Module 6

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Metamorphism: A Process of Change

Metamorphism:

Solid-state transformation of existing rocks (protoliths)

High heat, pressure, & chemically active fluids

Occurs deep in Earth's crust

Changes rocks minerology and texture without melting - causes recrystallization, increased density, & foliation (layering)

Two main agents of change in metamorphic rocks:

Heat

Pressure

Protolith: original, unmetamorphosed parent rock from which a metamorphic rock is formed

Examples: Protolith - limestone ; metamorphic equivalent - marble

Conglomerate/Metaconglomerate

Sandstone/Quartzite

Shale/Garnett Schist

Limestone/Pure or Coarse Marble (coarse made by "impure" marble)

Granite/Gneiss

Textural changes due to heat in metamorphism:

Denser more compact rocks by being bent, folded, crushed flattened & sheared

Recrystallization: larger more stable crystals

Non-foliated texture: (contact metamorphism) granular, tightly-packed crystals of same dimensions

Foliation & Alignment: (under intense heat/pressure) layered or banded textures (schist & gneiss)

Loss of original texture

Normal Stress: force acts perpendicular to the surface

Shear Stress: force acts parallel to the surface

Contact Metamorphism involves which agent of change?

Heat (due to magma) ex. quartz, marble

Regional Metamorphism ("mountain belt" metamorphism) involves which agent of change?

Pressure & Temperature

Creates foliated rocks

Caused by large geologic processes - mountain building

Foliation: repetitive layering in metamorphic rocks; caused by shearing /differential pressure

Lowest grade of foliation & what rock it's typically seen in:

Slate, made from shale that's given low-grade pressure/heat

"Metamorphic grade": increasing amounts of heat/pressure have been applied to metamorphose the rock

How does gneiss end up with bands/layers of separate minerals?

High amounts of heat/pressure crystallized into bands

Foliation

Polymorphs (kyanite, andalusite, sillimanite)

Indicators of pressure and temperature in metamorphic rocks

Minerals that can exist in more than one crystal form under certain conditions (normally heat/pressure)

Order of metamorphic indicators from low grade to high grade:

Chlorite

Biotite

Garnet

Stuarolite

Kyanite

Silliminite

Patterns of metamorphism: low to high

Provide clues of past mountain-building events, used to create maps of past mountain ranges

Example: Northern Appalachian Mountains ; metamorphic grade progression (low-grade to high-grade)

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