Metamorphic Rock
Metamorphic rock forms when existing rock is changed by heat, pressure, or chemically active fluids without melting.
Definition
Metamorphic rock is rock that has been changed by heat and pressure deep underground without melting; the word "metamorphic" means "changed form." A parent rock (protolith) is subjected to temperatures and pressures high enough to recrystallize its minerals or grow new ones, but below the melting point. Foliated rocks such as slate, schist, and gneiss have layered or banded textures from directed pressure, while nonfoliated rocks such as marble and quartzite do not. Metamorphism occurs from roughly $150$ to $200^\circ C$ up to the onset of partial melting near $700$ to $900^\circ C$, at pressures from a few kilobars to more than $20$. Regional metamorphism at convergent boundaries produces foliation as platy minerals align perpendicular to maximum stress, and contact metamorphism next to intrusions produces hornfels. Index minerals (chlorite, biotite, garnet, staurolite, kyanite, sillimanite) map increasing grade, and metamorphic facies define pressure-temperature fields.
Example
Marble starts out as limestone, and slate, used for roof tiles and old chalkboards, starts out as a mudstone called shale. Much of the North Carolina Piedmont, including the ground beneath Charlotte, is metamorphic rock formed when ancient volcanic islands collided with North America, and the Blue Ridge Mountains expose gneiss and schist that were buried kilometers deep. Blueschist, containing the sodium amphibole glaucophane, forms only under high pressure and low temperature, conditions found in subduction zones. Its presence in the California Coast Ranges records rock dragged tens of kilometers down and returned to the surface.
Key Insight
Think of a metamorphic rock as a rock that has been baked and squeezed, like dough becoming bread, but never turned into liquid. Metamorphism is a one-way trip up in grade: shale becomes slate, then phyllite, then schist, then gneiss as heat and pressure increase. Metamorphic mineral assemblages work as pressure-temperature gauges. By matching the minerals in a rock to laboratory phase diagrams, geologists reconstruct the depth and thermal history of mountain belts that eroded away long ago.