Bury a rock deep, heat it, squeeze it, and it changes, even though it never melts. That is a metamorphic rock ("changed form"). The heat and pressure come from deep burial, from nearby magma, or from plates pushing together. The starting rock can be igneous, sedimentary or even another metamorphic rock.
| Starting rock | Metamorphic rock | Texture |
|---|---|---|
| Shale | Slate, then phyllite, schist and gneiss as heat and pressure rise | Foliated: minerals lined up in layers or bands |
| Granite | Gneiss | Foliated: light and dark bands |
| Limestone | Marble | Not foliated: interlocking calcite crystals |
| Sandstone | Quartzite | Not foliated: very hard, quartz grains fused |
Key idea
Metamorphic rock forms when heat and pressure change a rock's minerals and texture while it stays solid. Squeezing often lines minerals up into layers (foliation).
Why it works
Atoms in a solid can slowly move and rearrange. At high temperature and pressure, some minerals are no longer stable, and their atoms regroup into new minerals that are, such as mica or garnet. When rock is squeezed harder in one direction, flat minerals like mica grow at right angles to the push, so they line up in sheets. That is why slate splits into flat slabs.
Worked example: From limestone
Limestone near a hot granite intrusion is baked. What rock forms?
- Limestone is made of calcite.
- Heat recrystallizes the calcite into larger, interlocking crystals: marble.
Answer: Marble
Worked example: Rising grade
Order these by how much heat and pressure they formed under, lowest first: schist, slate, gneiss.
- Slate forms at low grade from shale: tiny mica crystals.
- Schist forms at higher grade: mica crystals big enough to sparkle.
- Gneiss forms at the highest grade: minerals sorted into bands.
Answer: Slate, schist, gneiss
Common mistake
Thinking metamorphic rock forms by melting. If a rock melts, it becomes magma, and when that cools it is igneous rock. Metamorphism happens while the rock stays solid. Check: metamorphic rocks still show bent layers and stretched pebbles, which melting would erase.