Contact, Regional, Hydrothermal & Burial

All metamorphism involves heat, pressure, and fluids, but the balance of these agents varies dramatically. Geologists classify metamorphism by the dominant agent and the tectonic setting:

• Contact — heat dominates near an intrusion.
• Regional — directed pressure + heat in mountain belts.
• Hydrothermal — hot fluids drive chemical alteration.
• Burial — confining pressure + geothermal gradient in deep basins.

The rocks produced in each setting differ in texture, mineralogy, and extent — and those differences are clues to the ancient environment.

When magma intrudes shallow crust, it radiates intense heat into the surrounding country rock. This creates a contact aureole — a halo of metamorphosed rock that gradually fades outward as temperature drops with distance from the intrusion.

Within the aureole, shale becomes hornfels, limestone becomes marble, and sandstone becomes quartzite. The rocks are typically fine-grained and non-foliated because heat is the dominant agent and directed pressure is minimal at shallow depths.

In carbonate rocks near iron-rich intrusions, skarns form — coarse metamorphic rocks rich in garnet and pyroxene, often associated with ore deposits.

Regional metamorphism is the large-scale transformation of rock in the cores of mountain ranges. It is driven by the combination of directed pressure (from tectonic compression) and heat (from deep burial and radiogenic decay).

This is the type that produces the classic sequence of foliated rocks: slate → phyllite → schist → gneiss. It is most intense at convergent plate boundaries, where continents collide and crust is thickened to 60–80 km. The Himalaya, Appalachians, and Scottish Highlands all expose regional metamorphic rocks.

At mid-ocean ridges and in volcanic geothermal systems, seawater or groundwater circulates through hot rock and drives hydrothermal metamorphism. The hot fluids chemically alter the original minerals, replacing them with new assemblages stable at lower temperatures.

Oceanic basalt is particularly susceptible. As hot fluids percolate through fractures, they convert the original igneous minerals (pyroxene, olivine, Ca-plagioclase) into chlorite, epidote, and actinolite — the signature minerals of greenschist facies. This altered crust is what gets subducted, carrying water and volatiles back into the mantle.

In thick sedimentary basins, layers of sediment are buried progressively deeper by ongoing deposition. The weight of overlying sediment provides confining pressure, while the geothermal gradient supplies gentle heat. The result is burial metamorphism.

Because there is little directed pressure, burial metamorphism does not produce strong foliation. Instead, it drives diagenesis into low-grade metamorphism, producing zeolite and prehnite-pumpellyite assemblages. It is essentially the bridge between sediment compaction and true metamorphism.