Geology Final: Metamorphic Rocks

Primary agents driving metamorphism

Temperature, pressure, chemically active fluids, and original rock type (protolith)

Protolith

The original parent rock before metamorphism occurs.

Influence of temperature on metamorphism

Causes recrystallization and growth of new metamorphic minerals.

Effect of pressure on metamorphic rocks

Leads to compaction and alignment of minerals (foliation).

Role of fluids in metamorphism

Enhance chemical reactions and transport ions, speeding mineral changes.

Geotherm

A line showing how temperature increases with depth, varying by tectonic setting.

Foliation

Planar alignment of platy minerals due to directed pressure.

Lineation

Linear alignment of elongated minerals or structures within foliation.

Phyllite texture

Fine-grained with a silky sheen; platy minerals visible but small.

Schist texture

Medium to coarse-grained; visible aligned micas; clearly foliated.

Gneiss texture

Coarse-grained with compositional banding of light/dark minerals.

Migmatite

A partially melted rock showing both metamorphic and igneous textures.

Contact metamorphism

Heat-driven metamorphism near igneous intrusions, with low pressure; produces hornfels.

Regional (dynamothermal) metamorphism

Occurs over large areas under medium-high pressure and temperature during orogeny.

Hydrothermal metamorphism

Caused by hot fluids—common at mid-ocean ridges.

Impact (shock) metamorphism

Short-duration extreme P‑T changes from meteorite impacts; can produce coesite or stishovite.

Index minerals

Minerals stable only under specific P‑T conditions, used to gauge metamorphic grade.

Common index minerals in pelitic (mudstone) rocks

Chlorite, biotite, garnet, staurolite, kyanite, sillimanite.

Common index minerals in mafic rocks

Actinolite, hornblende, pyroxene, garnet.

Isograd

A line marking first appearance of a given index mineral on a map.

Isograd maps

Illustrate zones of increasing metamorphic grade across regions.

Greenschist facies

Low‑to‑medium grade; chlorite, actinolite, epidote; pronounced schistosity.

Amphibolite facies

Medium to high grade; hornblende + plagioclase; typical in orogenic belts.

Granulite facies

High grade; high T, medium‑high P; minerals like pyroxene + feldspar.

Blueschist facies

High P but low T; glaucophane presence indicates subduction conditions.

Eclogite facies

Very high P (rarely exposed); garnet + omphacite; forms deep in subduction zones.

Metamorphic rock progression from shale

Slate → Phyllite → Schist → Gneiss.

Hydrothermal metamorphism location

Most common at mid-ocean ridges - produces greenschist‑facies serpentinites.

Environment forming contact metamorphic hornfels

Adjacent to igneous intrusions with low pressure.

Index mineral signaling highest T conditions

Sillimanite.

Mineral indicating very high pressure

Omphacite in eclogite or glaucophane in blueschist.