Metamorphic Rocks

 

Overview of the process

Start with any rock

Subjected to different environment conditions

Commonly due to burial or subsidence of the crust due to tectonics

Heat and pressure usually involved

Generally at depth in the crust

Mineralogy ALWAYS changes in an attempt to restore equilibrium

Solid-State process - explain the reality of what this means

Iso-chemical process - explain the reality of what this means

Contact vs. Regional metamorphism

Litho-static vs. directed stress environments

Foliated vs. non-foliated rocks

 

The metamorphic process

Foliated metamorphic rocks

Usually associated with regional metamorphism

Result of heat and directed pressure

Therefore will generally exhibit a distinct layering

There is a fairly specific progression through the main metamorphic sequence

For example, starting with Shale - a common sedimentary rock

Very fine grain

Add HEAT and PRESSURE and it metamorphoses to...

Slate - little or no significant visible change

Still microscopic grains

Mineralogy begins to change

Usually to mica

Add more HEAT and PRESSURE and it metamorphoses to...

Phyllite - begin to see mineral grains

Commonly lots of mica - gives rock a shinny look

Add more HEAT and PRESSURE and it metamorphoses to...

Schist - significant change

Foliation of micaceous minerals (muscovite and/or biotite)

Other minerals begin to form based on composition of original rock and new conditions

Use additional minerals as modifier of name

EX: Hornblende schist, quartz schist, etc.

Add more HEAT and PRESSURE and it metamorphoses to...

Gneiss - high grade metamorphic rock

Color banding of light and dark minerals

Add more HEAT and PRESSURE and it metamorphoses to...

Migmatite - Partial melting of felsic minerals

Remember Bowen's Reaction Series?

The felsic minerals will melt at lower temperatures

Results in a rock with layers of "granite" and high grade mafic gneiss

Add enough HEAT and PRESSURE and it ultimately melts to form magma...

Granite or ??? - a new igneous rock after complete melting

 

Non-Foliated metamorphic rocks

Usually associated with contact metamorphism

Result of heat and litho-static pressure

Therefore will generally not exhibit a distinct layering

Marble - metamorphosed limestone

Relatively soft and will pass the fizz test

Generally coarsely crystalline

But can also be fine grained

Can be quite beautiful - many colors

Commonly used in carving

Carrera Marble - Italy

Very vine grain and pure

Quartzite - metamorphosed sandstone

Generally very hard and resistant

 


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