Chapter 8 – Metamorphism and Metamorphic Rocks

Metamorphism

The process meaning "to change form"; rocks are baked and squeezed deep underground but do not melt, forming new solid rocks.

Solid-State Change

Metamorphism occurs in the solid state; minerals recrystallize into new stable forms without melting.

Parent Rock

The original rock that undergoes metamorphism; also called the protolith.

Examples of Parent to Metamorphic Rocks

Shale → Slate → Schist → Gneiss; Limestone → Marble; Sandstone → Quartzite.

Purpose of Heat in Metamorphism

Acts as the “baker”; increases temperature causing minerals to rearrange into more stable forms.

Sources of Heat in Metamorphism

Includes Earth’s geothermal gradient (25–30°C per km) and nearby magma intrusions.

Purpose of Pressure in Metamorphism

Acts as the “squeezer”; compacts rocks and can cause foliation when pressure is uneven.

Confining Pressure

Equal pressure from all sides due to deep burial; compacts and increases rock density.

Differential Stress

Unequal pressure greater in one direction; aligns minerals and creates foliation (banded texture).

Chemically Active Fluids

Hot, ion-rich fluids that move through rock, speeding up mineral growth and causing chemical changes.

Metasomatism

Process where chemically active fluids change the composition of a rock during metamorphism.

Foliated Texture

Texture where minerals are aligned in layers or bands due to directional pressure.

Non-Foliated Texture

Texture without layers; forms when pressure is equal or minerals cannot align.

Porphyroblastic Texture

Texture with large crystals (porphyroblasts) growing in a fine-grained matrix, like garnet in schist.

Slate

A fine-grained, low-grade foliated rock with flat sheets and dull surfaces.

Phyllite

A foliated rock with a shiny, wavy surface caused by growing mica minerals; medium-low grade.

Schist

A medium- to coarse-grained foliated rock with visible mica flakes; shows schistosity texture.

Gneiss

A high-grade foliated rock with light and dark mineral bands showing gneissic texture.

Marble

A non-foliated rock formed from limestone or dolostone; composed of calcite; fizzes with acid.

Quartzite

A hard, non-foliated rock formed from quartz-rich sandstone; glassy and resistant to weathering.

Hornfels

A fine-grained, non-foliated rock formed by contact metamorphism near magma bodies.

Contact Metamorphism

Occurs when rock is heated by nearby magma; high temperature and low pressure; produces non-foliated rocks.

Regional Metamorphism

Occurs during mountain building at convergent boundaries; high temperature and pressure; forms foliated rocks.

Hydrothermal Metamorphism

Occurs when hot, mineral-rich fluids circulate through rock; common at mid-ocean ridges.

Burial Metamorphism

Results from deep burial in sedimentary basins; low-grade metamorphism from confining pressure and heat.

Subduction Zone Metamorphism

Occurs where tectonic plates descend; high pressure and low temperature; produces blueschist rocks.

Shock (Impact) Metamorphism

Forms from meteorite impacts; extreme pressure creates high-pressure minerals like coesite.

Metamorphic Grade

The intensity of metamorphism determined by temperature and pressure conditions.

Low-Grade Metamorphism

Produces rocks like slate and phyllite at temperatures around 200–300°C.

Medium-Grade Metamorphism

Produces schist with visible minerals such as garnet and staurolite.

High-Grade Metamorphism

Produces rocks like gneiss and migmatite; occurs above 600°C.

Index Minerals

Minerals that form only under specific pressure and temperature conditions; used to estimate metamorphic grade.

Examples of Index Minerals

Low-grade: chlorite, muscovite; Medium-grade: garnet, staurolite; High-grade: sillimanite, kyanite.

Prograde Metamorphism

Metamorphism during increasing temperature and pressure, usually during burial or mountain formation.

Retrograde Metamorphism

Metamorphism during cooling and uplift; minerals may rehydrate or revert to low-temperature forms.

Metamorphic Facies

Groups of minerals that form under similar temperature and pressure ranges, such as greenschist or blueschist facies.

Migmatite

A rock that is partially melted; a mixture of igneous and metamorphic features, often derived from gneiss.

Dynamic Metamorphism

Metamorphism caused mostly by mechanical deformation along faults; produces mylonite.

Preferred Orientation

The alignment of platy or elongated minerals like micas and amphiboles under directional stress.