Definition · Plain-language
Metamorphic rock
Metamorphic rock forms when an existing rock is transformed by heat, pressure or chemically active fluids, while staying solid.
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Rock changed without melting
Metamorphic rock is rock that has been transformed in the solid state. When a pre-existing rock — the parent or protolith — is subjected to high temperature, high pressure, or hot mineral-rich fluids, its minerals become unstable and recrystallise into new ones better suited to the new conditions. Crucially, the rock does not melt; if it did, it would become magma and eventually igneous rock. The word “metamorphic” comes from Greek roots meaning “change of form”, capturing how the rock’s texture and mineral make-up are reorganised while it remains solid.
Foliated and non-foliated
Metamorphic rocks fall into two textural groups. Foliated rocks form under directed pressure that flattens and aligns minerals into visible layers or bands — slate, schist and gneiss are examples, with bands becoming coarser as conditions intensify. Non-foliated rocks form where pressure is more uniform or the parent mineral does not align; they look more even and crystalline, like marble (from limestone) or quartzite (from sandstone). The presence and style of layering is therefore a clue to the pressures the rock endured during its transformation.
Where metamorphism happens
Metamorphism occurs where Earth supplies enough heat or pressure. Regional metamorphism affects large areas during mountain-building, as colliding tectonic plates bury and squeeze huge volumes of rock — this produces most foliated rocks. Contact metamorphism happens locally where hot magma intrudes and “bakes” the surrounding rock, changing it without much directed pressure. In the rock cycle, metamorphic rock can itself be uplifted and weathered into sediment, or heated further until it melts into magma, showing that no rock type is a permanent end state.
Key facts
At a glance
- Definition: existing rock transformed by heat and pressure without melting
- Name meaning: from Greek for “change of form”
- Starts from: igneous, sedimentary or older metamorphic rock (the protolith)
- Two textures: foliated (banded) and non-foliated (even)
- Examples: marble (from limestone), slate (from shale), gneiss, quartzite
- Settings: regional (mountain-building) and contact (near magma)
Common misconceptions
What people often get wrong
Often heard: Metamorphic rock forms by melting an existing rock.
Actually: Metamorphism happens in the solid state. If a rock actually melts it becomes magma, which crystallises into igneous rock instead. Metamorphic change recrystallises minerals without full melting.
Often heard: Only sedimentary rocks can turn into metamorphic rocks.
Actually: Any rock can be metamorphosed — igneous, sedimentary or even older metamorphic rock. Marble comes from sedimentary limestone, but gneiss can form from igneous granite.
Often heard: All metamorphic rocks have visible bands or layers.
Actually: Only foliated rocks are banded. Non-foliated metamorphic rocks like marble and quartzite are evenly crystalline, because they formed under uniform pressure or from minerals that do not align.
