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Metamorphic rocks form when existing rocks are changed by heat, pressure, or both — without melting. The word "metamorphic" comes from Greek meaning "change of form." When a rock is buried deep underground or pushed against hot magma, the minerals inside it rearrange and recrystallize into new patterns. Shale becomes slate, limestone becomes marble, and sandstone becomes quartzite. The original rock's identity is transformed, but it never became liquid — that is what separates metamorphism from the melting that creates igneous rocks.
Show transformation sequences: shale → slate → schist → gneiss, with increasing levels of metamorphism. Let students feel how slate splits into flat sheets (foliation from pressure) compared to the random texture of marble (no foliation). Use a modeling clay analogy: squeeze clay between your hands to show how pressure aligns flat minerals into layers. A piece of bread in a panini press illustrates how pressure changes texture without changing ingredients.
You have seen how igneous rocks form from melted rock cooling down and how sedimentary rocks form from pieces piling up and cementing together. Metamorphic rocks take a completely different path — they form when an existing rock is transformed by heat, pressure, or both, all while remaining solid.
Imagine a rock buried kilometers underground by tectonic forces. The deeper it goes, the hotter and more compressed it becomes. At these extreme conditions, the minerals inside the rock start to change. They do not melt — the temperature is high but not quite high enough for that. Instead, atoms rearrange within the solid rock, breaking old mineral structures and forming new ones that are stable at the higher temperature and pressure. Flat, platy minerals like mica align themselves perpendicular to the direction of pressure, creating a layered appearance called foliation — visible in rocks like slate and schist. This is why slate splits so neatly into thin sheets: all its mineral grains were squeezed into parallel alignment.
Not all metamorphic rocks are foliated, though. When limestone is metamorphosed, it becomes marble. Limestone's tiny calcium carbonate grains recrystallize into larger, interlocking crystals, producing marble's smooth, even texture. Since the mineral (calcite) is roughly the same shape in all directions, there are no flat grains to align, so marble does not develop layers. Similarly, sandstone metamorphoses into quartzite — the quartz grains fuse together so tightly that the rock breaks through the grains rather than around them, making quartzite extremely hard and durable.
The critical boundary to remember is between metamorphism and melting. As long as the rock stays solid and its minerals simply rearrange, the process is metamorphism and the product is a metamorphic rock. The moment the temperature rises enough to actually melt the rock, you have crossed into igneous territory — the liquid magma, when it cools, will produce an igneous rock. This boundary is not always sharp in nature, but the distinction matters: metamorphism is transformation without destruction, like reshaping clay on a potter's wheel without dissolving it back into mud.