Metamorphic rocks are rocks that have been transformed from pre existing rocks by heat, pressure, chemically active fluids or combinations of these factors without completely melting. The original rock is called the protolith. The protolith may be igneous, sedimentary or an older metamorphic rock. During metamorphism, minerals can recrystallise, new minerals can form, grains can change shape and rocks can develop new textures and structures. Because the rock remains largely solid, metamorphism differs from melting and crystallisation.
Slate, schist, gneiss, marble and quartzite are common examples. Metamorphism occurs over a wide range of temperatures and pressures and can take place deep within Earth's crust, around intruding magma bodies or along zones of intense tectonic deformation. Metamorphic rocks therefore preserve evidence of geological conditions that existed after the original rock formed but before the metamorphic rock was exposed at the surface.
Heat is a major driver of metamorphism because increasing temperature allows atoms and ions within minerals to rearrange. Regional metamorphism commonly occurs when rocks are buried deeply during mountain building and experience elevated temperature and pressure over large areas. Contact metamorphism occurs around hot magma intrusions, where surrounding rocks are heated and chemically altered. Burial metamorphism can occur in deeply buried sedimentary sequences where increased pressure and temperature cause mineral transformations.
Hydrothermal metamorphism involves hot chemically active fluids moving through rocks, commonly near magma bodies and mid ocean ridges. Shock metamorphism can occur during meteorite impacts, where extremely high pressures and temperatures act over very short periods. These different settings produce distinct metamorphic textures and mineral assemblages.
Pressure in metamorphism can arise from the weight of overlying rocks or from directed tectonic forces. Lithostatic pressure acts approximately equally in all directions, whereas differential stress can deform rocks preferentially. Under directed stress, minerals may rotate, recrystallise or grow in orientations that produce foliation. Slate, schist and gneiss can display progressively stronger foliation depending on metamorphic grade and protolith. Foliation is particularly common in rocks containing platy or elongated minerals such as mica. Not all metamorphic rocks are foliated. Marble and quartzite can develop non foliated crystalline textures because their major minerals are relatively equidimensional or because the conditions do not produce strong mineral alignment. Metamorphic texture therefore reflects both mineral composition and the physical forces acting during transformation.
Metamorphic reactions can create entirely new minerals that were not stable in the original rock. Clay minerals in shale can transform progressively into mica rich assemblages as pressure and temperature increase. Carbonate rich limestone can recrystallise into marble, while quartz rich sandstone can recrystallise into quartzite. In more intense conditions, minerals stable at lower pressures and temperatures may break down and form minerals stable under new conditions.
Index minerals such as garnet, staurolite, kyanite and sillimanite can provide clues about metamorphic pressure and temperature. Mineral assemblages are interpreted using experimental petrology and thermodynamic principles. Because different minerals are stable under different conditions, metamorphic rocks can function as geological records of the pressure temperature histories experienced by their parent rocks.
Metamorphic grade describes the intensity or degree of metamorphism, broadly reflecting changes in temperature, pressure and deformation. Low grade metamorphism may produce relatively subtle mineralogical and textural changes, while high grade metamorphism can produce substantial recrystallisation and new mineral assemblages. Metamorphic grade is not simply a single temperature scale because pressure, fluid composition, protolith chemistry and deformation also matter. Two rocks exposed to the same temperature can undergo different transformations if their compositions or pressures differ. Geologists therefore use mineral assemblages, textures and structural relationships to reconstruct metamorphic conditions. Metamorphic rocks can preserve multiple generations of minerals and deformation, revealing that a rock may have passed through several metamorphic stages during a long tectonic history.
Metamorphic rocks are especially important in mountain belts and plate boundary regions. During continental collision, rocks can be buried to considerable depths, compressed and heated before being uplifted and exposed by erosion. Metamorphic minerals can therefore reveal the depth and temperature conditions reached during mountain building. Subduction zones can carry rocks to high pressures while maintaining relatively low temperatures compared with other metamorphic environments, producing distinctive high pressure mineral assemblages. Rocks may subsequently be exhumed and brought back towards the surface through tectonic uplift, faulting and erosion. The metamorphic record can therefore preserve evidence of plate convergence, crustal thickening, subduction and exhumation.
Metamorphic rocks have practical importance as building and decorative materials. Marble has been widely used for sculpture, architecture and interior surfaces because it can be polished and has distinctive patterns. Slate can be split into thin sheets and has traditionally been used for roofing and other applications. Quartzite can be extremely hard and resistant to weathering and is used as construction stone and aggregate.
Metamorphic processes can also modify the distribution of economically important minerals by concentrating or remobilising elements through recrystallisation and fluid movement. The geological interpretation of metamorphic terrains is consequently important for mineral exploration and engineering. The strength and fracture behaviour of metamorphic rocks vary according to mineral composition, foliation, alteration and structural history.
Metamorphic rocks remain part of the rock cycle and can eventually be melted to form magma, uplifted and weathered into sediment, or undergo further metamorphism under changing conditions. A metamorphic rock can therefore have a complex ancestry. For example, a sedimentary shale can become slate, then schist and eventually gneiss as metamorphic conditions intensify, although actual geological pathways vary. A common misconception is that metamorphism means simply heating a rock until it melts.
Complete melting produces magma, and the subsequent crystallisation of that magma forms an igneous rock. Metamorphism instead involves solid state transformation, although small amounts of partial melting can occur at very high grades and produce complex mixed rocks. Metamorphic rocks are valuable because they preserve evidence of the changing physical and chemical conditions experienced by Earth's crust through geological time.
