ions. When these fluids introduce new elements or remove existing ones, the process is called metasomatism, which alters the rock’s chemical makeup.
Metamorphism Defined by Geologic Setting
The specific geological environment where these three agents interact determines the type of metamorphism that occurs. Regional metamorphism is the most widespread type, affecting vast volumes of rock over hundreds or even thousands of square kilometers. It is intrinsically linked to mountain-building events, such as continental collisions at convergent plate boundaries, where both high temperature and intense directed stress are simultaneously applied. This combination of deep burial and tectonic squeezing results in the characteristic deformation and mineral alignment observed in the cores of mountain ranges.
Contact metamorphism is a localized phenomenon that is almost entirely heat-driven, occurring at relatively low pressure in the upper crust. This type occurs when an intrusion of magma heats the surrounding country rock, effectively baking it. The zone of alteration is confined to a narrow band, called a metamorphic aureole, which surrounds the igneous body. Since the stress is minimal, the resulting rocks are typically changed in mineralogy but not structurally deformed.
Dynamic metamorphism is characterized by extremely high shear stress and localized deformation. This type is restricted to narrow zones along major faults, where rocks are crushed, pulverized, and ground into fine-grained fragments by frictional movement. The resulting rock, known as a mylonite or fault breccia, records the mechanical energy of the movement.
Burial metamorphism takes place when thick sequences of sedimentary or volcanic rocks accumulate in large basins. The sheer weight of the overlying material creates the confining pressure necessary to initiate low-grade metamorphic changes, though the temperatures remain relatively low.
Hydrothermal metamorphism involves the pervasive circulation of hot, ion-rich water, which is particularly common at mid-ocean ridges where new oceanic crust is formed. Here, seawater penetrates deep into the crust, is heated by magma, and reacts chemically with the basaltic rock, leading to significant chemical alteration.
Metamorphic Classification Based on Texture
Metamorphic rocks are also categorized by their resulting physical appearance, or texture, which provides direct evidence of the forces involved. The two major textural categories are foliated and non-foliated rocks. Foliation describes a planar arrangement of mineral grains or structural features, giving the rock a layered or banded look. This texture forms when the rock is subjected to intense directed stress, which causes platy or elongated minerals like mica and chlorite to align themselves perpendicular to the maximum stress.
The degree of foliation correlates with the intensity of metamorphism. A low-grade example is slate, which develops a fine, planar layering called slaty cleavage, easily splitting into thin sheets. With increasing temperature and pressure, the minerals grow larger, progressing through phyllite, which has a silky sheen due to slightly larger mica crystals, to schist, where visible, aligned mica flakes dominate the texture. The highest-grade foliated rock is gneiss, characterized by distinct bands of light and dark minerals, which have separated due to extreme temperature and pressure.
Non-foliated rocks lack this preferred mineral alignment and typically exhibit a massive, homogeneous appearance. This texture usually results from metamorphism where confining pressure was dominant over directed stress, such as in contact or burial settings. It also occurs when the protolith was composed of minerals that are not platy or elongated. Marble, formed from the metamorphism of limestone, is a common non-foliated rock where the calcite crystals recrystallize into a dense, interlocking mosaic. Similarly, quartzite forms from quartz-rich sandstone, with the quartz grains fusing together to create an extremely hard rock.