Metamorphism needs heat, pressure or hot fluids, and different places on Earth supply them in different mixes. Knowing which rock formed and at what pressure and temperature lets geologists say where it formed, and even which kind of plate boundary once stood there.
- Contact metamorphism: heat from an intrusion bakes the rock beside it in a zone (an aureole) meters to kilometers wide. High temperature, low pressure; it can happen just a few kilometers down. Makes hornfels and marble.
- Regional metamorphism: when plates collide, huge regions of crust are buried, heated and squeezed together. Medium to high pressure and temperature, with directed stress. Makes slate, schist and gneiss.
- Subduction-zone metamorphism: a cold ocean plate sinks faster than heat can warm it, so it meets high pressure at low temperature. Makes blueschist, colored by a blue amphibole.
- Hydrothermal metamorphism: hot water circulating through rock, especially new ocean crust at ridges, changes its minerals.
- Impact metamorphism: a meteorite strike delivers enormous pressure for an instant, leaving shocked quartz.
Why it works
Each setting has its own ratio of temperature to depth. Divide a rock's formation temperature by its depth (from $P = \rho g h$): around $10^\circ\text{C/km}$ or less means rock went deep while staying cold, which happens only where a slab is sinking. About 20 to $35^\circ\text{C/km}$ is the normal heating of thickened crust in a mountain belt. Much higher means a heat source close by: magma.
Key idea
Contact metamorphism bakes rock beside magma (hot, shallow); regional metamorphism comes with mountain building (hot and deep, with directed stress); subduction makes high-pressure, low-temperature rocks such as blueschist.
Worked examples
Worked example: Reading the gradient
A rock formed at $350^\circ\text{C}$ and 1.2 GPa. With rock of $2,800\,\text{kg/m}^3$, what setting fits?
- Depth: $h = \dfrac{1.2 \times 10^9}{2,800 \times 9.8} \approx 43.7\,\text{km}$.
- Gradient: $350 \div 43.7 \approx 8^\circ\text{C/km}$: very cold for its depth.
- Only a sinking slab goes that deep while staying that cool: a subduction zone (blueschist conditions).
Answer: A subduction zone.
Worked example: A baked zone
Shale changes to hornfels only within 300 m of a granite stock that cooled 3 km down. Which setting?
- The change is limited to a narrow zone around the intrusion, at shallow depth.
- That is contact metamorphism: the heat came from the magma, not from deep burial.
Answer: Contact metamorphism.
Common mistake
It is easy to think all metamorphism needs deep burial. A hot intrusion can bake the rock beside it only a few kilometers down, or even right at the surface under a thick lava flow. Check the extent: metamorphism that fades within a zone around an intrusion is contact metamorphism, whatever the depth.