During metamorphism, rocks undergo changes in their minerals, texture, density, and sometimes chemical composition, all while remaining solid. These changes happen at temperatures above 200°C and pressures beyond about 300 MPa, and they stop once the rock begins to melt (at which point the process becomes igneous). Understanding which changes occur helps distinguish metamorphic processes from other geological forces.
New Minerals Replace Old Ones
One of the most significant changes during metamorphism is the growth of entirely new minerals from existing ones. As temperature and pressure increase, the atoms in a rock’s original minerals rearrange into crystal structures that are more stable under the new conditions. A clay-rich shale, for instance, develops mica crystals as it transforms into slate, then schist. At extremely high pressures (around 35 kilobars), even common quartz can transform into coesite, a denser version of the same chemical compound. At high pressures and relatively low temperatures, minerals like glaucophane (a blue amphibole) and jadeite form, giving blueschist its distinctive color.
These “index minerals” serve as geological thermometers and pressure gauges. Garnet, kyanite, and sillimanite each form within specific temperature and pressure windows. Kyanite, the densest form of aluminum silicate, appears under high-pressure conditions. Rocks metamorphosed at extreme temperatures above 1,000°C, like those found in Antarctica’s Napier Complex, contain rare minerals such as sapphirine and osumilite that simply don’t form under milder conditions.
Crystal Size Increases Through Recrystallization
Even when no new mineral species form, existing crystals can grow larger through recrystallization. This process rearranges atoms at grain boundaries without the rock ever melting. In quartz-rich rocks, the specific recrystallization mechanism depends on temperature: below about 400°C, small new grains bulge out from existing crystal boundaries. Between 400°C and 500°C, crystals rotate and subdivide. Above 500°C, grain boundaries migrate freely, producing noticeably larger crystals.
This is why low-grade metamorphic rocks like slate have crystals too small to see, while high-grade rocks like gneiss contain visible, sometimes centimeter-scale mineral grains. The rock never liquefied. It simply spent enough time at high enough temperatures for atoms to slowly migrate and crystals to coarsen.
Minerals Align to Create Foliation
When rocks are squeezed under directed pressure (not uniform pressure from all sides), their minerals physically reorient. Platy minerals like mica and elongated minerals like amphibole grow with their long axes perpendicular to the direction of greatest stress. This alignment produces foliation: the layered, sheet-like texture visible in many metamorphic rocks.
Foliation develops in a progressive sequence. At low grades, you get slaty cleavage, where microscopic clay and mica grains align into smooth, flat planes that the rock splits along easily. With more heat and pressure, phyllite forms with a wavy, silky sheen as mica crystals grow slightly larger. At higher grades, schist develops with clearly visible mica flakes and sometimes large crystals of garnet or feldspar. At the highest grades, minerals separate into distinct light and dark bands, producing gneiss.
Not all metamorphic rocks develop foliation. When pressure is uniform (as in contact metamorphism near a magma body), or when the rock lacks platy minerals, the result is a non-foliated texture. Limestone becomes marble, sandstone becomes quartzite, and various rocks become hornfels, all without developing any directional alignment.
Density Increases and Volume Decreases
As burial pressure rises, rocks become denser. Porosity drops as pore water is squeezed out during the early stages of heating and compression. At a chemical level, increasing pressure favors minerals with more tightly packed atomic structures. Kyanite forms instead of the less dense andalusite precisely because its crystal structure packs atoms closer together, making it the stable form of aluminum silicate at high pressure.
Dehydration reactions during progressive metamorphism release water from minerals, which reduces the solid volume of the rock. This water loss is a one-way process under increasing temperature. During retrograde metamorphism, when conditions cool and water is reintroduced, hydration reactions do the opposite: the solid volume increases as water molecules are incorporated back into mineral structures. These volume changes are significant enough that they affect fluid pressure and rock porosity, and they’re thought to contribute to earthquake generation in subduction zones.
Chemical Composition Can Change
Most metamorphism is roughly isochemical, meaning the rock’s overall chemical composition (aside from water and carbon dioxide) stays about the same even as its minerals completely change. The atoms simply rearrange into different mineral combinations without being added or removed.
The exception is metasomatism, where hot fluids infiltrate the rock and carry new elements in or existing elements out. These externally derived fluids can change the rock’s bulk chemistry significantly, sometimes faster than minerals within the rock can adjust. Contact metamorphism sometimes involves metasomatism when hot fluids escape from a magma intrusion and penetrate surrounding rock along fractures, depositing new minerals and altering the original composition.
The Progressive Sequence From Shale to Gneiss
All of these changes, mineral growth, recrystallization, foliation, density shifts, and potential chemical alteration, work together during metamorphism. The classic example is the transformation of shale through increasing grades of regional metamorphism: shale becomes slate, then phyllite, then schist, then gneiss. At each step, crystals grow larger, foliation becomes more pronounced, and the mineral assemblage shifts to reflect higher temperatures and pressures. Regional metamorphism covers conditions from about 200°C to 750°C and depths of roughly 5 to 35 kilometers.
If temperatures continue rising beyond the gneiss stage, the rock begins to partially melt, producing a migmatite, a swirled mixture of solid metamorphic rock and pockets of melt. Once melting is complete, the process is no longer metamorphic. It has crossed into the igneous realm. That boundary, the onset of melting, is the upper limit of metamorphism by definition.